PNAS plant immunity: XA21-mediated bacterial infection resistance in rice

In a classic evolutionary game, both the pathogen and the host devise and deploy a series of strategies to infect or resist their fellows
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Pathogens secrete a series of molecular factors designed to manipulate host biology and suppress immune responses
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Plants, in turn, have developed a set of immune receptors that recognize these molecules or their activities and initiate mechanisms to destroy pathogens, which then try to counteract these mechanisms
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Decades of work on the rice immune receptor XA21-RaxX system have provided valuable insights into the molecular genetic basis of this evolutionary race
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However, there is still progress in understanding the physiological functions of pathogen-secreted factors, the molecular and structural requirements for their interaction with cell surface plant immune receptor complexes, the mechanistic significance of non-RD motifs in these receptors, and the transduction of downstream immune signaling.
There is a gap
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A more complete understanding of pathogen-plant interactions at the cell surface will help us tip the balance in favor of the plant host
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 Recently, Pamela Ronald's team at the University of California, Davis, published a review article entitled "Plant immunity: Rice XA21-mediated resistance to bacterial infection" in the journal PNAS
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The article describes developments in the field of plant immunity, from early studies of disease resistance genetics to our growing understanding of how plant receptors interact with their microbial ligands, with a focus on the rice immune receptor XA21 and its bacterial ligands
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In the 1990s, laboratories around the world made dramatic discoveries, using genetic methods to isolate the first putative immune genes
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Based on their structures and predicted functions, immune genes are broadly classified into five categories, including genes encoding detoxification enzymes, intracellular kinases, intracellular receptors, cell surface receptors and cell surface receptor kinases (Fig.
1)
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The fifth class of resistance genes is represented by the rice Xa21 gene, which confers resistance to the Gram-negative bacterium Bacterial blight oryzae (Xoo)
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Compared with previously cloned genes, the structure of the XA21 protein represents a novel class of plant disease resistance genes encoding receptor-like kinases (RLKs)
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Figure 1.
Immunoreceptor structure
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Studies have shown that plant RLK XA21 has specific functions for pathogen recognition and response, as well as robust broad-spectrum resistance to different races of bacteria
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The XA21 immunoreceptor specifically recognizes RaxX (required for the activation of XA21-mediated immune X), a sulfated microbial peptide secreted by the Gram-negative bacterium Bacterial blight oryzae
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Figure 3.
Biosynthetic pathway of RaxX, a tyrosine sulfate RiPP
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Figure 4.
raxX-raxSTAB divergent transcriptional control region
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Sequence analysis of different plant genomes led to the discovery that the C-terminus of proRaxX is similar to the peptide hormone PSY (plant peptide containing sulfated trisaccharides) (Fig.
5)
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The peptide hormone PSY has the functions of promoting cell proliferation and expansion, regulating cell size, and promoting root elongation
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Figure 5.
Microbially derived tyrosine sulfated peptides mimic phytopeptide hormones
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Sequence similarity between RaxX and plant peptides containing sulfated tyrosines (PSYs)
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Rice plants carrying XA21 can mount a defense response to pathogens but not to the highly similar endogenous PSY peptide hormone, which is predicted to be required for normal rice growth and development
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Therefore, XA21 is a highly selective immunoreceptor capable of specifically recognizing bacterial mimics
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Figure 6.
Model of XA21 immune function
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Both RaxX and PSY require tyrosine sulfation for full activity
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Tyrosine sulfation is an important post-translational modification of certain extracellular protein-protein interactions
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Plants and animals use tyrosine sulfated proteins to regulate growth, development, immunity and other biological processes
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In addition to PSY, plants produce four other classes of tyrosine-sulfated peptides that bind LRR receptor kinases: phytosulfonamides (PSK), root meristem growth factor (RGF), Kjeldahl integrity factor (CIF), and Helix Seed 1 (TWS1), which shares sequence similarity with CIF
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Like PSY, PSK, RGF, CIF, and TWS1 are processed, secreted, and function in various processes involved in the regulation of plant growth and development (Fig.
7)
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Despite recent progress, our knowledge of the composition of sulfated complexes governing these reactions remains limited
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Figure 7.
Functional diversity of post-translationally modified peptides
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Since the characterization of the first RLKs with known functions, rice XA21 and cruciferous SRKs, research into the phylogenetic and functional studies of RLKs has exploded
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Comparison of conserved LRR kinase domain sequences with sequences from each species identified 19 distinct subfamilies of LRR-RLK genes (Fig.
8), supporting a previous phylogeny of Arabidopsis LRR-RLK genes Analysis
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Furthermore, evidence accumulated over the past 30 years suggests that LRR receptor subfamily XI recognizes intrinsic peptides (such as phytopeptide hormones), while receptor subfamily XII recognizes exogenous peptides (such as microbial molecules) (Fig.
8)
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Figure 8.
Phylogenetic tree comparison of LRR-RLK genes in rice (O.
sativa), Arabidopsis thaliana, moss (Physcomitrium patens) and lycopate (Selaginella moellendorffii)
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In rice, RLP XA21D confers a partial resistance response to Xoo
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The core receptor for XA21D-mediated immunity has not been identified
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Unlike SOBIR1-interacting RHPs, which both have membrane anchors, XA21D is predicted to be a secreted RLP lacking a transmembrane domain, similar to the secreted S-site glycoprotein (SLG), which mediates pollen-stigma interactions specificity
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Although much progress has been made since the discovery and characterization of XA21, XA21D, and other plant RLKs and RLPs, how these ligand/receptor pairs interact with other co-receptors to exert their developmental effects, and how they interact closely with The related defense signaling pathway interactions remain to be further understood
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Pamela Ronald's team was particularly interested in how the receptors discriminate between related RaxX and PSY peptides
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The researchers hypothesized that the as yet unidentified PSY receptor regulates different developmental processes through multiple PSY peptides in a tissue-specific manner
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Such a model would explain how PSY has a powerful effect on root development in response to PSY treatment, whereas Xoo may infect other tissues
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Growth and immunity are highly interconnected processes, as reflected in a growing number of reports suggesting crosstalk between regulatory genes involved in the control of both processes
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These observations suggest that RaxX, acting as a mimetic of growth-promoting hormones, may alter plant development in a way that favors bacterial infection
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Isolation and characterization of putative PSY receptors (and co-receptors) followed by structural and functional studies comparing RaxX-XA21 and RaxX to putative PSY receptors or PSY/PSY receptor complexes will provide insight into control ligands.
mechanism of somatic recognition and will help us understand how PSY and RaxX peptides are sensed in plants to induce root growth, promote infection, and elicit immune responses
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