Introduction

Phosphorus (P) is an essential element required for optimal plant growth1. Approximately 75% of the P in grains is stored as phytic acid (PA; myo-inositol 1,2,3,4,5,6-hexakisphosphate)2. Phytic acid is poorly digested by animals and humans, leading to low mineral bioavailability and phosphate pollution of soil and water. These problems may be addressed by the development and cultivation of crops with low PA contents2,3.

To minimize the adverse effects of PA, many low phytic acid (lpa) mutants have been generated via mutagenesis and biotechnological methods in different crops, including soybean (Glycine max L. Merr.), rice (Oryza sativa L.), barley (Hordeum vulgare L.), maize (Zea mays L.), and wheat (Triticum aestivum L.)4,5. Decreases in the PA contents are related to the various types of mutated genes. Previous research revealed that cross and selective breeding of different soybean mutants and pyramiding different gene mutations is an effective strategy for obtaining progeny soybean lpa lines that produce seeds with stable and substantial decreases in PA levels. For example, decreases in PA contents ranging from 79 to 88% were observed in double lpa soybean mutants with two IPK1 mutations on chromosomes 6 and 146. More recently, a 63% lowering of the PA content was detected in the progeny resulting from a cross between a rice mutant with a mutation to the myo-inositol kinase gene (OsMIK) and a rice mutant with a mutated multidrug resistance-associated protein ABC transporter 5 gene (OsMRP5)7.

The lpa soybean lines Gm-lpa-TW-1, which has a non-lethal mutation in MIPS1 (2-bp deletion in the third exon)8,9, and Gm-lpa-ZC-2, which has a G-A point mutation in IPK1, were selected to generate a double lpa soybean mutant with mutated MIPS1 and IPK1. An earlier investigation proved myo-inositol phosphate synthase (MIPS; EC 5.5.1.4), which is the first and rate-limiting enzyme in the inositol and PA biosynthesis pathways, catalyzes the conversion of glucose 6-phosphate to myo-inositol 3-phosphate10. The IPK1 gene encodes an enzyme (EC 2.7.1.158) that converts inositol 1,3,4,5,6-pentakisphosphate (InsP5) to PA. An advanced generation recombinant inbred line, 2mlpa (mips1/ipk1), and a sibling line designated 2MWT (MIPS1/IPK1) with homozygous non-mutant alleles and a normal PA content were generated. The decrease in the PA content (up to 87%) in the double lpa line 2mlpa was significantly greater than that expected for the single mutants (approximately 50% decrease in phytate contents). The 2mlpa line accumulated less inositol phosphate isomers (InsP4 and InsP5) than the mutant with only the IPK1 mutation11.

The PA biosynthesis pathway is vital for plant growth4,5. Moreover, loss-of-function mutations in MIPS1 and IPK1 disrupt the PA biosynthesis pathway, resulting in dramatic decreases in the PA concentration. However, the mechanisms regulating PA levels and downstream gene expression in these lpa soybean lines are poorly characterized. Thus, the complete regulatory network associated with PA metabolism must be explored. In this study, mRNA-sequencing (RNA-seq) was used to study the effects of mutations in MIPS1 and IPK1 on global changes in the gene expression profiles of developing soybean seeds. The biological processes and pathways revealed by the functional enrichment analyses of differentially expressed genes (DEGs) identified in this study will further clarify the regulation of PA contents in soybean.

Results and discussion

Library sequencing and assembly

An advanced generation recombinant inbred line (2mlpa) with a low PA content and a sibling line (2MWT) with homozygous non-mutant alleles and a normal PA level were analyzed during five soybean seed developmental stages [7, 12, 17, 22, and 27 days after flowering (DAF)]. To comprehensively characterize the transcriptome changes in lpa soybeans, 30 cDNA libraries (three replicates) were sequenced. A total of 195.7 Gb of 150-bp paired-end clean reads were obtained. Each sample yielded up to 6.3 Gb of clean data with a Q20 value exceeding 98%. We mapped 85.71% of the clean reads to the annotated Glycine_max_v2.0 soybean reference genome. The number of mapped reads ranged from 34,971,326 to 39,249,412 (Table 1). These results suggest that the data were adequate for subsequent analyses. All raw data were deposited in the NCBI database (accession number PRJNA522338; http://www.ncbi.nlm.nih.gov/bioproject/).

Table 1 Summary of the library read analysis.
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Sequence annotation and DEG analysis

The reads per kilobase per million fragments (RPKM) values were used to estimate gene expression levels and conduct DEG analyses. For each mapped soybean gene, its RPKM value was calculated for all 30 libraries. The results of a principal component analysis identified the seed developmental stage and genotype as the major contributors to the data variations (Fig. 1A). Additionally, the biological replicates of each soybean sample clustered together, suggesting there was little variance between replicates. Similarities and differences between individual sample libraries were visualized using a heatmap presenting sample-to-sample distances (Fig. 1B). The 2mlpa-1, 2mlpa-2, and 2MWT-1 samples were relatively similar in the early seed developmental stages, whereas 2mlpa-4, 2mlpa-5, 2MWT-4, and 2MWT-5 were relatively similar during the late stages. However, there were also differences in gene expression among the examined samples. More specifically, 7,945 genes were significantly differentially expressed between 2mlpa and 2MWT during the five seed developmental stages (Fig. 2). A Venn diagram analysis was performed using the online OmicShare tools (http://www.omicshare.com/tools). Most of the DEGs were expressed during the early (stage 1) and late (stage 5) seed developmental stages. Moreover, 1380 DEGs were only differentially expressed in stage 1, whereas 4730 DEGs were only differentially expressed in stage 5. Of the identified DEGs, 96 were differentially expressed in all five seed developmental stages.

Figure 1
figure 1

Biological sample variability. (A) Principal component analysis plots for 30 2mlpa and 2MWT samples. (B) Sample clustering heatmap presenting sample-to-sample distances.

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Figure 2
figure 2

Venn diagram of the co-expressed and uniquely expressed DEGs between 2mlpa and 2MWT in five seed developmental stages. The number of DEGs between 2mlpa and 2MWT in each stage is also presented.

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Validation of DEGs through reverse-transcription quantitative PCR (RT-qPCR)

Eight randomly selected DEGs associated with biological processes were analyzed by RT-qPCR (Fig. 3). In most cases, the gene expression trends revealed by RT-qPCR were similar to those detected by the RNA-seq analysis, although the fold-changes varied. The RNA-seq and RT-qPCR expression profiles were similar for Glyma.16G065700 (Fig. 3C), Glyma.08G109300 (Fig. 3E), Glyma.12G232500 (Fig. 3F), and Glyma.16G214900 (Fig. 3G). However, in contrast to the corresponding RT-qPCR expression profiles, the RNA-seq analysis revealed the down-regulated expression of the following genes during specific seed developmental stages: Glyma.11G238800 (Fig. 3A) in stage 3, Glyma.14G072200 (Fig. 3B) in stage 5, Glyma.12G210600 (Fig. 3D) in stage 2, and Glyma.09G073600 (Fig. 3H) in stage 3. These results suggest the RT-qPCR data were relatively consistent with the RNA-seq data for 8 of the DEGs.

Figure 3
figure 3

Verification of the RNA-seq results by RT-qPCR. The y-axis represents the log2(ratio) (2mlpa/2MWT) for (A) Glyma.11G238800, (B) Glyma.14G072200, (C) Glyma.16G065700, (D) Glyma.12G210600, (E) Glyma.08G109300, (F) Glyma.12G232500, (G) Glyma.16G214900, and (H) Glyma.09G073600 in five seed developmental stages.

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Functional enrichment analyses of DEGs

We annotated 4,980 genes that were differentially expressed between 2mlpa and 2MWT (62.67% of all DEGs) during the five seed developmental stages with at least one Gene Ontology (GO) term. The GO terms included 345 associated with biological processes (Supplementary Table S1). Some terms were included in more than one stage, whereas others were exclusive to one stage. For example, establishment of localization (23% of DEGs in stage 1), single-organism metabolic process (13.39% of DEGs in stage 1), and localization (8.05% of DEGs in stage 1) were specific to early seed developmental stages (stages 1 and 2). In contrast, organic substance metabolic process (28.95% of DEGs in stage 5), macromolecule metabolic process (21.23% of DEGs in stage 5), and response to abiotic stimulus (4.44% of DEGs in stage 5) were exclusive to the late seed developmental stage (stage 5).

The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis is useful for elucidating the significantly enriched metabolic and signal transduction pathways among DEGs as well as the associated biological functions12. A total of 3316 DEGs (41.74% of all DEGs) between the 2mlpa and 2MWT lines in five seed developmental stages were involved in 128 metabolic and signal transduction pathways (Supplementary Table S2). The highly represented pathways included metabolic pathways, plant–pathogen interaction, plant hormone signal transduction, photosynthesis, flavonoid biosynthesis, phenylpropanoid biosynthesis, and starch and sucrose metabolism (Table 2). The DEGs assigned to KEGG pathways varied among the seed developmental stages. For example, none of the DEGs in stages 2 and 3 were associated with KEGG pathway (ko00196), whereas 22 DEGs in stage 5 were.

Table 2 Number of DEGs in enriched KEGG pathways in five seed developmental stages.
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The GO and KEGG pathway functional enrichment analyses indicated that the enriched DEGs were mainly involved in PA metabolism, photosynthesis, starch and sucrose metabolism, and defense mechanisms. These findings were somewhat consistent with those of earlier investigations on other plants, including Arabidopsis thaliana (Arabidopsis)13,14, rice15, poplar16, and sweet potato17. Moreover, the experimental design and results of a study by Redekar et al.18 were similar to those of this study. However, Redekar et al. analyzed a triple lpa mutant (mips1/mrp-l/mrp-n), but our investigation was completed with 2mlpa (mips1/ipk1). Thus, the mips1 mutation was common to both studies, whereas the other mutations differed (mrp-l/mrp-n vs ipk1). Both IPK1 and MRP function at very similar stages: the last step of the PA biosynthesis pathway (IPK1) as well as immediately after PA is synthesized, when PA is transported to the vacuole (MRP). Many of the DEGs revealed by the two studies are involved in similar pathways (e.g., photosynthesis and defense), which might be related to the common mutation (mips1) in the analyzed plants. Some of the DEGs are involved in different pathways, including glucan metabolism and apoptosis in the study by Redekar et al., which might be associated with the different mutations in the examined plants. The two studies are further compared in the following sections.

Low phytate contents often have undesirable effects on seed development and germination potential13,14,16. We observed that the 2mlpa plants grew and developed more poorly (lower emergence, thinner stems, and greater susceptibility to soybean mosaic virus) than the 2MWT plants under field conditions. This is consistent with the results of previous studies13,14,16 and is likely related to the biological processes discussed in the following section.

DEGs involved in PA metabolism

The PA biosynthesis pathway is critical for plant growth. Plants possess two parallel PA biosynthesis pathways, with both starting with the production of inositol 3-phosphate (InsP3)19. A total of 36 genes involved in PA metabolism and inositol phosphate metabolism (ko00562) were differentially expressed between 2mlpa and 2MWT by more than twofold in the late seed developmental stage (stage 5) (Supplementary Table S3, Fig. 4A). One of the mutated genes in 2mlpa was MIPS1 (Glyma.11G238800), and its expression was down-regulated in 2mlpa throughout the seed developmental period (Table 3). The 2mlpa plants were derived from a cross between Gm-lpa-TW-1 (mutated MIPS1) and Gm-lpa-ZC-2 (mutated IPK1). We previously determined that MIPS1 is expressed at low levels in developing Gm-lpa-TW-1 seeds20. Accordingly, cross-breeding with Gm-lpa-ZC-2 did not alter MIPS1 expression. Soybean includes three MIPS1 orthologs (Glyma.18G018600, Glyma.05G180600, and Glyma.08G138200)21,22. Interestingly, Glyma.18G018600 and Glyma.05G180600 expression levels were up-regulated in 2mlpa in the late seed developmental stage (Table 3). In a previous study, we observed that MIPS1 expression increased as seed organs developed in Gm-lpa-ZC-2 and the wild-type parents of Gm-lpa-TW-1 and Gm-lpa-ZC-2, peaking at 22 DAF20. It seems that the up-regulated expression of Glyma.18G018600 and Glyma.05G180600 in 2mlpa compensates for a mutated MIPS1 (Glyma.11G238800) to enhance MIPS expression in the late seed developmental stage. We hypothesized that these DEGs encoding MIPS are functionally redundant.

Figure 4
figure 4

Fold-change (2mlpa over 2MWT) ratios in five seed developmental stages for the DEGs associated with (A) PA metabolism, (B) photosynthesis-antenna proteins, (C) photosynthesis, (D) starch and sucrose metabolism, and (E) defense mechanisms. Rows represent DEGs between the 2mlpa and 2MWT soybean lines, whereas columns represent the five seed developmental stages. Blue and red indicate up-regulated and down-regulated expression (2mlpa over 2MWT), respectively.

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Table 3 Differential expression of MIPS1 and IPK1 orthologs in five seed developmental stages.
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Several studies confirmed that MIPS regulates pathways influencing seed development and plant growth9,23,24. For example, in Arabidopsis, the disruption of MIPS severely decreases the inositol and PA levels, resulting in smaller plants, programmed cell death, or hypersensitivity to high-intensity light stress25,26,27. In an earlier study, the Gm-lpa-TW-1 mutant exhibited acceptable agronomic and nutritional traits, with the exception of poor field emergence. In this study, the field emergence of 2mlpa was relatively low, but it was substantially better than that of Gm-lpa-TW-1. Hence, the 2mlpa seed traits may improve following a hybridization with Gm-lpa-ZC-2, which exhibits normal field emergence8.

The other mutated gene was IPK1 (Glyma.14g07880), which encodes the enzyme (InsP5 2-kinase) that catalyzes the final step of the PA biosynthesis pathway15. The expression of this gene was unaffected in 2mlpa, which is consistent with the RT-qPCR results for Gm-lpa-ZC-2. There are reportedly no obvious differences in the relative expression levels of IPK1 (Glyma.14g07880) between Gm-lpa-ZC-2 and its wild-type parent28. We determined that the single G-A point mutation in Gm-lpa-ZC-2 results in a deletion of 37 amino acids that disrupts an important motif. Thus, the mutation does not affect IPK1 expression, but it alters the functionality of the encoded enzyme28. This may explain the lack of significant changes in IPK1 expression in Gm-lpa-ZC-2 and 2mlpa. Moreover, IPK1 expression in 2mlpa might not be affected by cross-breeding. The expression of IPK1 orthologs (Glyma.04G032400 and Glyma.06G033100) was also not significantly affected in 2mlpa seeds (Table 3), in contrast to the significant up-regulated expression of Glyma.06G033100 in Gm-lpa-ZC-2 seeds at 15 DAF28. This difference may be related to a series of chain reactions induced by the MIPS1 mutation. Alternatively, it may be due to the low IPK1 ortholog expression levels revealed by the transcriptome analysis.

In rice, RNAi-mediated seed-specific silencing of IPK1 significantly decreases phytate levels and increases inorganic phosphate contents15. Although the constitutive suppression of the enzymes involved in phytate biosynthesis may be detrimental to plant growth and development23,29,30,31,32, we did not find any defects induced by the IPK1 mutation in Gm-lpa-ZC-2. Furthermore, some of the traits of Gm-lpa-TW-1 with a single mutation to MIPS1 were improved by the hybridization with Gm-lpa-ZC2.

We also observed that the expression levels of some inositol polyphosphate kinase and phosphatase genes involved in PA metabolism were up-regulated only at the late seed developmental stage (stage 5). These genes included two ITPK1 genes (inositol-1,3,4-trisphosphate 5/6-kinase), two PI4KA genes (phosphatidylinositol 4-kinase A), and five INPP5B/F genes (inositol polyphosphate 5-phosphatase). These results imply that the mutations in 2mlpa mainly affect PA biosynthesis in the late seed developmental stage and induce complex chain reactions related to PA metabolism. An earlier investigation indicated that Gm-lpa-ZC-2 (one of the parents of 2mlpa) and 2mlpa mutants have significantly more inositol triphosphate (InsP3) and inositol tetraphosphate (InsP4) than the wild-type control33. Both InsP3 and InsP4 are intermediary metabolites in PA metabolism. The up-regulated expression of the DEGs encoding inositol polyphosphate kinases and phosphatases is consistent with the increase in InsP3 and InsP4 in lpa lines. The effects of a single mutated gene on the expression of these PA metabolism-related genes remain to be characterized.

DEGs involved in photosynthesis in the late seed developmental stage

Photosynthesis and photosynthesis-antenna protein (ko00195 and ko00196) pathways were enriched across the five 2mlpa seed developmental stages, but especially in stage 5 (Supplementary Table S4). A total of 117 DEGs were associated with photosynthesis and photosynthesis-antenna proteins. The expression levels of most of the photosynthesis-related DEGs (81 of 94 DEGs) in stage 5 were lower in 2mlpa than in 2MWT (Fig. 4B,C). These DEGs encode different subunits of photosystems I and II, the cytochrome b6/f complex, the photosynthetic electron transport system, F-type ATPase, and the light-harvesting chlorophyll protein complex (LHC). The down-regulated expression of genes encoding the photosystem complex subunits may lead to decreased photosynthetic activities in 2mlpa mutant plants, similar to what has been observed in other lpa soybean mutants18,34. The initial reaction of the myo-inositol biosynthesis pathway is catalyzed by MIPS. The subsequent dephosphorylation of L-myo-inositol 1-phosphate results in the production of myo-inositol. This compound helps maintain the photosynthesis in Mesembryanthemum crystallinum, which is consistent with the observed changes in myo-inositol contents and the down-regulated expression of photosynthesis-related DEGs in 2mlpa33,34.

The photosynthesis-antenna protein pathway modulates plant defense responses induced by abiotic stimuli (e.g., light and temperature)15,35,36. The LHC affects the photosynthesis-antenna protein pathway. The chief function of the LHC is to collect and transfer light energy to photosynthetic reaction centers36. In this study, LHC-associated DEGs were significantly enriched in the late soybean seed developmental stage (stage 5). The expression levels of 16 DEGs encoding chlorophyll a/b-binding proteins related to 9 of 11 LHC subunits were down-regulated only in the late seed developmental stage (stage 5) in 2mlpa (Fig. 4B). Such diminished expression may inhibit light harvesting and energy production. These results suggest the mutations in 2mlpa modulate the photosynthesis-antenna protein pathway during the late seed developmental stage. Furthermore, light provides energy for photosynthesis, while also serving as a signal that influences plant growth and development. Arabidopsis mips1 mutants undergo light intensity-dependent cell death26,37. We observed that most of the photosynthesis-related DEGs had down-regulated expression levels in 2mlpa in stage 5, in accordance with the results of previous research. In a study by Redekar et al. (2015), photosynthesis (GO:0015979) was an enriched GO term in the late seed developmental period (stages 3–5), and the expression of most of the photosynthesis-related DEGs in these stages was down-regulated in the 3mlpa (mips1/mrp-l/mrp-n) mutant18. These findings imply that a mutation in MIPS1 or the lack of PA may decrease photosynthesis during the late seed developmental stages. The down-regulated expression of the genes involved in photosynthesis might contribute to some of the unfavorable traits in lpa mutants (e.g., decreased field emergence).

DEGs involved in starch and sucrose metabolism

In higher plants, sucrose is the main carbohydrate that is transported to provide the carbon and energy needed for growth and the synthesis of storage reserves38. Furthermore, sucrose is a signaling molecule affecting plant developmental processes, including growth, defense, and the regulation of flowering and the maturation of storage organs3,38,39. The 2mlpa soybean mutant exhibits a high-sucrose phenotype. In the current study, 67 DEGs related to starch and sucrose metabolism (ko00500) were more highly expressed in 2mlpa than in 2MWT almost exclusively in the late seed developmental stage (Fig. 4D, Supplementary Table S5). These DEGs mainly encode key enzymes involved in starch biosynthesis and degradation, sucrose metabolism, sugar and starch interconversion, and transport. The expression of a series of genes associated with starch and sucrose metabolism was altered in 2mlpa. Additionally, myo-inositol is an important intermediate in pathways mediating PA biosynthesis and sucrose metabolism. It also helps facilitate the reversible addition of galactose units from galactinol to sucrose during raffinosaccharide biosynthesis40. Variations in myo-inositol levels may inhibit raffinosaccharide biosynthesis, thereby increasing sucrose levels39,40. A mutation in MIPS1 decreases the PA level, induces a series of changes in the intermediate metabolites of the PA biosynthesis pathway (e.g., myo-inositol), and concomitantly decreases the raffinosaccharide content, which may increase the sucrose level in Gm-lpa-TW-139. The mutation in MIPS1 in 2mlpa might be responsible for the observed increased sucrose content and the up-regulated expression of 67 starch and sucrose metabolism (ko00500)-related DEGs. However, none of the DEGs in 2mlpa were involved in the raffinose family oligosaccharide pathway. The Gm-lpa-TW75-1 mutant, which is one of the parents of 2mlpa, has lower raffinose contents (74.2% to 84.3% lower) than the wild-type cultivar Taiwan 7539. The mips mutation reportedly down-regulates the expression of genes in the raffinose family oligosaccharide pathway25,37,41,42. Redekar et al.18 observed the same phenomenon in 3mlpa (mips1/mrp-l/mrp-n) soybean seeds. Thus, we speculated that mutations in IPK1 and MIPS1 gene have a complementary role affect in the raffinose family oligosaccharide pathway while the mutation in MIPS1 gene down-regulates the raffinose family oligosaccharide pathway.

Cell wall synthesis is modulated in soybean lpa mutants18. β-Glucosidase is a cell wall-associated enzyme43,44. In Arabidopsis leaves, β-glucosidase may play a vital role in the breakdown of cell wall polysaccharides, which provides the soluble sugars for remobilization and the completion of energy-dependent senescence45. In this study, the expression levels of 13 DEGs encoding β-glucosidase were higher in 2mlpa than in 2MWT in the late seed developmental stage. Interestingly, decreased photosynthesis is accompanied by a significant increase in the activity of cell wall-bound β-glucosidase in Arabidopsis45. This is consistent with the down-regulated expression of DEGs related to photosynthesis and the up-regulated expression of DEGs encoding β-glucosidase in 2mlpa soybean seeds. A precise characterization of the β-glucosidase function in the 2mlpa mutant is beyond the scope of this study; however, the mutations in 2mlpa may enhance the degradation of cell wall polysaccharides.

The DEGs directly associated with sucrose biosynthesis encoded the following two enzymes: sucrose-phosphate synthase (SPS; EC 2.4.1.14), which catalyzes the biosynthesis of sucrose 6-phosphate from UDP-glucose and fructose 6-phosphate, and sucrose synthase (SuSy; EC 2.4.1.13)46,47,48,49. We identified one up-regulated DEG (Glyma.13G161600) encoding SPS and two up-regulated DEGs (Glyma.09G073600 and Glyma.15G182600) encoding SuSy in 2mlpa in the late seed developmental stage (stage 5). These up-regulated DEGs may contribute to the high sucrose content of 2mlpa soybean seeds. Another lpa soybean line, V99-5089 (mutation in MIPS1), also produces seeds with low PA and high sucrose contents50. A mutation in MIPS1 or low PA levels may increase the sucrose content of soybean seeds, which is consistent with the high-sucrose phenotype of lpa mutant seeds39.

DEGs involved in defense mechanisms

The DEGs associated with the plant–pathogen interaction (ko04626) pathway and annotated with the response to stress (GO:0006950) GO term were enriched in 2mlpa in the five seed developmental stages, but especially in stages 1 and 5. A total of 604 DEGs were related to defense mechanisms, and the expression levels of most of these genes were exclusively regulated in one stage (Fig. 4E, Supplementary Table S6). For example, most of the DEGs in the early seed developmental stages (105 of 146 DEGs) were not differentially expressed in the late seed developmental stage (stage 5). Moreover, most of the DEGs in stage 5 (366 of 447 DEGs) were differentially expressed exclusively in this stage. These results suggest that the genes affected by the lpa mutations differ depending on the seed developmental stage.

Phytic acid is a natural antioxidant4,51. In plants, PA is also a key signaling molecule generated in response to the drought stress-related hormone abscisic acid, which stimulates Ca2+ release in guard cells4,5,52. Furthermore, PA is important for maintaining the basal resistance to plant pathogens4,5,53. Earlier research confirmed that InsP3 and InsP4 are intermediate metabolites in PA metabolism that act as second messengers during the regulation of cytosolic Ca2+ levels54. Hence, the decrease in the PA level and the increase in InsP3 and InsP4 contents in lpa soybean may induce a series of complex chain reactions influencing defense mechanisms33,39. In the current study, 604 DEGs were related to defense mechanisms, with about a third of these genes encoding plant disease resistance (R) proteins, including leucine-rich repeat (LRR) receptor-like serine/threonine-protein kinase, calmodulin, transcription factors, peroxidases, and mitogen-activated proteins kinases (MAPKs).

Plant R proteins are important components underlying the stress resistance mechanism, and several R genes confer resistance to diverse plant pathogens55. In this study, 64 R genes were more highly expressed in 2mlpa than in 2MWT in the late seed developmental stage. The most prevalent domain among the characterized R proteins is the LRR, which is the major pathogen recognition determinant. The LRR receptor-like R protein comprises an extracellular LRR domain and a transmembrane motif56. On the basis of the data generated in this study, the expression of 21 DEGs encoding the LRR receptor-like serine/threonine protein kinase FLS2 was up-regulated in the late seed developmental stage. In Arabidopsis, FLS2 is a transmembrane receptor kinase that activates antimicrobial defense responses57. Brassinosteroid insensitive 1-associated receptor kinase 1 (BAK1) is also an LRR receptor-like protein kinase and is involved in the signaling by FLS258. Plants in which BAK1 is mutated exhibit inhibited responses to various microbial elicitors and cold shock proteins58,59. In this study, the expression levels of 28 DEGs encoding BAK1 were up-regulated in 2mlpa in stage 5. The role of LRR-encoding genes during 2mlpa soybean seed development remains unknown, and additional studies are required to confirm the effects of the up-regulated expression of LRR genes on the 2mlpa soybean line. Similar results were reported by Redekar et al.18. Defense-related DEGs were enriched in the 3mlpa (mips1/mrp-l/mrp-n) soybean seeds, and some of the DEGs encoded LRRs or LRR-containing disease resistance proteins. However, Redekar et al. reported that apoptosis (GO:0006915) is an enriched GO term in the early seed developmental stages, which was in contrast to our results for 2mlpa. These findings are apparently associated with the IPK1 mutation in 2mlpa or the MRP mutations in 3mlpa18.

Some genes encoding NAC, MYC2, and WRKY transcription factors that regulate defense-related genes were also differentially expressed at seed developmental stage 5 in the 2mlpa mutant, including two WRKY70-encoding DEGs, which were up-regulated. In Arabidopsis, WRKY70 negatively modulates cell wall-associated defenses against pathogens and leaf senescence60,61,62. The MAPKs are key enzymes mediating adaptive responses to various abiotic and biotic stresses63. In the current study, 10 DEGs encoding MAPKs were differentially regulated between 2mlpa and 2MWT in the five examined seed stages. These defense-related DEGs may be affected by changes in the abundance of PA or its intermediary metabolites. Interactions between plants and pathogens involve bidirectional recognition. In response to a pathogen infection, plant defense mechanisms modulate the expression of many genes64. Our functional characterization revealed 418 DEGs involved in plant–pathogen interactions in 2mlpa soybean seeds, suggesting that MIPS1- and IPK1-induced processes share some common features with the defense responses to pathogens in plants. Overall, our results verify the correlation between PA and defense mechanisms.

Conclusions

In this study, an analysis of the 2mlpa mutant line revealed low PA contents as well as changes in multiple primary and secondary metabolites. We used a transcriptomics approach to compare gene expression profiles in five soybean seed developmental stages in a lpa soybean line (2mlpa) carrying two mutations (mips1 and ipk1) and in a non-mutant line (2MWT). A total of 7,945 genes were identified as differentially expressed between 2mlpa and 2MWT across the five seed developmental stages, with DEGs mainly related to 128 KEGG pathways (e.g., metabolic pathways, biosynthesis of secondary metabolites, starch and sucrose metabolism, photosynthesis, flavonoid biosynthesis, and photosynthesis-antenna proteins). The results presented herein have clarified the regulation of these metabolic events in 2mlpa. We also identified multiple DEGs involved in defense mechanisms, suggesting that MIPS1- and IPK1-induced processes and defense responses to abiotic and biotic stresses share common features in plants. On the basis of a comparison between the results of this study and those of an earlier investigation by Redekar et al.18, we conclude that the mutation in IPK1 compensates for the mutation in MIPS1 in the raffinose family oligosaccharide pathway. Furthermore, the mutation in MIPS1 is a major factor adversely affecting photosynthesis. The findings of the current study further clarify the metabolic regulatory network of the 2mlpa mutant during seed development.

Methods

Genetic materials and backgrounds

A homozygous double-mutant soybean line with low PA contents, designated 2mlpa (mips1/ipk1), and a sibling line with homozygous non-mutant alleles and normal PA levels, designated 2MWT (MIPS1/IPK1), were produced in the present study. These lines were developed by crossing Gm-lpa-TW-1 with Gm-lpa-ZC-2. Gm-lpa-TW-1 was developed earlier via the gamma irradiation of Taiwan 75 (a soybean cultivar widely grown in Zhejiang province, China), resulting in a 2-bp deletion in GmMIPS18. A comparison with the commercial wild-type cultivar Zhechun No. 3 detected a G-A point mutation in GmIPK1 in the Gm-lpa-ZC-2 soybean line28. The F1 plants generated by the cross were grown and harvested individually. The F2 plants (as well as the original parents) were grown and genotyped on the basis of a high inorganic P test and a high-resolution melting curve analysis8,28. Additionally, the F2 plants were classified as homozygous wild-type (HWT), homozygous lpa mutant (HM), or heterozygous plants. The F3 seeds from five heterozygous F2 individuals were sown and the resulting plants were genotyped to identify heterozygous F3 plants. The F4 seeds from five heterozygous F3 plants were sown to produce the F4 population, which was examined to detect homozygous wild-type and lpa mutant plants (Fig. 5). The method used to select F5–F6 plants was similar to that used for identifying F3–F4 plants. The F7 population was developed from five heterozygous F6 plants and segregated, after which their genotypes were determined. The HWT and HM seeds from 10 F7 plants were used to produce five HWT and five HM F8 lines, which were subsequently analyzed. The genotypes of the F8 plants were determined following a high-resolution melting curve analysis, after which the lpa and non-lpa lines were selected65. The seed PA and inositol phosphate isomer contents of the F8 lines were measured, which revealed the PA contents of the 2mlpa and 2MWT lines were 2.32 mg/g and 18.2 mg/g, respectively11.

Figure 5
figure 5

Flowchart of soybean material production via the cross-breeding of Gm-lpa-TW-1 and Gm-lpa-ZC-2. HM homozygous lpa-mutant, HW homozygous wild-type, HET heterozygous.

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Plant growth and RNA extraction

For each of the experimental F8 lines (2mlpa and 2MWT), 50 plants were grown in a greenhouse at 25 °C with a 14-h light/10-h dark photoperiod. Seed length was used as the criterion for defining the following five soybean seed developmental stages: stage 1, 7 DAF; stage 2, 12 DAF; stage 3, 17 DAF; stage 4, 22 DAF; and stage 5, 27 DAF28,39. For each stage, seeds were collected from 50 plants and mixed to produce one sample. Three parallel samples collected in each stage served as biological replicates for the RNA extraction. The collected seeds were immediately ground to a fine powder, from which total RNA was extracted using the E.Z.N.A. Plant RNA kit (Omega Bio-tek, Inc., USA). Genomic DNA contaminants were eliminated using RQ1 RNase-Free DNase (Promega, USA).

Library construction and sequencing

The quality of the extracted total RNA was assessed using the 2100 Bioanalyzer (Agilent, Santa Clara, CA, USA) as well as by agarose gel electrophoresis. The concentrations of all RNA samples were determined and then adjusted to 500 ng/μl. High-quality RNA samples were used to construct sequencing libraries. A total of 30 cDNA libraries were generated using the SMART™ cDNA Library Construction Kit (Takara Bio-tek, Inc., Japan) and tested using the Agilent 2100 Bioanalyzer and the ABI StepOnePlus Real-Time PCR System (Thermo Fisher, USA). The libraries were then sequenced using the Illumina HiSeq 4000 platform (Illumina, USA) at iGENE Biological Technology (Hangzhou, China).

Sequence assembly

The raw sequencing reads were filtered by removing low-quality sequences and rRNA reads. The clean reads were assembled using SOAPaligner/SOAP266. The TopHat67 program (http://ccb.jhu.edu/software/tophat/index.shtml) was used to align the clean reads for each sample with the reference genome sequence (Glycine_max_v2.0). Mismatches of no more than five bases were allowed for the alignment.

Gene annotation and analysis of DEGs

All genes were analyzed by aligning them with sequences in the NR (NCBI non-redundant protein), Swiss-Prot, GO, COG (Clusters of Orthologous Groups), and KEGG databases using BLAST2GO (https://www.blast2go.com/).

Gene expression levels were calculated and normalized as RPKM values. The RPKM method can eliminate the influence of gene length and sequence quantity on gene expression data. The calculated gene expression levels can be used directly to reveal differences in gene expression between samples. Differentially expressed genes were analyzed using the DESeq function of the Bioconductor software (http://www.bioconductor.org). The false discovery rate (FDR) was determined by correcting the P value according to the Benjamini–Hochberg method68. The criteria used for identifying significant DEGs were as follows: FDR ≤ 0.05 and |log2(ratio)| ≥ 1.

The TopGO function of the Bioconductor software was used for the GO enrichment analysis of the DEGs, with a P value threshold of 0.0169. Additionally, hypergeometric tests were used for the KEGG pathway enrichment analysis of the DEGs, with 0.05 FDR corrections70.

RT-qPCR

The RNA-seq data were validated by analyzing the expression of eight DEGs in a RT-qPCR assay. First-strand cDNA was synthesized from the high-quality total RNA using the RevertAid First Strand cDNA Synthesis Kit (Thermo Fisher Scientific Inc., Rockford, Illinois, USA). Gene-specific primers were designed using the Primer-Blast tool available on the NCBI website (https://www.ncbi.nlm.nih.gov/tools/primer-blast/). The housekeeping gene ACT11 was used to normalize gene expression levels. All primers are listed in Supplementary Table S7. The RT-qPCR analysis of the eight selected genes was conducted in triplicate using the SYBR Green Real-time PCR Master Mix (Toyobo, Osaka, Japan) and the LightCycler 480 Real-Time PCR System (Roche Diagnostics GmbH, Mannheim, Germany). Relative gene expression levels were calculated according to the corrected relative 2−ΔΔCT method.