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Upon encountering a susceptible plant host, a bacterial pathogen expresses specific virulence factors. For example, in planta, the Xanthomonas HrpX protein activates transcription of roughly 150 genes encoding components of the type III secretion system or its translocated effectors, as well as other secreted proteins implicated in pathogenesis. Here, we show that X. oryzae pv. oryzae growth in planta or in HrpX-inducing XOM2 media resulted in HrpX-dependent transcription of the raxX and raxST genes that control production of the RaxX sulfopeptide, exported through a type I secretion system. The RaxX protein is required for activation of XA21-mediated immunity in Xa21+ rice lines. We identified potential plant-inducible promoter elements upstream of the likely 5′ ends of the raxX and raxST transcripts. Deletions and nucleotide substitutions confirmed that these elements are required for HrpX-dependent expression of raxX and raxST. We conclude that raxX-raxST gene expression is induced by HrpX during growth in planta and, therefore, is coordinately expressed with other genes required for pathogenesis. [Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

The photoperiodic response is critical for plants to adjust their reproductive phase to the most favorable season. Wheat heads earlier under long days (LD) than under short days (SD) and this difference is mainly regulated by thePHOTOPERIOD1(PPD1) gene. Tetraploid wheat plants carrying thePpd-A1aallele with a large deletion in the promoter head earlier under SD than plants carrying the wildtypePpd-A1ballele with an intact promoter. PhytochromesPHYBandPHYCare necessary for the light activation ofPPD1, and mutations in either of these genes result in the downregulation ofPPD1and very late heading time. We show here that both effects are reverted when thephyBmutant is combined with loss-of-function mutations inEARLY FLOWERING 3(ELF3), a component of the Evening Complex (EC) in the circadian clock. We also show that the wheat ELF3 protein interacts with PHYB and PHYC, is rapidly modified by light, and binds to thePPD1promoterin planta(likely as part of the EC). Deletion of the ELF3 binding region in thePpd-A1apromoter results in its upregulation at dawn, similar toPPD1alleles with intact promoters in theelf3mutant background. The upregulation ofPPD1is correlated with the upregulation of the florigen geneFLOWERING LOCUS T1(FT1) and early heading time. Loss-of-function mutations inppd1result in the downregulation ofFT1and delayed heading, even when combined with theelf3mutation. Taken together, these results indicate that ELF3 operates downstream ofPHYBas a direct transcriptional repressor ofPPD1, and that this repression is relaxed both by light and by the deletion of the ELF3 binding region in thePpd-A1apromoter. In summary, the regulation of the light mediated activation ofPPD1by ELF3 is critical for the photoperiodic regulation of wheat heading time.Author SummaryThe coordination of reproductive development with the optimal season for seed production is critical to maximize grain yield in crop species. Plants can perceive the length of the day or night (photoperiod) and use this information to anticipate seasonal changes. In most eudicot plants,CONSTANSplays a central role in the perception of photoperiod, but in wheat the main photoperiod gene isPHOROPERIOD1(PPD1). In this study, we show that the clock geneEARLY FLOWERING 3(ELF3) regulates the phytochrome-mediated light activation ofPPD1. Loss-of-function mutations inELF3result in the upregulation ofPPD1at and dawn, and in early heading under both long and short days, even in the absence ofPHYB. A deletion in thePPD1promoter including an ELF3 binding region also results in earlier heading under short days, indicating that ELF3 acts as a direct transcriptional repressor ofPPD1. This study shows thatELF3plays a critical role in the wheat photoperiod pathway by regulating the light signal between the phytochromes andPPD1. ELF3 provides an additional entry point to engineer heading time in wheat, an important trait for the development of better adapted varieties to a changing environment.

Plants deploy cell-surface and intracellular leucine rich-repeat domain (LRR) immune receptors to detect pathogens1. LRR receptor kinases and LRR receptor proteins at the plasma membrane recognize microorganism-derived molecules to elicit pattern-triggered immunity (PTI), whereas nucleotide-binding LRR proteins detect microbial effectors inside cells to confer effector-triggered immunity (ETI). Although PTI and ETI are initiated in different host cell compartments, they rely on the transcriptional activation of similar sets of genes2, suggesting pathway convergence upstream of nuclear events. Here we report that PTI triggered by the Arabidopsis LRR receptor protein RLP23 requires signalling-competent dimers of the lipase-like proteins EDS1 and PAD4, and of ADR1 family helper nucleotide-binding LRRs, which are all components of ETI. The cell-surface LRR receptor kinase SOBIR1 links RLP23 with EDS1, PAD4 and ADR1 proteins, suggesting the formation of supramolecular complexes containing PTI receptors and transducers at the inner side of the plasma membrane. We detected similar evolutionary patterns in LRR receptor protein and nucleotide-binding LRR genes across Arabidopsis accessions; overall higher levels of variation in LRR receptor proteins than in LRR receptor kinases are consistent with distinct roles of these two receptor families in plant immunity. We propose that the EDS1–PAD4–ADR1 node is a convergence point for defence signalling cascades, activated by both surface-resident and intracellular LRR receptors, in conferring pathogen immunity. The authors provide mechanistic insights into the crosstalk between signalling components of pattern-triggered immunity and effector-triggered immunity and their molecular linkers.

In this article, we describe the development of the plant immunity field, starting with efforts to understand the genetic basis for disease resistance, which ∼30 y ago led to the discovery of diverse classes of immune receptors that recognize and respond to infectious microbes. We focus on knowledge gained from studies of the rice XA21 immune receptor that recognizes RaxX (required for activation of XA21 mediated immunity X), a sulfated microbial peptide secreted by the gram-negative bacterium Xanthomonas oryzae pv. oryzae. XA21 is representative of a large class of plant and animal immune receptors that recognize and respond to conserved microbial molecules. We highlight the complexity of this large class of receptors in plants, discuss a possible role for RaxX in Xanthomonas biology, and draw attention to the important role of sulfotyrosine in mediating receptor-ligand interactions.

Activation of plant pattern-triggered immunity (PTI) relies on the recognition of microbe-derived structures, termed patterns, through plant-encoded surface-resident pattern recognition receptors (PRRs). We show that proteobacterial translation initiation factor 1 (IF1) triggers PTI in Arabidopsis thaliana and related Brassicaceae species. Unlike for most other immunogenic patterns, IF1 elicitor activity cannot be assigned to a small peptide epitope, suggesting that tertiary fold features are required for IF1 receptor activation. We have deployed natural variation in IF1 sensitivity to identify Arabidopsis leucine-rich repeat (LRR) receptor-like protein 32 (RLP32) as IF1 receptor using a restriction site-associated DNA sequencing approach. RLP32 confers IF1 sensitivity to rlp32 mutants, IF1-insensitive Arabidopsis accessions and IF1-insensitive Nicotiana benthamiana, binds IF1 specifically and forms complexes with LRR receptor kinases SOBIR1 and BAK1 to mediate signaling. Similar to other PRRs, RLP32 confers resistance to Pseudomonas syringae, highlighting an unexpectedly complex array of bacterial pattern sensors within a single plant species.

Summary The rice XA21‐mediated immune response is activated on recognition of the RaxX peptide produced by the bacterium Xanthomonas oryzae pv. oryzae (Xoo). The 60‐residue RaxX precursor is post‐translationally modified to form a sulfated tyrosine peptide that shares sequence and functional similarity with the plant sulfated tyrosine (PSY) peptide hormones. The 5‐kb raxX‐raxSTAB gene cluster of Xoo encodes RaxX, the RaxST tyrosylprotein sulfotransferase, and the RaxA and RaxB components of a predicted type I secretion system. To assess raxX‐raxSTAB gene cluster evolution and to determine its phylogenetic distribution, we first identified rax gene homologues in other genomes. We detected the complete raxX‐raxSTAB gene cluster only in Xanthomonas spp., in five distinct lineages in addition to X. oryzae. The phylogenetic distribution of the raxX‐raxSTAB gene cluster is consistent with the occurrence of multiple lateral (horizontal) gene transfer events during Xanthomonas speciation. RaxX natural variants contain a restricted set of missense substitutions, as expected if selection acts to maintain peptide hormone‐like function. Indeed, eight RaxX variants tested all failed to activate the XA21‐mediated immune response, yet retained peptide hormone activity. Together, these observations support the hypothesis that the XA21 receptor evolved specifically to recognize Xoo RaxX.

Background Pathogens often secrete molecules that mimic those present in the plant host. Recent studies indicate that some of these molecules mimic plant hormones required for development and immunity. Scope and conclusion This Viewpoint reviews the literature on microbial molecules produced by plant pathogens that functionally mimic molecules present in the plant host. This article includes examples from nematodes, bacteria and fungi with emphasis on RaxX, a microbial protein produced by the bacterial pathogen Xanthomonas oryzae pv. oryzae. RaxX mimics a plant peptide hormone, PSY (plant peptide containing sulphated tyrosine). The rice immune receptor XA21 detects sulphated RaxX but not the endogenous peptide PSY. Studies of the RaxX/XA21 system have provided insight into both host and pathogen biology and offered a framework for future work directed at understanding how XA21 and the PSY receptor(s) can be differentially activated by RaxX and endogenous PSY peptides.

Summary The biotrophic pathogen Xanthomonas oryzae pv. oryzae (Xoo) produces a sulfated peptide named RaxX, which shares similarity to peptides in the PSY (plant peptide containing sulfated tyrosine) family. We hypothesize that RaxX mimics the growth-stimulating activity of PSY peptides. Root length was measured in Arabidopsis and rice treated with synthetic RaxX peptides. We also used comparative genomic analyses and reactive oxygen species burst assays to evaluate the activity of RaxX and PSY peptides. Here we found that a synthetic sulfated RaxX derivative comprising 13 residues (RaxX13-sY), highly conserved between RaxX and PSY, induces root growth in Arabidopsis and rice in a manner similar to that triggered by PSY. We identified residues that are required for activation of immunity mediated by the rice XA21 receptor but that are not essential for root growth induced by PSY. Finally, we showed that a Xanthomonas strain lacking raxX is impaired in virulence. These findings suggest that RaxX serves as a molecular mimic of PSY peptides to facilitate Xoo infection and that XA21 has evolved the ability to recognize and respond specifically to the microbial form of the peptide.

Xanthomonas axonopodis pv. manihotis (Xam) causes cassava bacterial blight, the most important bacterial disease of cassava. Xam, like other Xanthomonas species, requires type III effectors (T3Es) for maximal virulence. Xam strain CIO151 possesses 17 predicted T3Es belonging to the Xanthomonas outer protein (Xop) class. This work aimed to characterize nine Xop effectors present in Xam CIO151 for their role in virulence and modulation of plant immunity. Our findings demonstrate the importance of XopZ, XopX, XopAO1 and AvrBs2 for full virulence, as well as a redundant function in virulence between XopN and XopQ in susceptible cassava plants. We tested their role in pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI) using heterologous systems. AvrBs2, XopR and XopAO1 are capable of suppressing PTI. ETI suppression activity was only detected for XopE4 and XopAO1. These results demonstrate the overall importance and diversity in functions of major virulence effectors AvrBs2 and XopAO1 in Xam during cassava infection.

A hallmark of multicellular organisms is their ability to maintain physiological homeostasis by communicating among cells, tissues, and organs. In plants, intercellular communication is largely dependent on plasmodesmata (PD), which are membrane-lined channels connecting adjacent plant cells. Upon immune stimulation, plants close PD as part of their immune responses. Here, we show that the bacterial pathogen Pseudomonas syringae deploys an effector protein, HopO1-1, that modulates PD function. HopO1-1 is required for P. syringae to spread locally to neighboring tissues during infection. Expression of HopO1-1 in Arabidopsis (Arabidopsis thaliana) increases the distance of PD-dependent molecular flux between neighboring plant cells. Being a putative ribosyltransferase, the catalytic activity of HopO1-1 is required for regulation of PD. HopO1-1 physically interacts with and destabilizes the plant PD-located protein PDLP7 and possibly PDLP5. Both PDLPs are involved in bacterial immunity. Our findings reveal that a pathogenic bacterium utilizes an effector to manipulate PD-mediated host intercellular communication for maximizing the spread of bacterial infection.

The rice immune receptor XA21 is activated by the sulfated microbial peptide required for activation of XA21-mediated immunity X (RaxX) produced by Xanthomonas oryzae pv. oryzae (Xoo). Mutational studies and targeted proteomics revealed that the RaxX precursor peptide (proRaxX) is processed and secreted by the protease/transporter RaxB, the function of which can be partially fulfilled by a noncognate peptidase-containing transporter component B (PctB). proRaxX is cleaved at a Gly-Gly motif, yielding a mature peptide that retains the necessary elements for RaxX function as an immunogen and host peptide hormone mimic. These results indicate that RaxX is a prokaryotic member of a previously unclassified and understudied group of eukaryotic tyrosine sulfated ribosomally synthesized, posttranslationally modified peptides (RiPPs). We further demonstrate that sulfated RaxX directly binds XA21 with high affinity. This work reveals a complete, previously uncharacterized biological process: bacterial RiPP biosynthesis, secretion, binding to a eukaryotic receptor, and triggering of a robust host immune response.

The rice immune receptor XA21 is activated by the sulfated microbial peptide RaxX (required foractivation ofXA21-mediated immunityX) produced byXanthomonas oryzaepv.oryzae(Xoo). Mutational studies and targeted proteomics revealed that RaxX is processed and secreted by the protease/transporter RaxB, whose function can be partially fulfilled by a noncognatepeptidase-containing transporterB(PctB). RaxX is cleaved at a Gly-Gly motif, yielding a mature peptide that retains the necessary elements for RaxX function as an immunogen and host peptide hormone mimic. These results indicate that RaxX is a founding member of a previously unclassified and understudied group of tyrosine sulfated RiPPs (ribosomally synthesized,post-translationally modifiedpeptides). We further demonstrate that sulfated RaxX directly binds XA21 with high affinity. This work reveals a complete, previously uncharacterized biological process: bacterial RiPP biosynthesis, secretion, binding to a eukaryotic receptor and triggering of a robust host immune response.

Bacterial species in the genus Xanthomonas infect virtually all crop plants. Although many genes involved in Xanthomonas virulence have been identified through molecular and cellular studies, the elucidation of virulence-associated regulatory circuits is still far from complete. Functional gene networks have proven useful in generating hypotheses for genetic factors of biological processes in various species. Here, we present a genome-scale co-functional network of Xanthomonas oryze pv. oryzae (Xoo) genes, XooNet (www.inetbio.org/xoonet/), constructed by integrating heterogeneous types of genomics data derived from Xoo and other bacterial species. XooNet contains 106,000 functional links, which cover approximately 83% of the coding genome. XooNet is highly predictive for diverse biological processes in Xoo and can accurately reconstruct cellular pathways regulated by two-component signaling transduction systems (TCS). XooNet will be a useful in silico research platform for genetic dissection of virulence pathways in Xoo.

SummaryThe biotrophic pathogenXanthomonas oryzaepv.oryzae(Xoo) produces a sulfated peptide named RaxX, which shares similarity to peptides in the PSY (plant peptide containingsulfatedtyrosine) family. We hypothesize that RaxX functionally mimics the growth stimulating activity of PSY peptides.Root length was measured in Arabidopsis and rice treated with synthetic RaxX peptides. We also used comparative genomic analysis and Reactive Oxygen Species (ROS) burst assay to evaluate the activity of RaxX and PSY peptides.Here we found that a synthetic sulfated RaxX derivative comprising 13 residues (RaxX13-sY), highly conserved between RaxX and PSY, induces root growth in Arabidopsis and rice in a manner similar to that triggered by PSY. We identified residues that are required for activation of immunity mediated by the rice XA21 receptor but that are not essential for root growth induced by PSY. Finally, we showed that aXanthomonasstrain lackingraxXis impaired in virulence.These findings suggest that RaxX serves as a molecular mimic of PSY peptides to facilitateXooinfection and that XA21 has evolved the ability to recognize and respond specifically to the microbial form of the peptide.

The purpose of this study was to develop an assessment tool for the middle school student’s science writing and examine its reliability. The developed tool had the evaluative standards with three categories: understanding and analyzing (understanding quote and understanding thesis), reasoning (validity, accuracy, systemicity, and consistency), and creativity. With these evaluative standards two test materials were developed and were given to 98 eighth graders (50 boys and 48 girls) from a middle school in Seoul. Three science teachers assessed the students’ writings twice at the interval of two months for each test. The reliability examination showed that the Spearman’s rank correlation coefficients (rs) for the intra-rater reliability were .919 for the first and .906 for the second assessment and the Kendall’s coefficients of concordance (W) for the inter-rater reliability were .831 for the first and .819 for the second assessment. The results suggested that the assessment tool was highly reliable in assessing the middle school student’s science writing. key words: middle school student, structure and function of plant, science writing, evaluation, assesment tool, evaluative standard, intra-rater reliability, inter-rater reliability † Corresponding author (김영수) E-mail: yskim@snu.ac.kr 논문 접수(2013. 9. 4), 1차 통과(2013. 9. 9), 2차 통과(2013. 11. 7), 3차 통과(2013.12. 26), 최종 통과(2013. 12. 27) I. 서 론 우리나라 과학 교육의 목표는 자연 현상과 사물에 대 하여 흥미와 호기심을 가지고 탐구하여 과학의 기본 개 념을 이해하고, 과학 탐구 능력과 과학적 태도를 함양하 여 창의적이고 합리적으로 문제를 해결하는 데 필요한 과학적 소양을 기르는 것이다(Ministry of Education, Science and Technology, 2011b). 21세기 지식정보화 사 회는 과학적 소양을 갖춘 인재를 필요로 하고, 과학적 소양을 함양하기 위해서는 과학 개념과 지식에 대한 이 해를 바탕으로 과학의 방법과 본성에 관한 학습을 통해 과학 활동, 즉 탐구와 문제 해결에 참여하는 것을 요구 하고 있다(Hodson, 1998). 이러한 능력들은 기본적으로 과학적 사고력에 바탕을 두고 있는데, Hodson(1998)은 생물교육 제 41권 제4호, December 2013 676 기존 지식을 탐색, 검증, 강화, 개량하는 사고의 도구로 서 글쓰기를 활용할 수 있으며, 글쓰기를 통해 사고가 분명해지고 정교해진다고 하였다. 과학 글쓰기에서는 과 학 내용과 관련하여 과학의 본성을 반영하게 되며 학생 들은 글쓰기를 통한 문제 해결 과정에서 과학적 사고력 의 하나인 추론을 사용함으로써 과학적 사고력을 확장 시키게 된다(Keys, 1999). 따라서 과학적 소양을 갖추기 위한 기본적인 요소로 학생이 과학에 관한 글을 읽고, 이해하고, 말이나 글로 명확하게 표현하는 과정은 필수 적이라 할 수 있다(Lemke, 1990). 또한 과학을 배운다 는 것은 본질적으로 과학 언어를 읽고, 쓰고, 이해하는 것이기 때문에 과학 언어를 이해하지 못하면 과학을 배 울 수도 없으며, 나아가서는 과학을 할 수도 없다(Maeng et al., 2007). 글쓰기가 과학 학습에 기여한다는 연구 성 과는 국내외 연구에서 많이 보고되었다(Bea et al., 2009; Gibson & Bernhard, 2001; Hand et al., 2007; Hong & Chung, 2006; Keys, 1999; Nam et al., 2009; Park et al., 2007; Patterson, 2001; Son, 2010). 그 중 과학 논술은 과학적 현상이나 문제에 대해 자 신의 과학 지식을 바탕으로 창의적인 의견이나 해결 방 안을 논리적인 글로 표현하는 것이라 할 수 있으며(You & Kim, 2008), 과학 논술은 인지적 측면에서 과학 개념 이해 및 분화, 과학적 사고력과 문제 해결 능력을 증진 시킨다(Hand et al., 2004; Prain, 2006; Son, 2006). 따 라서 과학적 소양 함양을 위한 효과적인 교수 방법 중 하나가 과학 논술이고, 더 나아가 과학 논술은 과학적 사 고력과 문제해결력 같은 고등 정신 능력의 평가 도구로 도 그 의의가 있다고 할 수 있다. 또한 과학 논술을 통해 얻을 수 있는 이러한 능력은 단기간에 길러지는 것이 아 니기 때문에, 학생들의 발달 단계에 맞추어 그 수준에 합당한 글쓰기 훈련을 통해 꾸준하게 연습할 필요가 있 다. 특히, 중학생 시기는 글쓰기 능력 및 논리적 사고가 급격하게 발달하는 중요한 시기이다. 이 시기의 학습자 들은 자기 중심적 글쓰기에서 벗어나 사회적 글쓰기로 전 환하며, 입체적인 사고가 가능하여 창조적 의미를 창출 할 수 있는 능력을 지니게 되고, 재생적 사고 역량이 발 달하게 되므로, 중학생 시절에 논술 교육을 통해 제시된 글의 이해를 바탕으로 문제를 발견하고, 구체적인 해결 방안을 모색하여 논리적인 조직을 갖추어 서술하는 능력 을 기르는 것은 매우 중요한 일이라 할 수 있다(Choi, 2009). 이를 반영하여 국어과 교육 과정에서도 중학생 시 기에 논증하는 글을 쓰도록 독려하고 있다(Ministry of Education, Science and Technology, 2011a). 현대의 평가가 고도의 객관성과 평가의 효율성을 강 조하면서 간접적인 평가를 선호하지만, 논술 평가는 분명 다른 평가 방법을 통해서는 측정하기 어려운 사고력, 추 리력, 종합력, 비판력, 창의력 같은 고등 정신 기능의 측 정에 적합하다. 그런데 논술 평가가 학습자의 능력을 제 대로 측정해 주고, 학습을 바람직한 방향으로 이끈다는 데 많은 사람들이 동의하면서도 논술 평가를 주저하는 이유 는 전통적인 선택형 평가보다 평가자 확보와 평가에 드는 비용과 시간의 면에서 많은 어려움이 있기 때문일 것이 다(Korea Institute for Curriculum and Evaluation, 2005a). 과학 논술 평가가 이러한 고등 정신 기능을 측정하는 도 구로 쓰인다면 그 타당도는 오히려 보장받을 수 있음에 도 불구하고, 과학 논술을 평가의 도구로 사용하기를 꺼 리는 또 다른 이유 중의 하나는 검사 도구의 신뢰도 때문 이다. 따라서 과학 논술 평가에서 평가의 대상이 되는 수행이나 산출물의 질을 구별하는 지침을 제공하고, 평 가자의 일관성 있는 평가를 가능하게 하는 명확한 평가 기준이 필요하다. 평가 기준을 명확히 하는 것은 평가의 신뢰성을 확보하는 가장 기본적인 방법이기 때문이다. 교 육 현장에서 평가의 결과는 학습자에게 중요한 영향을 주기 때문에 신뢰할 수 있는 결과를 확보하는 것은 매우 중요하다(Seong, 2010). 또한 평가에서 학업 성취와 발 달 정도를 알아보는 발달적 평가가 중요하지만, 평가의 선발적 기능도 아주 무시할 수는 없다. 학생들의 학업 성 취가 상급 학교의 진학과 무관하지 않으므로 신뢰도가 보장된 평가 도구는 반드시 필요하다. 따라서 본 연구에서는 체계적인 과학 지식, 글의 논리 적 전개 능력, 나아가 창의성에 초점을 두고, 중학생의 학습 능력과 발달 단계에 적합한 과학 논술 평가 도구를 개발하고, 그 도구의 신뢰도를 검증하였다.

The Pseudomonas syringae type III effector HopU1 is a mono-ADP-ribosyltransferase that is injected into plant cells by the type III protein secretion system. Inside the plant cell it suppresses immunity by modifying RNA-binding proteins including the glycine-rich RNA-binding protein GRP7. The crystal structure of HopU1 at 2.7-Å resolution reveals two unique protruding loops, L1 and L4, not found in other mono-ADP-ribosyltransferases. Site-directed mutagenesis demonstrates that these loops are essential for substrate recognition and enzymatic activity. HopU1 ADP-ribosylates the conserved arginine 49 of GRP7, and this reduces the ability of GRP7 to bind RNA in vitro. In vivo, expression of GRP7 with Arg-49 replaced with lysine does not complement the reduced immune responses of the Arabidopsis thaliana grp7-1 mutant demonstrating the importance of this residue for GRP7 function. These data provide mechanistic details how HopU1 recognizes this novel type of substrate and highlights the role of GRP7 in plant immunity.

Pseudomonas fluorescens Q8r1-96 represents a group of rhizosphere strains responsible for the suppressiveness of agricultural soils to take-all disease of wheat. It produces the antibiotic 2,4-diacetylphloroglucinol and aggressively colonizes the roots of cereal crops. In this study, we analyzed the genome of Q8r1-96 and identified a type III protein secretion system (T3SS) gene cluster that has overall organization similar to that of the T3SS gene cluster of the plant pathogen Pseudomonas syringae . We also screened a collection of 30 closely related P. fluorescens strains and detected the T3SS genes in all but one of them. The Q8r1-96 genome contained ropAA and ropM type III effector genes, which are orthologs of the P. syringae effector genes hopAA1 - 1 and hopM1 , as well as a novel type III effector gene designated ropB . These type III effector genes encoded proteins that were secreted in culture and injected into plant cells by both P. syringae and Q8r1-96 T3SSs. The Q8r1-96 T3SS was expressed in the rhizosphere, but mutants lacking a functional T3SS were not altered in their rhizosphere competence. The Q8r1-96 type III effectors RopAA, RopB, and RopM were capable of suppressing the hypersensitive response and production of reactive oxygen species, two plant immune responses.

RNA-binding proteins (RBP) can control gene expression at both transcriptional and post-transcriptional levels. Plants respond to pathogen infection with rapid reprogramming of gene expression. However, little is known about how plant RBP function in plant immunity. Here, we describe the involvement of an RBP, Arabidopsis thaliana RNA-binding protein-defense related 1 (AtRBP-DR1; At4g03110), in resistance to the pathogen Pseudomonas syringae pv. tomato DC3000. AtRBP-DR1 loss-of-function mutants showed enhanced susceptibility to P. syringae pv. tomato DC3000. Overexpression of AtRBP-DR1 led to enhanced resistance to P. syringae pv. tomato DC3000 strains and dwarfism. The hypersensitive response triggered by P. syringae pv. tomato DC3000 avrRpt2 was compromised in the Atrbp-dr1 mutant and enhanced in the AtRBP-DR1 overexpression line at early time points. AtRBP-DR1 overexpression lines showed higher mRNA levels of SID2 and PR1, which are salicylic acid (SA) inducible, as well as spontaneous cell death in mature leaves. Consistent with these observations, the SA level was low in the Atrbp-dr1 mutant but high in the overexpression line. The SA-related phenotype in the overexpression line was fully dependent on SID2. Thus, AtRBP-DR1 is a positive regulator of SA-mediated immunity, possibly acting on SA signaling-related genes at a post-transcriptional level.

Posttranslational modification (PTM) of proteins and peptides is important for diverse biological processes in plants and animals. The paucity of heterologous expression systems for PTMs and the technical challenges associated with chemical synthesis of these modified proteins has limited detailed molecular characterization and therapeutic applications. Here we describe an optimized system for expression of tyrosine-sulfated proteins in Escherichia coli and its application in a bio-based crop protection strategy in rice.

A sulfated peptide activates a rice immune receptor., Surveillance of the extracellular environment by immune receptors is of central importance to eukaryotic survival. The rice receptor kinase XA21, which confers robust resistance to most strains of the Gram-negative bacterium Xanthomonas oryzae pv. oryzae (Xoo), is representative of a large class of cell surface immune receptors in plants and animals. We report the identification of a previously undescribed Xoo protein, called RaxX, which is required for activation of XA21-mediated immunity. Xoo strains that lack RaxX, or carry mutations in the single RaxX tyrosine residue (Y41), are able to evade XA21-mediated immunity. Y41 of RaxX is sulfated by the prokaryotic tyrosine sulfotransferase RaxST. Sulfated, but not nonsulfated, RaxX triggers hallmarks of the plant immune response in an XA21-dependent manner. A sulfated, 21–amino acid synthetic RaxX peptide (RaxX21-sY) is sufficient for this activity. Xoo field isolates that overcome XA21-mediated immunity encode an alternate raxX allele, suggesting that coevolutionary interactions between host and pathogen contribute to RaxX diversification. RaxX is highly conserved in many plant pathogenic Xanthomonas species. The new insights gained from the discovery and characterization of the sulfated protein, RaxX, can be applied to the development of resistant crop varieties and therapeutic reagents that have the potential to block microbial infection of both plants and animals.