Abstract
Microbiota benefit their hosts by improving nutrient uptake and pathogen protection. How host immunity restricts microbiota while avoiding autoimmunity is poorly understood. Here we show that the Arabidopsis phytosulfokine receptor 1 (pskr1) mutant displays autoimmunity (plant stunting, defence-gene expression and reduced rhizosphere bacterial growth) in response to growth-promoting Pseudomonas fluorescens. Microbiome profiling and microbiota colonization showed that PSKR1-mediated reduction in bacterial growth and stunting is largely specific to Pseudomonas. Transcriptional profiling demonstrated that PSKR1 regulates the growth–defence trade-off during Pseudomonas colonization: PSKR1 upregulates plant photosynthesis and root growth but suppresses salicylic-acid-mediated defences. Genetic epistasis experiments showed that pskr1 stunting and restriction of bacterial growth are salicylic acid dependent. Finally, we showed that Pseudomonas, but not other bacteria, induces PSKR1 expression in roots, suggesting that Pseudomonas might manipulate plant signalling to promote its colonization. Our data demonstrate a genetic mechanism to coordinate beneficial functions of the microbiome while preventing autoimmunity.
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Data availability
The raw data for 16S rRNA sequencing are available in the Sequence Read Archive database under BioProject PRJNA896256, accession numbers SRR22133746 to SRR22133785. The raw data for RNA sequencing are available in the Sequence Read Archive database under BioProject PRJNA897874, accession numbers SRR22177437 to SRR22177481.
Code availability
The code related to microbiome sequencing and RNA sequencing is available on the Haney lab GitHub site (https://github.com/haneylab/PSKR1_Song_etal_2023).
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Acknowledgements
We thank Y. Zhang and M. Sauter for sharing the seed stocks, and F. Menke and M. Draeger for providing the PSK peptide and constructive suggestions. This work was supported by an NSERC Discovery Grant and Accelerator Award (no. NSERC-RGPIN-2021-03587) and a Canada Research Chair salary award to C.H.H. Additional trainee support was provided by Chinese Scholarship Council Awards to S.S. Early stages of this work were supported by NIH grant no. R37 GM48707 and NSF grants no. MCB-0519898 and no. IOS-0929226 awarded to F. M. Ausubel.
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C.H.H. and S.S. conceptualized and designed the experiments. S.S. conducted the majority of the experiments and data analysis. Z.M.M. conducted the 16S rRNA sequencing data analysis and differential gene expression analysis for the RNA-seq data. A.L.B. helped with the SynCom-related experiments. X.-C.Z. performed the hsm screen and the mapping of HSM7. A.C.D. helped with the characterization of the hsm7 mutant.
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Nature Plants thanks Yang Bai, Devin Coleman-Derr, and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Extended data
Extended Data Fig. 1 PSKR1 enriches rhizosphere P. fluorescens abundance independent of changes in root architecture.
(a) Root phenotype of 8-day-old Col-0, hsm7, 35S::PSKR1–GFP and pskr1-3 seedlings planted on 1⁄2 MS media plate with 2% sucrose. The red lines highlight the ending point of the primary roots. (b) Primary root length of 8-day-old Col-0, hsm7, pskr1-3 and 35S::PSKR1–GFP seedlings. Each dot indicates the primary root length of one plant. Letters denote significant difference by ANOVA and Tukey’s HSD (P < 0.05). Bars indicate means ± s.e.m. Data from two independent experiments are shown and colored by experiment. n = 5-10 independent plants per experiment. (c) A diagram showing the natural soil colonization system. Natural soil containing natural microbiota was collected from the UBC farm, and 6-day-old plant seedlings from 1/2 MS plates were transplanted. After 17 days growing under 12 h light/12 h dark in growth chambers, root rhizosphere soil was collected, weighed and buffer was added to 0.05 g/mL (7.5 mM MgSO4 and 20% glycerol). The homogenized rhizosphere soil in buffer was plated on King’s B media after dilution, after which P. fluorescens colonies were identified under an UV light source and counted.
Extended Data Fig. 2 pskr1-3 shows an autoimmune growth deficiency phenotype in response to commensal P. fluorescens treatment.
(a) Root phenotype of 18-day-old Col-0 and pskr1-3 seedlings with MgSO4 buffer or P. fluorescens WCS365 treatment. (b) Primary root length of 18-day-old Col-0 and pskr1-3 seedlings with MgSO4 buffer or P. fluorescens WCS365 treatment. Each dot indicates the primary root length of 1 plant. Data from each temporal replate is labelled with a different color. n = 5 plants per experiment. Data shown are pooled from two temporal replicates. Letters indicate statistical differences (P < 0.05) by ANOVA and Tukey’s HSD. Bars indicate means ± s.e.m.
Extended Data Fig. 3 Gene Ontology (GO) analysis of P. fluorescens WCS365 induced differentially expressed genes in Col-0 and pskr1-3.
Bar length and fill color represent p values for individual GO term. The hypergeometric test and Benjamini-Hochberg p-value correction algorithm were used to identify all ontology terms that contain a statistically greater number of genes in common with an input list than expected by chance through Metascape.
Extended Data Fig. 4 Gene set enrichment analysis (GESA) revealed plant defense or growth-related pathways dependent on PSKR1.
(a) GESA map of enriched GO terms for DEGs in 35S::PSKR1–GFP versus pskr1-3 48 hours after P. fluorescens inoculation (35S::PSKR1–GFP_WCS365 versus pskr1-3_365, Padj < 0.05, |log2FC| ≥ 2). Individual GO terms are plotted as one node; node size represents the number of genes belonging to each GO term; node border color indicates the GO term significance; node filling color suggests PSKR1 positively (red) or negatively (blue) regulated GO terms. Similar GO terms are clustered and circled; similar clusters form functional groups labelled defense, growth, photosynthesis, and secondary metabolism. The significance of enriched GO terms was determined through g:Profiler (Methods) by the cumulative hypergeometric test and adjusted by multiple testing correction g:SCS. (b), (c) Genes belonging to GO term defense response (GO:0006952) and photosynthesis (GO:0015979) specifically for Col-0, pskr1-3, 35S::PSKR1–GFP samples with or without P. fluorescens treatment are displayed in heatmaps.
Extended Data Fig. 5 PSKR1 upregulates growth and suppresses defense responses.
(a) KEGG search and color pathway analysis of PSKR1-regulated DEGs [35S::PSKR1–GFP_WCS365 versus pskr1-3_WCS365, adjusted p value (Padj < 0.05)] in the plant-pathogen interaction pathway by comparing P. fluorescens WCS365 treated 35S::PSKR1–GFP versus pskr1-3 samples. Red and green color indicate PSKR1 up- and down-regulated DEGs respectively. (b) The ridgeline plot visualizing expression distributions of PSKR1 regulated DEGs with the presence of commensals [35S::PSKR1–GFP_WCS365 versus pskr1-3_WCS365, adjusted p value (Padj < 0.05)] by comparing P. fluorescens WCS365 treated 35S::PSKR1–GFP versus pskr1-3 samples. The black circles indicate the Log10 fold change of individual DEG in each KEGG pathway. The p values of indicated KEGG pathways were determined by hypergeometric tests with NetworkAnalyst (Methods).
Extended Data Fig. 6 PSKR1 regulates multiple processes in addition to SA-mediated defense responses.
(a) Heatmap displaying the top 2000 variable genes from the RNAseq dataset of Col-0, pskr1-3, npr1 npr4-4D, and pskr1-3 npr1 npr4-4D plants treated with MgSO4 buffer or P. fluorescens for 48 hours, and Col-0 plants treated with 40 µM SA for 6 hours. Cluster A indicates SA-upregulated genes (Col-0_SA versus Col-0_buffer) that show epistasis by npr1 npr4-4D in the pskr1-3 npr1 npr4-4D triple mutant. Cluster B and Cluster C indicate PSKR1-regulated genes without a full epistasis effect by npr1 npr4-4D. (b) The Gene Ontology (GO) processes enriched in Cluster A, B and C genes by Metascape. The hypergeometric test and Benjamini-Hochberg p-value correction algorithm were used to identify all ontology terms that contain a statistically greater number of genes in common with an input list than expected by chance.
Extended Data Fig. 7 The SA inducible gene expression is inhibited in pskr1-3 npr1 npr4-4D.
(a), (b) Expression levels of WRKY51 (a) and WRKY70 (b) in Col-0, npr1, npr4-4D, npr1 npr4-4D and pskr1-3 npr1 npr4-4D as measured by qRT-PCR. Expression values were normalized to the expression of house-keeping gene EF1. n = 3 replicates with 10 plants per replicate. Letters indicate significance as determined by ANOVA and Tukey’s HSD (P < 0.05). Error bars indicate means ± s.e.m.
Extended Data Fig. 8 Induction of root PSKR1 expression is largely specific to Pseudomonas.
(a) PSKR1pro::GUS plants inoculated with 10 mM MgSO4 buffer, P. fluorescens WCS365 (OD600 = 0.002) for 24 hours. (b) Representative images of PSKR1pro::GUS plants inoculated with 10 mM MgSO4 buffer or indicated bacteria treatments. The experiment was repeated 3 times independently with similar results.
Extended Data Fig. 9 A working model for the role of PSKR1 in balancing plant growth-defense trade-off in association with the microbiome.
Pseudomonas upregulates root PSKR1 expression to hijack plant signaling. The activation of PSKR1 in turn enriches rhizosphere Pseudomonas abundance by upregulating photosynthesis and other growth-related events while suppressing SA-mediated defense responses.
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Song, S., Morales Moreira, Z., Briggs, A.L. et al. PSKR1 balances the plant growth–defence trade-off in the rhizosphere microbiome. Nat. Plants 9, 2071–2084 (2023). https://doi.org/10.1038/s41477-023-01539-1
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DOI: https://doi.org/10.1038/s41477-023-01539-1