The single-celled cotton fibers, produced from seed coat epidermal cells are the largest natural source of textile fibers. The economic value of cotton fiber lies in its length and quality. The multifunctional laccase enzymes play important roles in cell elongation, lignification and pigmentation in plants and could play crucial role in cotton fiber quality. Genome-wide analysis of cultivated allotetraploid (G. hirsutum) and its progenitor diploid (G. arboreum and G. raimondii) cotton species identified 84, 44 and 46 laccase genes, respectively. Analysis of chromosomal location, phylogeny, conserved domain and physical properties showed highly conserved nature of laccases across three cotton species. Gene expression, enzymatic activity and biochemical analysis of developing cotton fibers was performed using G. arboreum species. Of the total 44, 40 laccases showed expression during different stages of fiber development. The higher enzymatic activity of laccases correlated with higher lignin content at 25 DPA (Days Post Anthesis). Further, analysis of cotton fiber phenolic compounds showed an overall decrease at 25 DPA indicating possible incorporation of these substrates into lignin polymer during secondary cell wall biosynthesis. Overall data indicate significant roles of laccases in cotton fiber development, and presents an excellent opportunity for manipulation of fiber development and quality.
Cotton fiber is a single-celled seed trichome, developed from seed coat epidermal cells through four distinct, yet overlapping stages: 1) initiation (−3 to 5 DPA, days post anthesis); 2) elongation (5–25 DPA); 3) secondary cell wall (SCW) deposition (25–40 DPA) and 4) maturation (40–60 DPA). The widely cultivated cotton species, G. hirsutum (AD1-genome) is an allotetraploid originated from two diploid ancestor species, G. arboreum (A-genome) and G. raimondii (D-genome) through a natural hybridization and genome doubling process 1–2 MYA (Million Years Ago)1. Globally, cotton fiber is produced from G. hirsutum (~93–95%), G. barbadense (~5%) and G. arboreum (~2%) species2. The diploid G. arboreum is mainly cultivated in Indian subcontinent which accounts for ~16% of the total fiber production in South Asia2. Commercially cultivated white cotton fibers contain ~96% cellulose with less lignin (0.5–2.5%) and hemicelluloses (1.0–3.0%) however, naturally pigmented (brown and green) cotton fibers contain relatively less cellulose (~80%) and higher lignin (9–13%) and hemicelluloses (8.7–11%)3,4. Domestication, selection and breeding for white fibers have resulted in enhanced cellulose content with a simultaneous reduction in lignin, hemicellulose and phenolic compounds. The phenolic compounds and lignin content are believed to play an important roles in cotton fiber development and quality5.
Lignin is synthesized in specialized plant cells that undergo SCW deposition in addition to primary cell walls (PCW). Lignin, the second most abundant biopolymer is primarily composed of three canonical monomers namely, coniferyl (G), sinapyl (S) and p-coumaryl (H) alcohols6. Lignin monomers are synthesized in cytosol, exported to apoplastic region, oxidized and polymerized into lignin by a random coupling process. Laccases belong to multifunctional oxidase enzyme family, which carry out single electron oxidation of phenolic compounds resulting in resonance structures that undergo polymerization process7. Laccases are found to be predominant in fungal kingdom8,9 followed by bacteria10, plants11 and insects12. The plant laccases are involved in lignin13,14 and flavonoid15 biosynthesis while the microbial laccases are primarily involved in lignin breakdown16. Phenolic acids such as, benzoic (vanillic and sinapic acid) and cinnamic acid derivatives (p-coumaric, ferulic and caffeic acids) are part of lignin and flavonoid biosynthetic pathways7. These phenolic acids play an important role in cell elongation in plants17,18. In vitro studies showed laccase mediated oxidation of ferulic acid forms diferulate bridges between pectin polymers and arrest cell elongation indicating an important role of laccase enzymes in cell elongation through cell wall modification19.
Presence of wall linked phenolic acids such as ferulic, sinapic, vanillic and caffeic acids have been reported in G. hirsutum fibers and addition of 100 mM ferulic acid arrested fiber cell elongation in vitro20. In addition, phenolic compounds are associated with initiation21 and pigmentation of cotton fibers22. Laccases are associated with immature fiber phenotype23 and showed enhanced expression during fiber elongation and SCW biosynthetic stages24. Overall, emerging evidences indicate important roles of laccases in fiber development and quality through oxidation of phenolic compounds. Despite the importance of laccase genes in cotton fiber development and quality, there was no comprehensive report on laccase gene family in cotton. The present study focused on genome-wide identification and analysis of laccase genes in cultivated allotetraploid cotton (G. hirsutum) and its diploid progenitor species (G. arboreum and G. raimondii). The identified laccase genes were further analyzed for gene architecture, conserved domain profile, physical properties (protein size, isoelectric point and subcellular localization), chromosomal location, phylogenetic relationships and synteny within the A-, D-, AT- and DT genomes (T-tetraploid). Due to the presence large number of genes (84) in tetraploid G. hirsutum species and lack of fiber production in G. raimondii, the gene expression, enzyme activity and biochemical analysis (lignin and soluble/wall bound phenolic acids) were performed using G. arboreum fibers.
Genome-wide identification of cotton laccase genes from tetraploid and its progenitor diploid species
The availability of G. hirsutum, G. arboreum and G. raimondii genome sequences facilitated identification and analysis of laccase gene family from cultivated tetraploid and its diploid progenitor species25,26,27. The total coding and protein sequences of G. hirsutum, G. arboreum and G. raimondii genomes were downloaded to identify laccase gene family in these cotten species. Total proteomes of three cotton species were searched for laccase family members using blastP similarity search program (Supplementary Table S1). A total of 44 laccase proteins were identified from G. arboreum using Arabidopsis laccase protein sequences as query (Table 1, Supplementary Table S2). Similarly, 46 and 84 laccase proteins were identified from G. raimondii and G. hirsutum, respectively (Supplementary Table S2). The identified proteins were validated using NCBI conserved domain database for the presence of laccase specific conserved domains i.e. laccase (TIGR03389), CuRO_1_LCC_plant (cd13849), CuRO_2_LCC_plant (cd13875) and CuRO_3_LCC_plant (cd13897) in these proteins (Supplementary Table S3). These proteins were further confirmed using InterProScan, which showed presence of multicopper oxidase type 1 (IPR001117), multicopper oxidase type 2 (IPR011706), multicopper oxidase type 3 (IPR011707) and laccase (IPR017761) domains (Supplementary Table S3). All the identified laccase proteins from G. arboreum were named based on orthologous similarity with Arabidopsis laccase protein sequences (Table 1). The G. raimondii and G. hirsutum laccase proteins were named according to their phylogenetic closeness to G. arboreum laccases (Supplementary Table S4). Laccase groups with multiple members (eg. GaLAC04) were further classified as members within the group and numbered according to their phylogenetic closeness (eg. GaLAC04_1 to GaLAC04_8).
Analysis of conserved domain, gene architecture and subcellular localization of laccases
The conserved domain architecture of laccase proteins was analyzed using NCBI’s conserved domain database and InterProScan. Out of 44 proteins of G. arboreum, four characteristic domains (InterProScan: laccase, IPR001117, IPR011706 and IPR011707; NCBI CDD: laccase, CuRO_1_LCC, CuRO_2_LCC and CuRO_3_LCC) were present in all the identified proteins (Fig. 1, Supplementary Table S3). Similar pattern of conserved domains were present in G. raimondii and G. hirsutum laccase proteins (Supplementary Fig. S1A–C, Supplementary Table S3). Further, the exon-intron organization of laccase genes was analyzed by comparing coding sequences of laccase genes with their respective genomic sequences. The number of exons showed variation across the three species ranging from 2–8 (G. arboreum, 3–8; G. raimondii, 4–7 and G. hirsutum, A-subgenome 4–7, D-subgenome 2–7) (Fig. 1, Supplementary Fig. S1A–C). An average exon number of 5.75, 5.82, 5.85 and 5.7 was observed in A, D, AT-subgenome and DT-subgenome, respectively. All 44 laccase orthologous genes identified from G. arboreum were further analyzed in silico for their genomic and physical characteristics such as chromosomal location, gene size (genomic and coding sequence), protein molecular weight, pI (isoelectric point), and subcellular localizations (Table 1). Comparison of gene length among G. arboreum laccases showed GaLAC14_3 as the longest (11.061 kb) and GaLAC05_3 (1.994 kb) as the smallest based on genomic sequences while GaLAC017_5 (1.842 kb) as the longest and GaLAC11_5 (1.35 kb) as the smallest genes based on coding sequences (Table 1). The predicted molecular weight and isoelectric points of laccase proteins were found to be 49.982-68.113 kDa (pI, 4.76–9.85) in G. arboreum, 45.179–65.935 kDa (pI, 4.66–9.87) in G. raimondii, 30.954 to 66.534 kDa (pI, 4.98–9.89) in G. hirsutum A-subgenome and 36.416 to 66.56 kDa (pI, 4.84–9.87) in G. hirsutum D-subgenome (Table 1, Supplementary Table S2). The subcellular localization pattern of cotton laccase proteins were computationally predicted using online tool TargetP 1.1. The subcellular localization prediction showed majority of laccase proteins as secretory (33/44 G. arboreum; 43/46 G. raimondii; 80/84 G. hirsutum) and some as non-secretory (8/44 G. arboreum; 2/46 G. raimondii; 3/84 G. hirsutum) while a few others as mitochondrial localized (3/44 G. arboreum; 1/46 G. raimondii; 1/84 G. hirsutum) (Table 1, Supplementary Tables S2 and S5).
Phylogeny, chromosomal distribution and synteny analysis
Phylogenetic analysis was performed using 240 laccase protein sequences from five different plant species [A. thaliana (17), P. trichocarpa (49), G. arboreum (44), G. raimondii (46), G. hirsutum A-subgenome (42) and G. hirsutum D-subgenome (42)]. The constructed phylogenetic tree revealed clustering of cotton laccase proteins with Arabidopsis and poplar laccases into 12 different clades/groups (LAC02, LAC03, LAC04, LAC05, LAC06, LAC07, LAC09, LAC11, LAC12, LAC14, LAC15 and LAC17) (Fig. 2, Supplementary Fig. S2 and Supplementary Table S4). Laccases from all three cotton species represented in 10 of the 12 clades and the number of laccases within clades/groups varied from 1 (LAC09, LAC12 and LAC15) to 9 (LAC17) (Fig. 2, Supplementary Table S4). Together with conserved domain analysis, these results confirmed the conserved nature of laccase proteins across different plant species. Chromosomal distribution analysis of the identified laccase genes was performed in three cotton species. In G. arboreum, the identified 44 laccase genes were distributed across 10 chromosomes except chromosome numbers 1, 3 and 5 (Fig. 3A). The G. arboreum chromosomes 7 and 13 contained highest number of laccase genes, with 8 on each, while, chromosomes 2 and 11 showed least number of laccase genes with only one on each (Fig. 3A). Similarly, in G. raimondii, the identified 46 laccase genes were found to be distributed across all the chromosomes except on chromosome 1 (Fig. 3B) with highest (11) on chromosomes 9 and lowest (1 each) on chromosomes 4, 5, 6 and 8 (Fig. 3B). In G. hirsutum A-subgenome, laccase genes were found on all the chromosomes except chromosomes 7 and 8. In G. hirsutum A-subgenome, chromosome 5 has the highest number (11) of laccase genes whereas seven of 42 laccase genes were found to be present on scaffolds related to 3rd, 8th and 13th chromosomes (Fig. 3D). Laccase genes of G. hirsutum D-subgenome were distributed among 12 chromosomes except chromosome 7 while chromosome 5 showed highest number (6) of laccase genes (Fig. 3D). Six of 42 laccase genes from D-subgenome were found to be present on scaffolds related to 5th, 6th, 10th and 13th chromosomes (Fig. 3C). Tandem duplication analysis among laccase gene family members in three species showed, a total of 5 tandem duplication events involving 13 genes, 6 events involving 19 genes, 6 events involving 14 genes and 4 events involving 10 genes in G. arboreum, G. raimondii, G. hirsutum A-subgenome and D-subgenome, respectively (Fig. 3, Table 1 and Supplementary Table S2). Similarly, 12, 17, 14 and 12 independent segmental duplications of laccase genes were identified in G. arboreum, G. raimondii, G. hirsutum A- and D-subgenomes, respectively (Fig. 3A–D).
Laccases with miRNA target sites in G. arboreum
Laccase gene expression has been shown to be post-transcriptionally regulated by three different miRNA families in Arabidopsis28. To predict the role of miRNAs in regulating cotton laccase genes, all 44 genes from G. arboreum have been analyzed for the presence of miRNA target sites. The three reported laccase targeting miRNAs (miR397, miR408a and miR857) from Arabidopsis, Brachypodium, poplar, rice, corn and sorghum (Supplementary Table S6) were used as query sequences to predict the target sites on cotton laccase genes using online target prediction tool psRNATarget with stringent criteria. Out of 44 laccase genes, 22 were predicted to have miRNA target sites for either miR397 (miR397a and miR397b) or miR408a (Table 2, Supplementary Table S6). Among the 22 targeted genes, 18 were targeted by miR397 family, 4 were targeted by miR408, while none were predicted to be targeted by miR857 (Table 2).
Expression of laccase genes during G. arboreum fiber development
The lignin and phenolic acid content and composition are influenced by several factors including spatio-temporal expression of monolignol biosynthetic and laccase genes13, hence it is important to analyze the laccase gene expression during fiber development. The present study performed gene expression analysis of 44 laccase genes at five developmental stages of G. arboreum cotton fiber; elongation stage (5 DPA), transition stage (10 and 15 DPA) and SCW deposition stage (20 and 25 DPA). Out of 44 laccase genes, four (GaLAC11_5, GaLAC11_4, GaLAC05_2 and GaLAC04_8) were not detectable in any of the five fiber developmental stages (Fig. 4) while, 40 genes showed differential expression during cotton fiber development. The gene expression (fold change) was calculated using ∆∆Ct method. In order to compare expression of individual gene across different fiber developmental stages, the individual gene expression was normalized to 5 DPA. Hence the fold change is a relative higher or lower expression of a particular gene in relation to its expression at 5 DPA. The highly abundant laccase genes at different developmental stages were identified based on their relative abundance at a particular stage compared to all other stages. The abundant genes at different stages are described in the following sub-headings.
Laccase genes with higher relative expression during elongation stage (5 DPA)
Since laccase genes are involved in cell elongation it is important to identify the genes that are highly expressed during cotton fiber elongation stage. The elongation stage specific gene expression analysis showed highest expression of 11 laccases at 5 DPA compared to other stages (Fig. 4A). The highly expressed genes were GaLAC03_1, GaLAC03_2, GaLAC06_1, GaLAC07_1, GaLAC07_2, GaLAC07_5, GaLAC11_1, GaLAC12, GaLAC14_1, GaLAC14_4 and GaLAC14_5. Among these genes, the LAC11 group is involved in lignin deposition during SCW biosynthesis in Arabidopsis; however, the functions of LAC03, LAC06, LAC07, LAC12 and LAC14 have not been characterized. Further the 11 abundant laccases were compared for their relative expression levels at 5 DPA by normalizing with the lowest expressed gene (GaLAC03_2) and presented as log10 fold change in expression (Supplementary Fig. S3). The comparative expression among abundant 5 DPA laccases showed a higher expression of GaLAC14_4 and GaLAC14_5 followed by GaLAC11_1, GaLAC06_1, GaLAC07_2 and GaLAC03_1.
Laccase genes with higher relative expression during transition/onset of SCW stage (10 and 15 DPA)
Plant cell wall elongation arrest plays an important role in determining the cotton fiber length. Phenolic compounds such as ferulic and p-coumaric acids, which are oxidized by laccases, are involved in cell wall elongation arrest. The expression pattern of laccases during transition stage of cotton fiber development, which marks the cessation of elongation and initiation of secondary cell wall biosynthesis helps to identify laccases involved in the cell wall elongation arrest. At 10 and 15 DPA, 15 laccase genes showed higher relative expression compared to other stages (Fig. 4B). The highly expressed laccases were GaLAC02_1, GaLAC04_6, GaLAC04_7, GaLAC05_1, GaLAC05_3, GaLAC06_2, GaLAC07_3, GaLAC11_2, GaLAC11_3, GaLAC14_2, GaLAC17_1, GaLAC17_2, GaLAC17_3, GaLAC17_4 and GaLAC17_5. Among these laccases, LAC04, LAC11 and LAC17 have a major role in lignin deposition during secondary cell wall biosynthesis in Arabidopsis; however, the roles of LAC05, LAC06 and LAC07 have not been characterized. Among the 15 abundant 10 DPA laccases, the comparative expression analysis showed GaLAC11_2 to be highly expressed followed by GaLAC05_3, GaLAC04_6 and GaLAC05_1 while GaLAC02_1 was the least expressed at 10 DPA (Supplementary Fig. S3). The comparative expression of abundant laccases at 15 DPA showed higher expression of GaLAC11_2 and GaLAC14_2, followed by GaLAC05_1, GaLAC05_3, GaLAC17_5 and GaLAC04_6, while GaLAC17_2 showed least expression (Supplementary Fig. S3). GaLAC11_2, involved in lignin biosynthesis in Arabidopsis, showed higher expression among abundant genes both at 10 and 15 DPA.
Laccase genes with higher relative expression during active SCW stage (20 and 25 DPA)
The role of laccases in lignin deposition during SCW biosynthesis in plants is very well known. Interestingly, the cotton fiber, which deposits large amounts of cellulose during SCW biosynthesis, shows minimal lignin deposition. It is interesting to study the expression pattern of laccase genes during the SCW biosynthetic stage in cotton fiber to understand their role in fiber development. Gene expression analysis across different developmental stages showed (Fig. 4C) a higher relative expression of 14 laccase genes at SCW stage (GaLAC02_2, GaLAC02_3, GaLAC02_4, GaLAC04_1, GaLAC04_2, GaLAC04_3, GaLAC04_4, GaLAC04_5, GaLAC05_4, GaLAC07_4, GaLAC14_3, GaLAC14_6, GaLAC14_7 and GaLAC15). Among these highly abundant laccases, LAC04 group is involved in lignification of secondary cell wall and LAC15 group is involved in proanthocyanidin formation in seed coats of Arabidopsis. The comparative expression analysis among the 14 abundant laccases at 20 DPA showed GaLAC04_2 to be highly expressed followed by GaLAC04_3, GaLAC04_5 and GaLAC05_4, while GaLAC14_3 was the least expressed (Supplementary Fig. S3). At 25 DPA, the comparative expression analysis among highly abundant laccases showed GaLAC04_2 to be highly expressed followed by GaLAC04_5, GaLAC04_3 and GaLAC14_6 while GaLAC04_4 was the least expressed laccase at 25 DPA (Supplementary Fig. S3). The SCW stage showed lignification and proanthocyanidin related laccase expression apart from other laccases with unknown functions.
Overall laccase gene expression during cotton fiber development (5, 10, 15, 20 and 25 DPA)
Laccases are involved in free radical formation of monolignols during lignin biosynthesis. Individual laccases may have same or different substrates for free radical formation, however all the free radicals must be produced simultaneously to be incorporated into the cell wall or lignin polymer. Hence combined expression is important for lignin deposition during SCW biosynthesis. In order to understand the overall role of laccases during different developmental stages of cotton fiber, the combined expression level of all 40 laccases was calculated (Supplementary Fig. S4). The results indicate an 8 fold increase in laccase gene expression between 5 and 10 DPA, 8 fold further increase in expression between 10 and 15 DPA and an additional doubling from 15 DPA to 20 DPA followed by 0.6 fold decrease at 25 DPA. Overall expression of laccases increased during the cotton fiber development, which is consistent with their primary role in secondary cell wall biosynthesis.
Detection of laccase activity in developing G. arboreum fibers
Zymography is an efficient technique to identify the enzyme activity using non-denaturing native PAGE (Poly Acrylamide Gel Electrophoresis) with suitable substrate and optimum activity conditions29. Laccase activity was measured using zymography technique with ABTS [2,2′-Azinobis (3-ethylbenzothiazoline-6-sulfonic acid] as a substrate30. Fifty micrograms of total protein each from four different stages (10, 15, 20 and 25 DPA) of G. arboreum fibers was used to measure laccase enzyme activity using 1-dimensional zymogram. Laccase activity was detected in all four stages including positive control (stem sample of G. arboreum). Laccase activity showed a dynamic pattern with relatively moderate activity at 10 and 15 DPA followed by a weaker activity at 20 DPA and a sudden increase in activity at 25 DPA (Fig. 5). The activity at 25 DPA represents a combined activity of all laccases and not of an individual laccase enzyme. The presence of two activity bands (approximate size of 46 and 60 kDa) indicates presence of different laccase enzymes with different sizes, which is consistent with predicted protein sizes (Table 1). The laccase activity is consistent with laccase role in lignin deposition during SCW stage.
Increase in lignin content during fiber development
Lignification process involves the coordinated expression of phenylpropanoid pathway genes for the monolignol biosynthesis and laccases for free radical formation6,31. Since laccases are involved in lignin formation, we estimated the lignin content in different stages of cotton fiber development. Estimation of lignin content using Thioglycolic acid (TGA)32 method showed an overall increase in lignin content during fiber development (Fig. 6). The percentage of lignin in dried cell wall was found to be 0.1–0.2% of cell wall weight during 10 and 15 DPA with no increase at 20 DPA. Relatively, a significant increase in lignin content (1%) was observed during SCW biosynthetic stage at 25 DPA (Fig. 6). A similar range of cotton fiber lignin content (0.12–2.6%) has been reported in G. hirsutum based on Thioglycolate, Acetyl bromide and Klason estimation methods3.
Estimation of phenolic acid content during G. arboreum fiber development
Phenolic acids like caffeic acid, sinapic acid, vanillic acid and ferulic acid are substrates of laccases, which are ester linked to cell wall polysaccharides in developing cotton fiber3. These phenolic compounds are known to influence the cell elongation arrest and other cell wall properties in plants17,18. The wall bound and soluble phenolics were extracted from cotton fibers and estimated using HPLC (High Performance Liquid Chromatography). The absolute quantification of different compounds was calculated using internal control and the retention time of individual phenolic compounds were established by respective standards. The retention time of p-coumaric acid and sinapic acid standards were overlapping hence could not be distinguished, therefore represented as combination of p-coumaric/sinapic acids (Fig. 7 and Supplementary Fig. S5). The soluble phenolic compounds did not show any statistically significant variation in abundance during fiber development while the wall bound phenolics showed a clear statistically significant decrease in abundance with fiber development. The vanillic acid could not be detected in soluble phenolic fraction in any of the cotton fiber developmental stages (Fig. 7A). The sum of four soluble phenolic acids showed a dynamic change in the abundance (decreased from 10 DPA to 15 DPA and increased in 20 DPA followed by a decrease in 25 DPA) (Supplementary Fig. S6A). The analysis of wall bound phenolics showed a clear pattern of decrease in abundance with the fiber development (5 DPA towards 25 DPA) (Fig. 7B; Supplementary Fig. S6B). A similar pattern of decrease in wall bound phenolic acid abundance was reported in developing fibers of G. hirsutum5. Ferulic acid showed an interesting pattern during fiber development. At 10 DPA, ferulic acid was detectable in soluble fraction but not in wall bound fraction; however, appeared at 15 DPA onwards in wall bound fraction (Fig. 7A,B).
The multifunctional laccase enzymes are involved in cell elongation19, pigmentation15 and SCW biosynthesis13,33 in Arabidopsis. Cotton fiber, which undergoes a similar process including pigmentation, serves as an excellent single cell model system to study the functions of laccases. The established primary role of laccases in plants is lignin deposition during SCW biosynthesis which accounts for 20–30% of total plant biomass34; however, the lignin content of cotton fiber, which also undergoes SCW biosynthetic process is relatively very low (0.12–2.6%)3. The abundance and dynamic expression of different laccases at different stages of G. arboreum fiber development indicate a potential role of laccases in oxidizing phenolic compounds that are involved in other developmental functions besides cotton fiber lignin biosynthesis. The present study is focused on studying the relation between laccase gene expression, enzymatic activity and biochemical analyses during cotton fiber development.
Genome-wide identification and analysis of cotton laccase gene family
Genome-wide analysis of diploid G. arboreum26 and G. raimondii27 and allotetraploid G. hirsutum35 resulted in the identification of 44, 46 and 84 genes, respectively (Table 1, Supplementary Tables S1 and S2). The number of laccases in allotetraploid cotton was not equal to the sum of progenitor species, which is likely due to either gene loss in tetraploid species or gain in progenitor species after polyploidization event. The exon frequency (5–6 exons/gene) and largest exon (second last exon) analysis showed a similar pattern in cotton and Arabidopsis laccase genes indicating a conserved nature of gene structure between these two plant species (Fig. 1, Supplementary Fig. S1). The four signature conserved domains (laccase, CuRO_1_LAC, CuRO_2_LAC and CuRO_3_LAC ) of cotton laccases were similar to Arabidopsis laccases11, which also shows a highly conserved nature of laccase proteins and potential functions. Further, phylogenetic analysis of Arabidopsis, poplar and cotton laccases showed presence of cotton laccases in 12 out of 17 clades (Fig. 2). The phylogenetic clustering of cotton laccases further supports the orthology based nomenclature (Fig. 2 and Supplementary Table S4). Chromosomal localization analysis showed that laccase genes were distributed on almost all chromosomes within the genomes of three cotton species (Fig. 3). Though there are more numbers of laccase genes in cotton species, they are found to be orthologues of only 12 out of 17 Arabidopsis laccases. The synteny analysis showed that the increase in number of laccase genes in cotton was a result of segmental and tandem duplication events during cotton evolution (Fig. 3).
The physical properties of laccases such as molecular mass and isoelectric point showed variation between diploid and tetraploid species. The molecular mass of laccases in diploid progenitor cotton species is higher than tetraploid subgenomes indicating evolutionary preference of tetraploid species towards smaller laccase proteins. The isoelectric point (pI) determines the solubility, subcellular localization and interactions36,37 of proteins, interestingly majority (95.2%) of cotton laccases are basic proteins (pI > 7) while a very small percentage of them are acidic (pI < 7) which indicates common physical properties of majority of laccases (Table 1 and Supplementary Table S2). In addition, in silico analysis predicted that majority of cotton laccases as secretory proteins, similar to Arabidopsis11 laccases, which is consistent with their physical properties and primary role in cell wall biosynthesis. However, some laccase proteins are predicted to be non-secretory and while a few others are predicted to be mitochondrial localized. The function of non-secretory and mitochondrial laccases is not known in plants, characterization of these proteins might reveal novel functions, particularly the mitochondrial laccases.
Gene expression analysis during cotton fiber development
Laccases are multigene and multifunctional genes with different physical properties and localization patterns. To understand their role in cotton fiber development, it is important to study the expression pattern during different stages of cotton fiber development. Analyses of laccase gene expression of five different developmental stages of cotton fiber revealed differential expression of laccase genes during cotton fiber development. Active fiber elongation stage (5 DPA) showed a relatively higher expression of 6 out of 12 laccase groups (GaLAC03, GaLAC06, GaLAC07, GaLAC11, GaLAC12 and GaLAC14) (Fig. 4A); however, the functions of these laccases in cotton fiber elongation are not known. Since the laccases are involved in cell wall polymer bridge formation through oxidation of phenolic compounds, it is possible that these laccases might be playing an active role in the fiber elongation process. Transcriptome analysis of G. arboreum, showed that the stages between 10 and 15 DPA marks the transition/ onset of SCW stage38. The present study, supported by gene expression and biochemical analysis, is consistent with published report. The expression analysis at 10 and 15 DPA showed higher relative expression of 8 laccase groups (Fig. 4B). Among highly abundant genes, GaLAC04, GaLAC11 and GaLAC17 are homologues of Arabidopsis laccases (AtLAC04, AtLAC11 and AtLAC17) which are involved in SCW biosynthesis33. The expression pattern of GaLAC04 and GaLAC17 is consistent with previous reports on G. arboreum fiber development38 indicating their potential role in the onset of SCW stage.
Large amounts of crystalline cellulose is deposited during SCW biosynthetic stage leading to rapid increase in fiber thickness39. The published evidence shows that expression of key genes involved in SCW development in Arabidopsis xylem, are also expressed in cotton fiber SCW biosynthetic stage40. The gene expression analysis of G. arboreum at SCW stage showed an increased relative expression of 6 laccase groups (Fig. 4C). A comparison among the SCW expressed genes showed that GaLAC04_2, GaLAC04_3 and GaLAC04_5 are highly expressed (Fig. 4C and Supplementary Fig. S3). Overall gene expression analysis showed similar orthologous SCW laccases are commonly expressed in Arabidopsis and cotton fiber despite the amount of lignin deposited in cotton fiber is negligible compared to Arabidopsis stems. Though exact reason for low level of lignin deposition in cotton fiber is not known, it is possible that the availability of monolignols is a limiting factor in cotton fibers. Domestication and selection for white cotton fiber (~96% cellulose) must have resulted in gradual reduction in monolignol biosynthesis which is evident from reduction of lignin from ~8% in naturally colored cotton to ~2% in white fibers. In addition, total metabolomic analysis could not detect canonical monolignols in G. hirsutum fibers which indicated either absence or below detection levels41. Functional studies on these individual laccases using novel genome editing tools such as CRISPR\Cas9 with minimal off target effect42 will provide important information on role of individual laccase genes in cotton fiber development.
Laccase enzyme activity and relevant biochemical analysis
The cell elongation is a dynamic process that requires deposition and re-arrangement of cell wall components. Laccases regulate cell elongation by oxidizing phenolic acids such as ferulic and p-coumaric acids which arrest the cell elongation17,18,19. The present study performed laccase enzyme activity assay along with relevant biochemical analysis of developing cotton fibers to correlate laccase activity with biochemical analysis. The total soluble and wall bound phenolic compounds showed high abundance at 10 DPA followed by a steady decrease towards 25 DPA (Fig. 7B and Supplementary Fig. S6A,B). The high abundance of phenolic acids during elongation stage is consistent with their potential roles in fiber elongation. In addition, phenolic acids such as ferulic acid also serves as nucleation sites for lignification43, hence directly get incorporated into lignin polymer44. In pine xylem, a decrease in phenolic acid level is correlated with early xylem lignification process45. The present study showed a decrease in wall bound phenolic acid levels and a simultaneous increase in lignin content from 20 to 25 DPA (Figs 6 and 7), which could be due to the incorporation of phenolic acids into lignin or lignin like polymer in cotton fiber. Further, laccase activity assay showed a higher activity at 25 DPA, perfectly correlating with a rapid increase in lignin content (Figs 5 and 6). The overall combined laccase gene expression showed highest expression at 20 DPA; however, the total laccase activity was high at 25 DPA (Fig. 5 and Supplementary Fig. S4). From gene expression and activity assay results, it appears that there is a gap between transcription and laccase activity between 20 and 25 DPA. This could be due to either delayed translation46,47 and/or post-translation modifications (N-glycosylation, O-glycosylation and phosphorylation) of laccases11 expressed at 20 DPA. Further, some cotton laccases are targeted by miRNAs (Table 2) that are known to inhibit the translation48,49,50.
Overall, the present study provides comprehensive information on laccase gene family in cultivated tetraploid and its progenitor diploid species along with laccase gene expression, enzymatic and biochemical analysis of developing cotton fiber. The data from present study indicates potentially important roles of laccase genes (Fig. 8) and offers strategies for manipulation of cotton fiber development and properties using laccase genes.
Materials and Methods
Identification and nomenclature of cotton laccase family genes
The availability of reference genome sequence for two diploids (G. arboreum and G. raimondii) and one tetraploid species (G. hirsutum) offered an excellent source for the identification and analysis of the large multifunctional laccase gene family25,26,27. The in silico identification of cotton laccase gene family was performed by downloading the reference genomes (G. arboreum, G. raimondii and G. hirsutum) from CottonGen database (https://www.cottongen.org/data/download/genome)51. The cotton laccase genes were identified using Arabidopsis laccase family protein sequences retrieved from Tair10 (https://www.arabidopsis.org/)52. Genome-wide blastP similarity search (ftp://ftp.ncbi.nlm.nih.gov/blast/executables/blast+/LATEST/) was performed against cotton protein sequences from all three genomes using Arabidopsis laccase protein sequence as query. Default parameters were used to perform blastP with an e-value of 10−10, comparison matrix: BLOSUM62 and number of maximum alignment to show was 10053. After removing the redundant sequences, the identified sequences were analyzed for the presence of conserved laccase (TIGR03389), CuRO_1_LCC_plant (cd13849), CuRO_2_LCC_plant (cd13875) and CuRO_3_LCC_plant (cd13897) domains using NCBI’s Conserved Domains Database (CDD; http://www.ncbi.nlm.nih.gov/Structure/cdd/wrpsb.cgi)54. Further, InterProScan with default parameters was also used to validate the essential conserved domain profile55. Genome-wide reciprocal blastN similarity searches were performed by using validated cotton laccase gene sequences in all three cotton genomes. Naming of cotton laccase genes were performed according to the orthologous sequence similarity with Arabidopsis laccase proteins. Using individual Gossypium laccase sequences as query, Arabidopsis proteome was searched using blastP program to identify Arabidopsis homologue and named based on best match. Multiple cotton homologues were identified for some Arabidopsis laccase proteins. These multiple cotton homologues were sub grouped based on phylogenetic closeness between them (Fig. 2 and Supplementary Table S4).
Analysis of gene structure, conserved domains and subcellular localization of laccases
The identified laccases were further analyzed for gene structure, organization, domain analysis and subcellular localization. For the exon/intron structure determination, the genomic sequences were retrieved from the database using the SAMtools faidex program (http://samtools.sourceforge.net/)56. The coding sequence of each gene was compared with its genomic sequence using Gene Structure Display Server (GSDS 2.0; http://gsds.cbi.pku.edu.cn/)57 to obtain the exon/intron structure. Conserved domains in G. arboreum, G. raimondii and G. hirsutum laccase proteins were searched using web based NCBI’s Conserved Domain Database (CDD: http://www.ncbi.nlm.nih.gov/Structure/cdd/wrpsb.cgi)54 and represented using the online Illustrator tool for Biological Sequences (IBS: http://ibs.biocuckoo.org/)58. The theoretical pI and molecular weight of the laccase proteins were calculated using ExPASy server’s Compute pI/Mw tool (http://web.expasy.org/compute_pi/)59. Subcellular localization pattern of laccase genes were predicted using web based tool TargetP 1.1 server (http://www.cbs.dtu.dk/services/TargetP/)60. The default cut-off values were used with plant as organismal group to perform subcellular localization analysis.
Sequence retrieval and phylogenetic analysis
Phylogenetic analysis was performed using laccase family proteins of Arabidopsis and Poplar (Populus trichocarpa) downloaded from TAIR10 (https://www.arabidopsis.org/)52 and phytozome databases (http://phytozome.jgi.doe.gov/pz/portal.html)61, respectively. Multiple sequence alignment of 240 laccase proteins from 5 plant species (A. thaliana, P. trichocarpa, G. arboreum, G. raimondii and G. hirsutum) was performed using ClustalW sequence alignment program62 with default parameters (pairwise alignment gap open: 10 and gap extension: 0.1, multiple alignment gap open: 10 and gap extension: 0.2, protein weight matrix: BLOSUM). After exporting the alignment to MEGA file, phylogenetic tree was constructed with MEGA v6.2 (http://www.megasoftware.net/)63 using Neighbor-Joining (NJ) method with bootstrap test (replicated 1000 times) and other default parameters (phylogenetic reconstruction, substitution type: amino acids, model/methods: Jones-Taylor-Thornton (JTT) model, rates among sites: uniform rates and gap missing data treatment: partial deletion). Further, maximum likelihood method of MEGA v6.2 was also used to validate the results obtained from the NJ method.
Tandem duplication and synteny analysis
Chromosomal distribution of laccase genes was shown for G. arboreum, G. raimondii, G. hirsutum (AT- and DT- subgenomes) genomes by using their chromosomal coordinates. In A, D and AD1 cotton genomes, tandem duplication within laccase gene family was analyzed using genome co-ordinates of respective genomes. Adjacent laccase genes with no more than one intervening gene in between them were considered tandemly duplicated. For segmental duplication, synteny blocks within A, D, AT and DT genomes were analyzed using the MCSCAN (http://chibba.agtec.uga.edu/duplication/mcscan/)64 with the default parameters. The MCSCAN input files were prepared using blastP with e-value of 10−10 and filtered for redundancy using python program. Synteny blocks with laccase genes were manually identified and plotted using custom made perl program.
Computational prediction of laccase genes with miRNA target sites
Multiple genes of laccase family were targeted by miRNAs in Arabidopsis, rice and poplar49,50. Since, the expression analysis was performed in G. arboreum; we performed miRNA target analysis of laccase genes in this species. G. arboreum laccase genes were analyzed for the presence of miRNA target sites using psRNATarget server (http://plantgrn.noble.org/psRNATarget/)65 with stringent parameters (maximum expectation values of 2.0 were used in order to avoid the false positives, length for complementarity scoring: 20 bp, target accessibility-allowed maximum energy to unpair the target site (UPE): 25, flanking length around target site for target accessibility analysis: 17 bp upstream and 13 bp downstream, range of central mismatch leading to translational inhibition: 9–11 nucleotides).
Cotton plant growth, sample collection and preparation
Delinted G. arboreum seeds were imbibed in water overnight and germinated in soil pots (12 × 12 cm size) and grown in environmentally controlled growth chambers at 28 °C with 16 h light and 8 h dark conditions. Two week old seedlings (~15 cm height) were transplanted into bigger circular soil pots (30 cm in diameter) containing Metro mix 852®, transferred to green house and grown to maturity at 30 °C. Flowers were tagged at 0 DPA and 10 independent cotton bolls from four biological replicates (independent plants) were collected at 5, 10, 15, 20 and 25 DPA. The bolls were split opened, flash frozen immediately in liquid Nitrogen and stored in ultralow freezer at −80 °C. The fibers from four biological replicates were pooled for each DPA and a representative sample was used for RNA extraction. For all biochemical analysis, from the pooled biological replicate, three independent technical replicates of fiber samples were used from stages 10, 15, 20 and 25 DPA. After complete sample collection, the fibers were separated from the frozen ovules by gently grinding using mortar and pestle to separate the fibers from ovules. The 5 DPA fiber sample was very low due the small size of the fiber initials despite the collection of large number of ovules. Hence the samples from four biological replicates were pooled for all stages for a fair comparison of different stages. The pooled samples represent four independent plants, multiple bolls from each plant, multiple seeds from each boll and multiple fiber cells from single seed. The samples are pooled to reduce the variation arising from individual plants, position of bolls, ovules within bolls and fiber position on the seeds. The fibers were ground into fine powder using liquid Nitrogen in SPEX freeze mill 6870 (SPEX sample prep®, USA). The finely ground fiber powder was stored at −80 °C and used as a single source for gene expression, enzyme activity and biochemical analysis.
Primer design for quantitative PCR and specificity analysis
Primers for 44 G. arboreum laccase genes were designed using primer 3 software (http://bioinfo.ut.ee/primer3-0.4.0/)66 with melting temperatures above 60 °C and with no 3′ or 5′ end complementarity (to prevent primer dimers). Since laccase genes showed high sequence similarity, to distinguish individual genes, the forward primer was designed from 3′ coding region and reverse primer was designed from 3′ untranslated region (UTR) (Supplementary Table S7). All the primer pairs were validated for specificity using regular PCR reactions for their specificity and amplification efficiency under different annealing temperatures (data not shown) and optimized 56 °C was used as common annealing temperature for all primers for quantitative PCR (qPCR) experiments. Further, the amplified PCR products from 10 randomly selected G. arboreum laccase genes representing different stages of fiber development were sequenced using Sanger sequencing technology (Genewiz, USA). The sequencing results confirmed the specificity of the primers (Supplementary Fig. S7).
RNA isolation, cDNA synthesis and gene expression analysis using quantitative PCR
To perform the gene expression analysis using qPCR, total RNA was extracted from ground fiber samples using Sigma total RNA extraction kit (Sigma, USA) and on-column DNase (Sigma, USA) treatment was performed according to the manufacturer instructions. The RNA concentration was measured using UV spectrophotometer (BioSpectrophotometer® Kinetic, Eppendorf) at 260 nm and the integrity of RNA was analyzed on 1% agarose gels. One microgram of total RNA was used for first strand cDNA synthesis using Invitrogen superscript 2® reverse transcriptase kit (Invitrogen, USA) using oligodT primers. Briefly, total RNA was incubated with reverse transcriptase in presence of dNTP, oligodT and first strand buffer at 42 °C for 50 min followed by heat deactivation of enzyme at 70 °C for 50 min. The obtained cDNA was diluted 5 times and used for quantitative PCR reactions using Roche fast start Sybergreen® (Roche, USA) in LightCycler® 96 (Roche, USA) machine. All qPCR runs were performed at 56 °C annealing temperature with a fixed protocol and melt curve analysis was performed. The obtained Ct values were exported to excel file to calculate ∆Ct and ∆∆Ct values. ∆Ct was calculated by subtracting the Ct values of Histone 3 (internal control) with the target laccase gene within the same stage (for example 5 DPA) whereas ∆∆Ct was calculated by subtracting the ∆Ct values of fiber stages 10, 15, 20 and 25 DPA with that of 5 DPA. Hence the results were presented as relative expression of laccase genes in different stages normalized with the 5 DPA.
Analysis of soluble and wall bound phenolic compounds of developing cotton fiber
The freezer mill ground fiber powder from different stages was used for quantification of soluble and wall bound phenolic compounds. The soluble and wall bound phenolic acid extractions were performed following published protocols67. Briefly, 800 μl of 80% methanol was added to 100 mg of fiber powder, vortexed and centrifuged at 13300 rpm for 7 minutes at room temperature. The supernatant was transferred to a new tube and the extraction was repeated again with 800 μl of 80% methanol. Supernatants from the two methanol extraction steps were pooled and were used to analyze soluble phenolics. The pellet was processed for the analysis of wall bound phenolics. One ml of acetone was added to the pellet and vortexed and centrifuged at 13300 rpm for 7 minutes at room temperature. The pellet was air dried overnight at room temperature and the air dried pellet was dissolved in 1 ml of 0.5 M sodium hydroxide and incubated at 96 °C for 2 h to carry out mild saponification of phenolic acids from fiber cell walls. After incubation, the tubes were centrifuged at 13300 rpm for 10 minutes, supernatant was transferred to a fresh tube, neutralized with 250 μl of 2 M HCL and the pellet was dried stored for lignin extraction. The tubes with neutralized supernatant were centrifuged at 20800 rpm for 10 minutes and the resultant supernatant was transferred to a fresh tube. One ml of diethyl ether is added to the supernatant, vortexed and upper ether phase is transferred into a new tube. This step is repeated twice and the obtained 2 ml supernatant was dried in vaccufuge (Eppendorf). The dried pellet is dissolved in 1 ml of 10% methanol (pH: 2.1) and then analyzed using HPLC to detect wall bound phenolic acids. Agilent HPLC 1200 series with a standard Zorbax SB-C18 column (3.5 μl, 4.6*150 mm) was used. The mobile phase was 33% A (Acetonitrile) and 67% B (milli-Q water-pH 2.1) with a flow rate of 1.5 ml/minute. Appropriate standards, at a concentration of 1 mg/ml were used for caffeic acid, p-coumaric acid, ferulic acid, sinapic acid and vanillic acid. 1-Napthalene acetic acid (1 mg/ml) was used as internal control to calculate the concentration of phenolic acids, which was represented as the amount per gram of extracted cell wall weight in case of wall bound compounds and per gram of fresh cell wall weight in case of soluble compounds. The results obtained were analyzed for statistical significance using Student’s t-test program with p-value < 0.05.
Lignin content and composition of developing cotton fiber
The pellets obtained after soluble/wall bound phenolic extraction was pooled and used for TGA derivatization and lignin quantification. The lignin content was estimated using Thioglycolic acid (TGA) method32. TGA reagent (10%) was prepared using 3 M HCl. One ml of TGA reagent was added to the pellet and incubated at 80 °C for 3 h to add thioglycolic groups to the lignin structure to increase the solubility of lignin. All centrifugation steps from here onwards were carried out at 20000 g for 10 minutes at room temperature. After incubation, the tubes were centrifuged and the supernatant was discarded. The resultant pellet was then washed with 1 ml milli-Q water, centrifuged and supernatant was discarded. The pellet was vacuum dried and incubated with 1 ml of 1 M sodium hydroxide on thermal shaker at 37 °C for 24 h. The tube was centrifuged; the supernatant was acidified with 0.2 ml of concentrated HCL and incubated at 4 °C for 4 h to extract the trace amount of the derivatized lignin. After incubation, the tube was centrifuged, vacuum dried, solubilized in Dimethyl Sulfoxide (DMSO) to estimate lignin content using UV absorbance at 280 nm. A standard curve was plotted from different quantities of derivatized industrial lignin based on the absorbance at 280 nm. The quantity of lignin at different stages of G. arboreum cotton fiber was calculated using the established standard curve. The results obtained were analyzed for statistical significance using Student’s t-test program with p-value < 0.05.
Laccase activity assay of developing cotton fibers using 1-D zymography
The laccase activity was assayed using 1-D zymography method29. Briefly, 200 mg of ground fiber samples from 10, 15, 20 and 25 DPA and stem sample (control for laccase activity) from G. arboreum were homogenized in 400 μl of protein extraction buffer (100 mM sodium phosphate buffer, pH 8.5 and protease inhibitors) and incubated on ice for 1 h with intermittent mixing to allow complete and efficient lysis. The homogenate was centrifuged at 10000 g for 20 minutes at 4 °C and the supernatant was taken and stored in aliquots at −20 °C. Protein concentration was estimated using Bicinchoninic Acid (BCA) protein assay kit (Fisher Scientific, USA). 50 micrograms of total proteins from different stages were run on non-denaturing PAGE (Poly Acrylamide Gel Electrophoresis) (12% separating and 6% stacking) at 20 mA for 2 h and the gel was rinsed with milli-Q water. The gel was incubated in 2.5% TritonX-100 for 1 h at 4 °C, washed thoroughly with distilled water and incubated for 2 h at 25 °C in 50 mM sodium tartrate buffer (pH 4). Laccase activity (appearance of green bands) was visualized by incubating in 5 mM ABTS solution for 20 minutes. Simultaneously, the loading control gel was run, stained with Coomassie brilliant blue solution for 1 h, destained (50:40:10 methanol: water: acetic acid) for 6 h and scanned.
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This work was supported by the Department of Plant & Soil Science, Texas Tech University and USDA-FAS. We would like to thank Dr. Nagaraju Dharavath for helping with the zymography experiments.
The authors declare no competing financial interests.
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Balasubramanian, V., Rai, K., Thu, S. et al. Genome-wide identification of multifunctional laccase gene family in cotton (Gossypium spp.); expression and biochemical analysis during fiber development. Sci Rep 6, 34309 (2016). https://doi.org/10.1038/srep34309
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