The tomato transcription factor (TF) MYC2 and MYC2-targeted TFs form a hierarchical transcriptional cascade during jasmonate-mediated plant immunity that initia

In tomato (Solanum lycopersicum), as in other plants, the immunity hormone jasmonate (JA) triggers genome-wide transcriptional changes in response to pathogen and insect attack. These changes are largely regulated by the basic helix-loop-helix (bHLH) transcription factor MYC2. The function of MYC2 depends on its physical interaction with the MED25 subunit of the Mediator transcriptional coactivator complex. Although much has been learned about the MYC2-dependent transcriptional activation of JA-responsive genes, relatively less studied is the termination of JA-mediated transcriptional responses and the underlying mechanisms. Here, we report an unexpected function of MYC2 in regulating the termination of JA signaling through activating a small group of JA-inducible bHLH proteins, termed MYC2-TARGETED BHLH1 (MTB1), MTB2, and MTB3. MTB proteins negatively regulate JA-mediated transcriptional responses via their antagonistic effects on the functionality of the MYC2-MED25 transcriptional activation complex. MTB proteins impair the formation of the MYC2-MED25 complex and compete with MYC2 to bind to its target gene promoters. Therefore, MYC2 and MTB proteins form an autoregulatory negative feedback circuit to terminate JA signaling in a highly organized manner. We provide examples demonstrating that gene editing tools such as CRISPR/Cas9 open up new avenues to exploit MTB genes for crop protection.

To restrict pathogen entry, plants close stomata as an integral part of innate immunity. To counteract this defense, Pseudomonas syringae pv tomato produces coronatine (COR), which mimics jasmonic acid (JA), to reopen stomata for bacterial entry. It is believed that abscisic acid (ABA) plays a central role in regulating bacteria-triggered stomatal closure and that stomatal reopening requires the JA/COR pathway, but the downstream signaling events remain unclear. We studied the stomatal immunity of tomato (Solanum lycopersicum) and report here the distinct roles of two homologous NAC (for NAM, ATAF1,2, and CUC2) transcription factors, JA2 (for jasmonic acid2) and JA2L (for JA2-like), in regulating pathogen-triggered stomatal movement. ABA activates JA2 expression, and genetic manipulation of JA2 revealed its positive role in ABA-mediated stomatal closure. We show that JA2 exerts this effect by regulating the expression of an ABA biosynthetic gene. By contrast, JA and COR activate JA2L expression, and genetic manipulation of JA2L revealed its positive role in JA/COR-mediated stomatal reopening. We show that JA2L executes this effect by regulating the expression of genes involved in the metabolism of salicylic acid. Thus, these closely related NAC proteins differentially regulate pathogen-induced stomatal closure and reopening through distinct mechanisms.

Stomata are natural openings through which many pathogenic bacteria enter plants. Successful bacterial pathogens have evolved various virulence factors to promote stomatal opening. Here, we show that the Pseudomonas syringae type III effector protein AvrB induces stomatal opening and enhances bacterial virulence in a manner dependent on RPM1-INTERACTING4 (RIN4), which promotes stomatal opening by positively regulating the Arabidopsis plasma membrane H⁺-ATPase (AHA1), which is presumed to directly regulate guard cell turgor pressure. In support of a role of AHA1 in AvrB-induced stomatal opening, AvrB enhances ATPase activity in plants. Unexpectedly, AHA1 promotes the interaction between the jasmonate (JA) receptor CORONATINE INSENSITIVE1 (COI1) and JASMONATE ZIM-DOMAIN (JAZ) proteins and enhances JA signaling. JA signaling is required for optimum stomatal infection in AHA1-active plants. Similarly, AvrB also induces the COI1-JAZ9 interaction and the degradation of multiple JAZ proteins. AvrB-induced stomatal opening and virulence require the canonical JA signaling pathway, which involves the COI1 and NAC transcription factors. The findings thus point to a previously unknown pathway exploited by P. syringae that acts upstream of COI1 to regulate JA signaling and stomatal opening.

Summary: Incorporating male sterility into hybrid seed production reduces its cost and ensures high varietal purity. Despite these advantages, male‐sterile lines have not been widely used to produce tomato (Solanum lycopersicum) hybrid seeds. We describe the development of a biotechnology‐based breeding platform that utilized genic male sterility to produce hybrid seeds. In this platform, we generated a novel male‐sterile tomato line by clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR‐associated protein 9 (Cas9)‐mediated mutagenesis of a stamen‐specific gene SlSTR1 and devised a transgenic maintainer by transforming male‐sterile plants with a fertility‐restoration gene linked to a seedling‐colour gene. Offspring of crosses between a hemizygous maintainer and the homozygous male‐sterile plant segregated into 50% non‐transgenic male‐sterile plants and 50% male‐fertile maintainer plants, which could be easily distinguished by seedling colour. This system has great practical potential for hybrid seed breeding and production as it overcomes the problems intrinsic to other male‐sterility systems and can be easily adapted for a range of tomato cultivars and diverse vegetable crops.