Tissue-type immune response in Arabidopsis, lettuce, and tomato

As stomatal-based defense influences the ability of bacteria to internalize leaves and cause disease (phytopathogens) or plant contamination (human pathogens), a major goal of our research is to improve the understanding of the molecular mechanisms underlying this process (Melotto et al. 2017; Zhang et al. 2017). Using variable environmental conditions, we uncovered that stomatal movement in common bean and Arabidopsis is accompanied by regulation of the jasmonic acid (JA) and salicylic acid (SA) pathways in guard cells (Panchal et al. 2016a, Panchal and Melotto, 2017). Additionally, we found that functional attributes of coronatine, a phytotoxin produced by Pseudomonas syringae (Panchal et al. 2017) provide epidemiological advantages for this pathogen on the leaf surface (Panchal et al. 2016b).

Well-known regulators of jasmonic acid (JA) signaling and co-receptors of JA-Isoleucine and coronatine are member of the JAZ protein family. My lab has discovered that JAZ4 participates in the canonical (COI1/MYC) JA signaling pathway leading to plant defense response in addition to COI1/MYC‐independent functions in plant growth and development, supporting the notion that JAZ4‐mediated signaling may have distinct branches (Oblessuc et al. 2020b). The expression pattern of JAZ4 is tissue-specific and the protein is highly conserved among plant species (DeMott et al. 2021). We have confirmed that its orthologs in tomato and lettuce also function in plant responses to pathogens (manuscripts in preparation); thus, we are in the process of creating genome-edited lines via CRISPR/Cas9 to validate these observations in these agronomic relevant systems.

Molecular and genetic mechanisms of human bacterial pathogens colonization of edible leaves 

Unlike foods of animal origin, fresh or ready-to-eat produce, such as leafy vegetables, cannot undergo thermal processes to inactivate human pathogens. The lack of an efficient kill-step poses considerable risks of foodborne disease outbreaks and a great challenge for the fresh produce industry in California, the US, and worldwide. Although several procedures are in place to prevent food poisoning, additional control measures are still needed to reduce the number of outbreaks, including genetic resistance (Oblessuc et al. 2019, 2020a; Melotto et al., 2020). Our research has uncovered molecular and genetic mechanisms that allow E. coli O157:H7 and S. enterica to persist in the leaf environment, a trait that is largely controlled by genetic factors in both the plant host (Jacob and Melotto, 2020; Jacob et al., 2021) and the bacterial pathogen (Jeanine et al. 2020). Importantly, recent research efforts in our lab revealed genetic variability that controls bacterial internalization and persistence in leaves (Roy and Melotto 2019; Jacob and Melotto 2020; Oblessuc and Melotto 2020), providing the foundation to explore breeding strategies to mitigate foodborne illnesses.

Collaborators: Ivan Simko (USDA-ARS Salinas) and Richard Michelmore (UC Davis)

The balance between plant growth and defense

The antagonism between plant growth and plant defense has been a bottleneck for crop improvement in a long time. Uncovering the molecular nodes that connect these traits is crucial to achieve robust plant growth under less than optimum conditions (i.e., under stress). We are using metabolic engineering to decouple these processes, or at least alleviate the detrimental growth-defense trade offs.