Plant immunity often penalizes growth and yield. The transcription factor Ideal Plant Architecture 1 (IPA1) reduces unproductive tillers and increases grains per panicle, which results in improved rice yield. Here we report that higher IPA1 levels enhance immunity. Mechanistically, phosphorylation of IPA1 at amino acid Ser within its DNA binding domain occurs in response to infection by the fungus and alters the DNA binding specificity of IPA1. Phosphorylated IPA1 binds to the promoter of the pathogen defense gene and activates its expression, leading to enhanced disease resistance. IPA1 returns to a nonphosphorylated state within 48 hours after infection, resuming support of the growth needed for high yield. Thus, IPA1 promotes both yield and disease resistance by sustaining a balance between growth and immunity.

SUMMARYWe show that OsSERK2 is a regulator of innate immune signaling mediated by multiple non-RD receptor kinases (RKs) including XA21, XA3, and OsFLS2. OsSerk2-silenced rice lines are impaired in XA21-mediated immunity to Xoo PXO99, XA3-mediated immunity to Xoo PXO86, and OsFLS2-mediated defense responses. Thus, OsSERK2 is broadly involved in PRR-mediated immunity in rice.

Rice (Oryza sativa) is a staple food for more than half the world and a model for studies of monocotyledonous species, which include cereal crops and candidate bioenergy grasses. A major limitation of crop production is imposed by a suite of abiotic and biotic stresses resulting in 30%–60% yield losses globally each year. To elucidate stress response signaling networks, we constructed an interactome of 100 proteins by yeast two-hybrid (Y2H) assays around key regulators of the rice biotic and abiotic stress responses. We validated the interactome using protein–protein interaction (PPI) assays, co-expression of transcripts, and phenotypic analyses. Using this interactome-guided prediction and phenotype validation, we identified ten novel regulators of stress tolerance, including two from protein classes not previously known to function in stress responses. Several lines of evidence support cross-talk between biotic and abiotic stress responses. The combination of focused interactome and systems analyses described here represents significant progress toward elucidating the molecular basis of traits of agronomic importance.

Perception of extracellular signals by cell surface receptors is of central importance to eukaryotic development and immunity. Kinases that are associated with the receptors or are part of the receptors themselves modulate signaling through phosphorylation events. The rice (Oryza sativa L.) XA21 receptor kinase is a key recognition and signaling determinant in the innate immune response. A yeast two-hybrid screen using the intracellular portion of XA21, including the juxtamembrane (JM) and kinase domain as bait, identified a protein phosphatase 2C (PP2C), called XA21 binding protein 15 (XB15). The interaction of XA21 and XB15 was confirmed in vitro and in vivo by glutathione-S-transferase (GST) pull-down and co-immunoprecipitation assays, respectively. XB15 fusion proteins purified from Escherichia coli and from transgenic rice carry PP2C activity. Autophosphorylated XA21 can be dephosphorylated by XB15 in a temporal- and dosage-dependent manner. A serine residue in the XA21 JM domain is required for XB15 binding. Xb15 mutants display a severe cell death phenotype, induction of pathogenesis-related genes, and enhanced XA21-mediated resistance. Overexpression of Xb15 in an XA21 rice line compromises resistance to the bacterial pathogen Xanthomonas oryzae pv. oryzae. These results demonstrate that Xb15 encodes a PP2C that negatively regulates the XA21-mediated innate immune response.

Mixed-linkage glucan (MLG) is a cell wall polysaccharide containing a backbone of unbranched (1,3)-and (1,4)-linked b-glucosyl residues. Based on its occurrence in plants and chemical characteristics, MLG has primarily been associated with the regulation of cell wall expansion due to its high and transient accumulation in young, expanding tissues. The Cellulose synthase-like F (CslF) subfamily of glycosyltransferases has previously been implicated in mediating the biosynthesis of this polymer. We confirmed that the rice (Oryza sativa) CslF6 gene mediates the biosynthesis of MLG by overexpressing it in Nicotiana benthamiana. Rice cslf6 knockout mutants show a slight decrease in height and stem diameter but otherwise grew normally during vegetative development. However, cslf6 mutants display a drastic decrease in MLG content (97% reduction in coleoptiles and virtually undetectable in other tissues). Immunodetection with an anti-MLG monoclonal antibody revealed that the coleoptiles and leaves retain trace amounts of MLG only in specific cell types such as sclerenchyma fibers. These results correlate with the absence of endogenous MLG synthase activity in mutant seedlings and 4-week-old sheaths. Mutant cell walls are weaker in mature stems but not seedlings, and more brittle in both stems and seedlings, compared to wild type. Mutants also display lesion mimic phenotypes in leaves, which correlates with enhanced defense-related gene expression and enhanced disease resistance. Taken together, our results underline a weaker role of MLG in cell expansion than previously thought, and highlight a structural role for MLG in nonexpanding, mature stem tissues in rice.Plant primary cell walls are typically composed of cellulose, matrix polysaccharides, and proteins. Among vascular plants, grasses, such as rice (Oryza sativa), have unique primary cell walls with a low content of xyloglucan and pectins, and high levels of feruloylated arabinoxylans and (1,3; 1,4)-b-D-glucan (Carpita, 1996;Smith and Harris, 1999;Vogel, 2008), also known as mixed-linkage glucan (MLG). MLG is a polysaccharide containing a backbone of unbranched (1,3)-and (1,4)-linked b-glucosyl residues. In flowering plants, MLG is only found in cell walls of the taxonomic order Poales, which consists of 16 families including the Poaceae

The rice Xa21 gene, which confers resistance to the bacterial pathogen Xanthomonas oryzae pv. oryzae (Xoo), encodes a receptor-like kinase. Few components involved in transducing the Xa21-mediated defense response have yet been identified. Here, we report that XA21 binds to a WRKY transcription factor, called OsWRKY62. The OsWRKY62 gene encodes two splice variants (OsWRKY62.1 and OsWRKY62.2). OsWRKY62.1:smGFP2 and OsWRKY62.2:smGFP2 fusion proteins partially localize to the nucleus. Transgenic plants overexpressing OsWRKY62.1 are compromised in basal defense and Xa21-mediated resistance to Xoo. Furthermore, overexpression of OsWRKY62.1 suppresses the activation of defense-related genes. These results imply that OsWRKY62 functions as a negative regulator of innate immunity in rice, and serves as a critical mediator of both basal and race-specific defense responses.

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Cell-surface pattern recognition receptors (PRRs) are key components of the innate immune response in animals and plants. These receptors typically carry or associate with non-RD kinases to control early events of innate immunity signaling. Despite their importance, the mode of regulation of PRRs is largely unknown. Here we show that the rice PRR, XA21, interacts with XA21 binding protein 24 (XB24), a previously undescribed ATPase. XB24 promotes autophosphorylation of XA21 through its ATPase activity. Rice lines silenced for Xb24 display enhanced XA21-mediated immunity, whereas rice lines overexpressing XB24 are compromised for immunity. XB24 ATPase enzyme activity is required for XB24 function. XA21 is degraded in the presence of the pathogen-associated molecular pattern Ax21 when XB24 is overexpressed. These results demonstrate a function for this large class of broadly conserved ATPases in PRR-mediated immunity.non-RD kinase | pathogen-associated molecular pattern | pattern recognition receptor | ATPase | rice I nnate immunity is the first line of defense against pathogen attack and is activated rapidly following infection. In contrast to the adaptive immune system that depends on somatic gene rearrangements for the generation of antigen receptors with random specificities, the innate immune system uses a set of defined receptors for recognition of pathogen-associated molecular patterns (PAMPs) (1).Cell-surface pattern recognition receptors (PRRs) are key components of the innate immune response in animals and plants (2-8).In animals, recognition of PAMPs at the cell surface is largely carried out by the Toll-like receptor (TLR) family that contains leucine-rich repeats (LRRs) in the extracellular domain and a Toll-interleukin receptor (TIR) intracellular domain (9). Although TLRs recognize diverse molecules, they activate a common signaling pathway via association with non-RD (arginine-aspartic acid) kinases to induce a core set of defense responses (10, 11). For example, TLRs 1, 3, 5, 6, 7, 8, and 9 function through interleukin-1 receptor-associated kinase (IRAK1), whereas TLR3 and TLR4 function through receptor interacting-protein 1 (RIP1). Both IRAK1 and RIP1 are non-RD kinases (10). In plants, cell-surface recognition of PAMPs is carried out by PRRs that carry non-RD IRAK kinases integral to the receptor (ca. 35 in Arabidopsis and 328 in rice) (10). For example, rice XA21, XA26, Arabidopsis flagellin sensitive 2 (FLS2), elongation factor-Tu receptor (EFR), and barley Rpg1 (conferring stem rust-resistance) all contain an intracellular non-RD Ser/Thr kinase (8,(12)(13)(14)(15).Non-RD kinases typically carry a cysteine or glycine (in place of the arginine) before the catalytic aspartate residue and control early events of innate immunity signaling in animals and plants (16). Whereas RD kinases are regulated by autophosphorylation of the activation segment-a centrally located loop that sits close to the catalytic center-non-RD kinases are not autophosphorylated in the activation segment (17). These results s...