Elevating plant immunity by translational regulation of a rice WRKY transcription factor

Plants have intricate mechanisms that tailor their defence responses to pathogens. WRKY transcription factors play a pivotal role in plant immunity by regulating various defence signalling pathways. Many WRKY genes are transcriptionally activated upon pathogen attack, but how their functions are regulated after transcription remains elusive. Here, we show that OsWRKY7 functions as a crucial positive regulator of rice basal immunity against Xanthomonas oryzae pv. oryzae (Xoo). The activity of OsWRKY7 was regulated at both translational and post-translational levels. Two translational products of OsWRKY7 were generated by alternative initiation. The full-length OsWRKY7 protein is normally degraded by the ubiquitin-proteasome system but was accumulated following elicitor or pathogen treatment, whereas the alternate product initiated from the downstream in-frame start codon was stable. Both the full and alternate OsWRKY7 proteins have transcriptional activities in yeast and rice cells, and overexpression of each form enhanced resistance to Xoo infection. Furthermore, disruption of the main AUG in rice increased the endogenous translation of the alternate stabilized form of OsWRKY7 and enhanced bacterial blight resistance. This study provides insights into the coordination of alternative translation and protein stability in the regulation of plant growth and basal defence mediated by the OsWRKY7 transcription factor, and also suggests a promising strategy to breed disease-resistant rice by translation initiation control.

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.

Sucrose preferentially promotes expression of OsWRKY7 and OsPR10a to enhance defense response to blast fungus in rice

Sucrose controls various developmental and metabolic processes in plants. It also functions as a signaling molecule in the synthesis of carbohydrates, storage proteins, and anthocyanins, as well as in floral induction and defense response. We found that sucrose preferentially induced OsWRKY7, whereas other sugars (such as mannitol, glucose, fructose, galactose, and maltose) did not have the same effect. A hexokinase inhibitor mannoheptulose did not block the effect of sucrose, which is consequently thought to function directly. MG132 inhibited sucrose induction, suggesting that a repressor upstream of OsWRKY7 is degraded by the 26S proteasome pathway. The 3-kb promoter sequence of OsWRKY7 was preferentially induced by sucrose in the luciferase system. Knockout mutants of OsWRKY7 were more sensitive to the rice blast fungus Magnaporthe oryzae, whereas the overexpression of OsWRKY7 enhanced the resistance, indicating that this gene is a positive regulator in the plant defense against this pathogen. The luciferase activity driven by the OsPR10a promoter was induced by OsWRKY7 and this transcription factor bound to the promoter region of OsPR10a, suggesting that OsWRKY7 directly controls the expression of OsPR10a. We conclude that sucrose promotes the transcript level of OsWRKY7, thereby increasing the expression of OsPR10a for the defense response in rice.