Rice transcription factor WRKY114 directly regulates the expression of OsPR1a and Chitinase to enhance resistance against Xanthomonas oryzae pv. oryzae

The OsICS1 is directly regulated by OsWRKY6 and increases resistance against Xanthomonas oryzae pv. oryzae

KEY MESSAGE

OsICS1 but not OsICS1-L mediates the rice response to Xoo inoculation, with its overexpression increasing resistance against this pathogen. OsICS1 but not OsICS-L is directly upregulated by OsWRKY6. Rice (Oryza sativa) is a staple crop for about half of the global population and is particularly important in the diets of people living in Asia, Latin America, and Africa. This crop is continually threatened by bacterial leaf blight disease caused by Xanthomonas oryzae pv. oryzae (Xoo), which drastically reduces yields; therefore, it is needed to elucidate the plant's resistance mechanisms against Xoo. Isochorismate synthase (ICS1) generates salicylic acid (SA) and increases resistance against bacterial disease. The OsICS1 is differently annotated in rice genome databases and has not yet been functionally characterized in the context of Xoo infection. Here, we report that the expression of the OsICS1 is directly regulated by OsWRKY6 and increases plant resistance against Xoo. Inoculation with Xoo increased the expression of OsICS1 but not that of the long variant of OsICS1 (OsICS1-L). OsWRKY6 directly activated the OsICS1 promoter but not the OsICS1-L promoter. OsICS1 overexpression in rice increased resistance against Xoo through the induction of SA-dependent bacterial defense genes. These data show that OsICS1 promotes resistance against Xoo infection.

OsWRKY7 contributes to pattern-triggered immunity against Xanthomonas oryzae pv. oryzae

Rice is a staple crop continually threatened by bacterial and fungal pathogens. OsWRKY transcription factors are involved in various disease responses. However, the functions of many OsWRKYs are still elusive. In this study, we demonstrated that OsWRKY7 enhances rice immunity against Xanthomonas oryzae pv. oryzae (Xoo). OsWRKY7 localized in the nucleus, and gene expression of OsWRKY7 was induced by Xoo inoculation. The OsWRKY7-overexpressing lines showed enhanced resistant phenotype against Xoo, and gene expressions of OsPR1a, OsPR1b, and OsPR10a were significantly increased in the transgenic lines after Xoo inoculation. Moreover, OsWRKY7 activated the OsPR promoters, and the promoter activities were synergistically upregulated by flg22. Genetic- and cell-based analysis showed OsWRKY7 is involved in pattern-triggered immunity against Xoo. These results suggest that OsWRKY7 plays a role as a positive regulator of disease resistance to Xoo through pattern-triggered immunity.

OsWRKY114 Is a Player in Rice Immunity against Fusarium fujikuroi

Every year, invasive pathogens cause significant damage to crops. Thus, identifying genes conferring broad-spectrum resistance to invading pathogens is critical for plant breeding. We previously demonstrated that OsWRKY114 contributes to rice (Oryza sativa L.) immunity against the bacterial pathovar Xanthomonas oryzae pv. oryzae (Xoo). However, it is not known whether OsWRKY114 is involved in defense responses to other pathogens. In this study, we revealed that OsWRKY114 enhances innate immunity in rice against the fungal pathogen Fusarium fujikuroi, which is the causal agent of bakanae disease. Transcript levels of various gibberellin-related genes that are required for plant susceptibility to F. fujikuroi were reduced in rice plants overexpressing OsWRKY114. Analysis of disease symptoms revealed increased innate immunity against F. fujikuroi in OsWRKY114-overexpressing rice plants. Moreover, the expression levels of OsJAZ genes, which encode negative regulators of jasmonic acid signaling that confer immunity against F. fujikuroi, were reduced in OsWRKY114-overexpressing rice plants. These results indicate that OsWRKY114 confers broad-spectrum resistance not only to Xoo but also to F. fujikuroi. Our findings provide a basis for developing strategies to mitigate pathogen attack and improve crop resilience to biotic stress.

Engineering plant immune circuit: walking to the bright future with a novel toolbox

Plant pathogens destroy crops and cause severe yield losses, leading to an insufficient food supply to sustain the human population. Apart from relying on natural plant immune systems to combat biological agents or waiting for the appropriate evolutionary steps to occur over time, researchers are currently seeking new breakthrough methods to boost disease resistance in plants through genetic engineering. Here, we summarize the past two decades of research in disease resistance engineering against an assortment of pathogens through modifying the plant immune components (internal and external) with several biotechnological techniques. We also discuss potential strategies and provide perspectives on engineering plant immune systems for enhanced pathogen resistance and plant fitness.

Molecular basis for host responses to Xanthomonas infection

This review highlights the most relevant and recent updated information available on the defense responses of selected hosts against Xanthomonas spp. Xanthomonas is one of the most important genera of Gram-negative phytopathogenic bacteria, severely affecting the productivity of economically important crops worldwide, colonizing either the vascular system or the mesophyll tissue of the host. Due to its rapid propagation, Xanthomonas poses an enormous challenge to farmers, because it is usually controlled using huge quantities of copper-based chemicals, adversely impacting the environment. Thus, developing new ways of preventing colonization by these bacteria has become essential. Advances in genomic and transcriptomic technologies have significantly elucidated at molecular level interactions between various crops and Xanthomonas species. Understanding how these hosts respond to the infection is crucial if we are to exploit potential approaches for improving crop breeding and cutting productivity losses. This review focuses on our current knowledge of the defense response mechanisms in agricultural crops after Xanthomonas infection. We describe the molecular basis of host-bacterium interactions over a broad spectrum with the aim of improving our fundamental understanding of which genes are involved and how they work in this interaction, providing information that can help to speed up plant breeding programs, namely using gene editing approaches.

OsWRKY114 Inhibits ABA-Induced Susceptibility to Xanthomonas oryzae pv. oryzae in Rice

The phytohormone abscisic acid (ABA) regulates various aspects of plant growth, development, and stress responses. ABA suppresses innate immunity to Xanthomonas oryzae pv. oryzae (Xoo) in rice (Oryza sativa), but the identity of the underlying regulator is unknown. In this study, we revealed that OsWRKY114 is involved in the ABA response during Xoo infection. ABA-induced susceptibility to Xoo was reduced in OsWRKY114-overexpressing rice plants. OsWRKY114 attenuated the negative effect of ABA on salicylic acid-dependent immunity. Furthermore, OsWRKY114 decreased the transcript levels of ABA-associated genes involved in ABA response and biosynthesis. Moreover, the endogenous ABA level was lower in OsWRKY114-overexpressing plants than in the wild-type plants after Xoo inoculation. Taken together, our results suggest that OsWRKY114 is a negative regulator of ABA that confers susceptibility to Xoo in rice.

OsWRKY114 Negatively Regulates Drought Tolerance by Restricting Stomatal Closure in Rice

The WRKY family of transcription factors plays a pivotal role in plant responses to biotic and abiotic stress. The WRKY Group III transcription factor OsWRKY114 is a positive regulator of innate immunity against Xanthomonas oryzae pv. oryzae; however, its role in abiotic stress responses is largely unknown. In this study, we showed that the abundant OsWRKY114 transcripts present in transgenic rice plants are reduced under drought conditions. The overexpression of OsWRKY114 significantly increased drought sensitivity in rice, which resulted in a lower survival rate after drought stress. Moreover, we showed that stomatal closure, which is a strategy to save water under drought, is restricted in OsWRKY114-overexpressing plants compared with wild-type plants. The expression levels of PYR/PYL/RCAR genes, such as OsPYL2 and OsPYL10 that confer drought tolerance through stomatal closure, were also markedly lower in the OsWRKY114-overexpressing plants. Taken together, these results suggest that OsWRKY114 negatively regulates plant tolerance to drought stress via inhibition of stomatal closure, which would otherwise prevent water loss in rice.

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

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.

Identification of a novel NPR1 homolog gene, OsNH5N16, which contributes to broad-spectrum resistance in rice

Over half of the earth's population consumes rice as the primary food crop for dietary calories. However, severe loss of rice yield occurs due to bacterial blight caused by Xanthomonas oryzae pv. oryzae (Xoo) and bakanae disease caused by Fusarium fujikuroi (F. fujikuroi). Therefore, broad-spectrum resistance (BSR) to these pathogens is essential for rice cultivation. The Nonexpressor of Pathogenesis-Related Genes1 (NPR1), which is related to the signal molecule salicylic acid (SA) and the expression of pathogenesis-related (PR) genes, is a key regulator of systemic acquired resistance (SAR). Although five NPR1 homologs (NHs) have been identified in rice thus far, their cellular and biological functions remain largely unexplored. In this study, we identified a novel rice NH gene from Oryza sativa L. cv. Dongjin. The genetic variation of single nucleotide polymorphisms in OsNH5 caused a single amino acid substitution of asparagine for serine at residue 16. OsNH5N16 was mainly located in the nucleus, and its transcription was induced by Xoo. We generated transgenic rice lines constitutively expressing OsNH5N16 to investigate its function. Plants that overexpressed OsNH5N16 displayed enhanced BSR to Xoo and F. fujikuroi compared with wild varieties, and the transcription of PR genes such as OsPR1, GLUC, and CHIT2 was considerably upregulated. Moreover, we revealed that SA increases the transcription of OsNH5N16 and the promoter activity of OsPR1 regulated by OsNH5N16. These results showed that OsNH5N16 enhances BSR by regulating the expression of PR genes related to SAR and it is controlled by SA at the transcriptional and post-translational levels. This is the first report on the innate immune response conferring BSR associated with NH5.