The OsWRKY6 transcriptional cascade functions in basal defense and Xa1-mediated defense of rice against Xanthomonas oryzae pv. oryzae.

MAIN CONCLUSION: The rice protein OsWRKY6 directly activates OsWRKY45 and OsWRKY47 expression, and also activates OsPR1a and OsPR1b through the two OsWRKYs, and this transcriptional module participates in Xa1-mediated defense against the pathogen Xanthomonas oryzae pv. oryzae. Biotic stress, the pathogen Xanthomonas oryzae pv. oryzae (Xoo) in particular, negatively impacts worldwide productivity and yield in the staple crop rice (Oryza sativa). OsWRKY transcription factors are involved in various biotic stress responses in rice, and OsWRKY6 specifically acts as an important defense regulator against Xoo. However, the relationship between OsWRKY6 and other OsWRKYs, as well as its role in resistance (R) gene-mediated defense, have yet to be studied in depth. Here, we characterized a transcriptional cascade triggered by OsWRKY6 that regulated defense against Xoo infection mediated by the NBS-LRR protein Xa1. OsWRKY45 and OsWRKY47 were identified as direct transcriptional targets of OsWRKY6, and their two gene products reciprocally activated their two genes. Furthermore, OsWRKY6 activated OsPR1a and OsPR1b via the OsWRKY45 and OsWRKY47. Two OsWRKY6 RNAi knockdown lines showed significantly reduced defense even against an incompatible Xoo infection, and the expression of OsWRKY6 was not regulated by OsWRKY51 and OsWRKY88. This study reveals that a novel downstream transcriptional pathway activated by OsWRKY6 is involved in Xa1-mediated defense against Xoo.

TALE-triggered and iTALE-suppressed Xa1-mediated resistance to bacterial blight is independent of rice transcription factor subunits OsTFIIAγ1 or OsTFIIAγ5.

Xa1-mediated resistance to rice bacterial blight, caused by Xanthomonas oryzae pv. oryzae (Xoo), is triggered by transcription activator-like effectors (TALEs) and suppressed by interfering TALEs (iTALEs). TALEs interact with the rice transcription factor OsTFIIAγ1 or OsTFIIAγ5 (Xa5) to activate expression of target resistance and/or susceptibility genes. However, it is not clear whether OsTFIIAγ is involved in TALE-triggered and iTALE-suppressed Xa1-mediated resistance. In this study, genome-edited mutations in OsTFIIAγ5 or OsTFIIAγ1 of Xa1-containing rice 'IRBB1' and Xa1-transgenic plants of xa5-containing rice 'IRBB5' did not impair the activation or suppression of Xa1-mediated resistance. Correspondingly, the expression pattern of Xa1 in mutated OsTFIIAγ5 and OsTFIIAγ1 rice lines and 'IRBB1' rice was similar. In contrast, the expression of OsSWEET11 was repressed in rice lines mutated in OsTFIIAγ5 and OsTFIIAγ1. Bimolecular fluorescence complementation (BiFC) and co-immunoprecipitation assays showed that both TALE PthXo1 and iTALE Tal3a interacted with OsTFIIAγ1 and OsTFIIAγ5 in plant nuclei. These results indicated that TALE-triggered and iTALE-suppressed Xa1-mediated resistance to bacterial blight is independent of OsTFIIAγ1 or OsTFIIAγ5 in rice, and suggest that an unknown factor is potentially involved in the interaction of Xa1, TALEs and iTALEs.

WRKY10 transcriptional regulatory cascades in rice are involved in basal defense and Xa1-mediated resistance.

WRKY proteins play essential roles as negative or positive regulators of pathogen defense. This study explored the roles of different OsWRKY proteins in basal defense and Xa1-mediated resistance to Xanthomonas oryzae pv. oryzae (Xoo) infection in rice. Assays of disease in OsWRKY10KD and OsWRKY88KD lines following infection with an incompatible Xoo race, which induced Xa1-mediated resistance in wild-type plants, showed that OsWRKY10 and OsWRKY88 were positive regulators of Xa1-mediated resistance. OsWRKY10 also acted as a positive regulator in basal defense by directly or indirectly activating transcription of defense-related genes. OsWRKY10 activated the OsPR1a promoter by binding to specific WRKY binding sites. Two transcriptional regulatory cascades of OsWRKY10 were identified in basal defense and Xa1-mediated resistance. In the first transcriptional regulatory cascade, OsWRKY47 acted downstream of OsWRKY10 whereas OsWRKY51 acted upstream. OsWRKY10 activated OsPR1a in two distinct ways: by binding to its promoter and, at the same time, by indirect activation through OsWRKY47. In the second transcriptional regulatory cascade, OsWRKY47 acted downstream of OsWRKY10, and OsWRKY88 acted upstream. These OsWRKY10 transcriptional regulatory cascades played important roles in basal defense and Xa1-mediated resistance to enable the mounting of a rapid immune response against pathogens.

Rice Routes of Countering Xanthomonas oryzae.

Bacterial blight (BB) and bacterial leaf streak (BLS), caused by Xanthomonas oryzae pv. oryzae and Xanthomonas oryzae pv. oryzicola, respectively, are two devastating diseases in rice planting areas worldwide. It has been proven that adoption of rice resistance is the most effective, economic, and environment-friendly strategy to avoid yield loss caused by BB and BLS. As a model system for plant-pathogen interaction, the rice-X. oryzae pathosystem has been intensively investigated in the past decade. Abundant studies have shown that the resistance and susceptibility of rice to X. oryzae is determined by molecular interactions between rice genes or their products and various pathogen effectors. In this review, we briefly overviewed the literature regarding the diverse interactions, focusing on recent advances in uncovering mechanisms of rice resistance and X. oryzae virulence. Our analysis and discussions will not only be helpful for getting a better understanding of coevolution of the rice innate immunity and X. oryzae virulence, but it will also provide new insights for application of plant R genes in crop breeding.

Close linkage of a blast resistance gene, Pias(t), with a bacterial leaf blight resistance gene, Xa1-as(t), in a rice cultivar 'Asominori'.

It has long been known that a bacterial leaf blight-resistant line in rice obtained from a crossing using 'Asominori' as a resistant parent also has resistance to blast, but a blast resistance gene in 'Asominori' has not been investigated in detail. In the present study, a blast resistance gene in 'Asominori', tentatively named Pias(t), was revealed to be located within 162-kb region between DNA markers YX4-3 and NX4-1 on chromosome 4 and to be linked with an 'Asominori' allele of the bacterial leaf blight resistance gene Xa1, tentatively named Xa1-as(t). An 'Asominori' allele of Pias(t) was found to be dominant and difference of disease severity between lines having the 'Asominori' allele of Pias(t) and those without it was 1.2 in disease index from 0 to 10. Pias(t) was also closely linked with the Ph gene controlling phenol reaction, suggesting the possibility of successful selection of blast resistance using the phenol reaction. Since blast-resistant commercial cultivars have been developed using 'Asominori' as a parent, Pias(t) is considered to be a useful gene in rice breeding for blast resistance.

TAL Effectors with Avirulence Activity in African Strains of Xanthomonas oryzae pv. oryzae.

BACKGROUND: Xanthomonas oryzae pv. oryzae (Xoo) causes bacterial leaf blight, a devastating disease of rice. Among the type-3 effectors secreted by Xoo to support pathogen virulence, the Transcription Activator-Like Effector (TALE) family plays a critical role. Some TALEs are major virulence factors that activate susceptibility (S) genes, overexpression of which contributes to disease development. Host incompatibility can result from TALE-induced expression of so-called executor (E) genes leading to a strong and rapid resistance response that blocks disease development. In that context, the TALE functions as an avirulence (Avr) factor. To date no such avirulence factors have been identified in African strains of Xoo. RESULTS: With respect to the importance of TALEs in the Rice-Xoo pathosystem, we aimed at identifying those that may act as Avr factor within African Xoo. We screened 86 rice accessions, and identified 12 that were resistant to two African strains while being susceptible to a well-studied Asian strain. In a gain of function approach based on the introduction of each of the nine tal genes of the avirulent African strain MAI1 into the virulent Asian strain PXO99A, four were found to trigger resistance on specific rice accessions. Loss-of-function mutational analysis further demonstrated the avr activity of two of them, talD and talI, on the rice varieties IR64 and CT13432 respectively. Further analysis of TalI demonstrated the requirement of its activation domain for triggering resistance in CT13432. Resistance in 9 of the 12 rice accessions that were resistant against African Xoo specifically, including CT13432, could be suppressed or largely suppressed by trans-expression of the truncTALE tal2h, similarly to resistance conferred by the Xa1 gene which recognizes TALEs generally independently of their activation domain. CONCLUSION: We identified and characterized TalD and TalI as two African Xoo TALEs with avirulence activity on IR64 and CT13432 respectively. Resistance of CT13432 against African Xoo results from the combination of two mechanisms, one relying on the TalI-mediated induction of an unknown executor gene and the other on an Xa1-like gene or allele.

Effector Mimics and Integrated Decoys, the Never-Ending Arms Race between Rice and Xanthomonas oryzae.

Plants are constantly challenged by a wide range of pathogens and have therefore evolved an array of mechanisms to defend against them. In response to these defense systems, pathogens have evolved strategies to avoid recognition and suppress plant defenses (Brown and Tellier, 2011). Three recent reports dealing with the resistance of rice to Xanthomonas oryzae have added a new twist to our understanding of this fascinating co-evolutionary arms race (Ji et al., 2016; Read et al., 2016; Triplett et al., 2016). They show that pathogens also develop sophisticated effector mimics to trick recognition.