Tissue-type immune response in Arabidopsis, bean, tomato, and sugarcane

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. in preparation). We have confirmed that its orthologs in tomato and sugarcane also function in plant responses to pathogens (manuscripts in preparation); thus, we are in the process of creating CRSPR lines to validate these observations in these agronomic relevant systems.

Collaborators: Claudia BM Vitorello (ESALQ, University of Sao Paulo, Brazil)

Genetic resistance in crop plants

Genetic complementation of Arabidopsis mutants has been a valuable approach to gain insight of protein functions in other less tractable plant systems. We continue our efforts to understand the function of the Co-4 resistance locus of common bean and the putative COK-4 kinase residing in this locus. Our latest research  provided strong evidence that the COK-4 is involved in the control of growth-defense balance in plants (Azevedo et al. 2018). This finding has major implications as breeding programs worldwide still employ the Co-4 locus as an effective measure to control anthracnose in the field.

More recently, we developed a collaboration with Brazilian researchers to validate the molecular and biological functions of a putative transcription factor (TF) of citrus, an important commodity for the State of California. Together, we showed that the TF CrRAP2.2 confers resistance against Xylella fastidiosa in Arabidopsis and citrus (Pereira et al. 2019, 2020). Thus, our lab has been able to capitalize on Arabidopsis genetic and genomic resources (including yeast –two-hybrid screenings; Matiolli and Melotto 2018) to advance current understanding of genetic resistance against pathogens in important crops.

Collaborators: Alessandra Alves de Souza (Centro de Citricultura, Cordeiropolis, Brazil)

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) 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: Shlomo Sela (The Volcani Center), Michael McClelland (UC Irvine), and Richard Michelmore (UC Davis)

Plant and microbial indicators of soil health

To ensure the long-term sustainability of cropping systems, the well-being of both soil microorganisms and crops need to be taken into account. With the ultimate goal of informing management decisions, we are collaborating with other researchers at UC Davis to develop a holistic approach that combines traditional soil health assessments with sensitive biological indicators of stress in the soil microbiome and plants (tomato and corn) under varying management practices (conventional and organic).

This is a collaborative research that includes soil microbiology (led by Jorge Rodrigues, LAWR, UC Davis), nutrient management (led by Daniel Geisseler, LAWR, UC ANR), and plant physiology (led by Maeli Melotto, Plant Sciences, UC Davis).