OsCPK20 positively regulates Arabidopsis resistance against Pseudomonas syringae pv. tomato and rice resistance against Magnaporthe grisea

Calcium-dependent protein kinases are important decoders of calcium signals in plants, which are involved in plant immunity. We report isolation and functional characterization of a pathogen-responsive OsCPK20 gene in rice. The expression of OsCPK20 in rice was significantly induced following treatment with a Magnaporthe grisea elicitor. Overexpression of constitutively active OsCPK20 in Arabidopsis enhanced the resistance to infection with Pseudomonas syringae pv. tomato, associated with elevated expression of both SA- and JA-related defense genes. Similarly, transgenic rice plants containing constitutively active OsCPK20 exhibited enhanced resistance to blast fungus M. grisea. The enhanced resistance in the transgenic Arabidopsis and rice was associated with activated expression of both SA- and JA-related defense genes. We also found that OsCPK20 was significantly induced by drought stress, indicating that OsCPK20 might be involved in plant response to drought stress. Taken together, our results indicate that rice OsCPK20 positively regulates Arabidopsis resistance against Pseudomonas syringae pv. tomato and rice resistance against M. grisea, and that it may enhance disease resistance by activating both SA- and JA-dependent defense responses.     

Manipulating mRNA splicing by base editing in plants

Precursor-mRNAs(pre-mRNA) can be processed into one or more mature m RNA isoforms through constitutive or alternative splicing pathways. Constitutive splicing of pre-mRNA plays critical roles in gene expressional regulation, such as intronmediated enhancement(IME), whereas alternative splicing(AS) dramatically increases the protein diversity and gene functional regulation. However, the unavailability of mutants for individual spliced isoforms in plants has been a major limitation in studying the function of mRNA splicing. Here, we describe an efficient tool for manipulating the splicing of plant genes. Using a Cas9-directed base editor, we converted the 5′ splice sites in four Arabidopsis genes from the activated GT form to the inactive AT form. Silencing the AS of HAB 1.1(encoding a type 2 C phosphatase) validated its function in abscisic acid signaling, while perturbing the AS of RS31 A revealed its functional involvement in plant response to genotoxic treatment for the first time. Lastly,altering the constitutive splicing of Act2 via base editing facilitated the analysis of IME. This strategy provides an efficient tool for investigating the function and regulation of gene splicing in plants and other eukaryotes.     

Raphanusanin-mediated resistance to pathogens is light dependent in radish and Arabidopsis thaliana

Raphanusanin (Ra) is a light-induced inhibitor of hypocotyl growth that responds to unilateral blue light illumination in radish seedlings. We have previously shown that Ra regulates genes that are involved in common defense mechanisms. Many genes that are induced by Ra are also positively regulated by early blue light. To extend the understanding of the role of Ra in pathogen defense, we evaluated the effects of Ra on radish and Arabidopsis thaliana (A. thaliana) infected with the necrotrophic pathogen Botrytis cinerea (B. cinerea) and biotrophic pathogen Pseudomonas syringae (P. syringae). Radish and A. thaliana were found to be resistant to both pathogens when treated with Ra, depending on the concentration used. Interestingly, Ra-mediated resistance to P. syringae is dependent on light because Ra-treated seedlings exhibited enhanced susceptibility to P. syringae infection when grown in the dark. In addition to regulating the biotic defense response, Ra inhibited seed germination and root elongation and enhanced the growth of root hairs in the presence of light in radish and A. thaliana. Our data suggest that Ra regulates the expression of a set of genes involved in defense signaling pathways and plays a role in pathogen defense and plant development. Our results show that light may be generally required not only for the accumulation of Ra but also for its activation during the pathogen defense response.     

Methyl Salicylate Production and Jasmonate Signaling Are Not Essential for Systemic Acquired Resistance in Arabidopsis

Systemic acquired resistance (SAR) develops in response to local microbial leaf inoculation and renders the whole plant more resistant to subsequent pathogen infection. Accumulation of salicylic acid (SA) in noninfected plant parts is required for SAR, and methyl salicylate (MeSA) and jasmonate (JA) are proposed to have critical roles during SAR long-distance signaling from inoculated to distant leaves. Here, we address the significance of MeSA and JA during SAR development in Arabidopsis thaliana. MeSA production increases in leaves inoculated with the SAR-inducing bacterial pathogen Pseudomonas syringae; however, most MeSA is emitted into the atmosphere, and only small amounts are retained. We show that in several Arabidopsis defense mutants, the abilities to produce MeSA and to establish SAR do not coincide. T-DNA insertion lines defective in expression of a pathogen-responsive SA methyltransferase gene are completely devoid of induced MeSA production but increase systemic SA levels and develop SAR upon local P. syringae inoculation. Therefore, MeSA is dispensable for SAR in Arabidopsis, and SA accumulation in distant leaves appears to occur by de novo synthesis via isochorismate synthase. We show that MeSA production induced by P. syringae depends on the JA pathway but that JA biosynthesis or downstream signaling is not required for SAR. In compatible interactions, MeSA production depends on the P. syringae virulence factor coronatine, suggesting that the phytopathogen uses coronatine-mediated volatilization of MeSA from leaves to attenuate the SA-based defense pathway.