Melotto Lab at UCD

**Note: We are hiring post-docs to work on a newly funded project. Full announcement coming soon... stay tuned!

Lab Projects:

1. Dissecting the metabolic pathways that control plant growth and defense 

(has been funded by NIAID, Henry A. Jastro Research Award, and USDA-NIFA-AFRI)

Historically, plant biologists, breeders, and geneticists have struggled with crop yield penalties when improving plant defenses against pathogens and pests. The understanding of these growth-defense tradeoffs remains largely elusive. While interrogating the regulation of jasmonic acid (JA) signaling pathway, a metabolic process that controls both plant growth and defense, we identified a family of proteins that act as the JA-Ile hormone receptor as well as a regulator of gene transcription (Thines et al. 2007). My lab has discovered that a member of this family, named JAZ4, participates in the canonical JA signaling pathway leading to plant defense response, whereas it controls plant growth and development independently of JA signaling (Oblessuc et al. 2020). JAZ4 can independently control these biological processes due to its distinct temporal-spatial expression pattern (DeMott et al. 2021) and its interactions with specific transcription factors in different plant organs and tissues (Miccono et al. 2023). 

Our detailed analyses of JAZ4 functions revealed that it acts as a negative regulator of both growth and defense (Oblessuc et al. 2020). As such, plants lacking this protein become more resistant to several pathogens and show more vigorous vegetative growth (Miccono et al. 2023). These findings represent a paradigm shift in the field and a highly sought combination of traits for a resilient crop production.

The JAZ protein family is highly conserved among plant species (our original discovery is cited more than 2,600 times and it is now part of plant science textbooks); thus, creating the opportunity to translate this knowledge to many agronomically relevant systems. We have confirmed that JAZ orthologs of lettuce and tomato function in plant responses to pathogens and created corresponding knockout lines using CRISPR/Cas9 genome editing to confirm protein functionality. This research has paved the way for crop metabolic engineering to respond to novel environmental challenges. 

2. Employing strategies to disrupt plant reservoirs for human pathogens 

(has been funded by the Henry A. Jastro Research Award, CA Leafy Greens Research Board, and USDA-NIFA-AFRI)

Fresh produce, such as leafy vegetables, poses one of the greatest risks to foodborne illnesses because, unlike foods of animal origin, they cannot undergo thermal processes or harsh chemical treatment to inactivate human pathogens. Leafy greens (mainly produced in California and Arizona) have been estimated to be involved in approximately 2.3 million illnesses and a $5.28 billion cost annually in the US. Recent models placed lettuce as the vehicle for 75.7% of the leafy green foodborne illnesses and accounting for 70% of the cost, where Shiga toxin-producing Escherichia coli (STEC) O157 is the most frequent etiological agent. Disease outbreaks mainly originate from plant contamination in the field, becoming clear that edible leaves can be reservoirs for these pathogens until they reach the human host. My research is focused on prevention of environmental contamination to improve food safety. 

We have shown that clinical isolates of E. coli O157:H7 and Salmonella enterica can survive on/in lettuce leaves, or even multiply in certain plant genotypes (Jacob and Melotto 2020). The survival of these pathogens in the leaf environment is a trait that is largely controlled by genetic factors of both the plant host (Jacob and Melotto 2020) and the bacterial pathogen (Montano et al. 2020), as well as the environmental conditions under which the plants are grown (Student et al. 2024). Importantly, we found genetic variability for this trait (Roy and Melotto 2019; Jacob and Melotto 2020; Oblessuc and Melotto 2020), phenotyped mapping populations (Bridges et al. in preparation), generated several omics resources (Jacob et al. 2021), and created CRISPR-edited lettuce lines (Yang et al. in preparation) to study genes and metabolites involved in bacterium growth in edible leaves. These efforts are providing the foundation for breeding strategies to mitigate foodborne illnesses related to leaf greens (Melotto et al. 2022). I have played a key role in driving federal research funding in this direction by organizing a NIFA workshop at UC Davis with experts in related fields for a comprehensive analysis of the status and opportunities to improve crop safety (Melotto et al. 2020).

3. Elucidating microbiome functions controlled by lettuce immunity to biotic stresses  

(has been funded by NIFA-NNF and Graduate Student Fellowships)

Owing to co-occurrences of diseases in the field, resistance to multiple biotic stresses is a highly desirable trait for the lettuce industry. However, the complex nature of this trait (i.e., broad resistance) has prevented significant advancements in that direction. I am leading collaborative research to decipher the contributions of plant immunity, microbiome functions, and current agricultural practices to the full expression of broad resistance, promoting crop resilience under biotic stress while still achieving crop quality and safety. The team, Dr. Jorge Rodrigues at LAWR, Dr. Ivan Simko at USDA-ARS, and myself bring expertise in microbiology, breeding and genetics, and environmental plant science to this project. 

Capitalizing on the abundance of lettuce genetic and genomic resources developed by our collaborator Dr. Richard Michelmore, we created mutants for four, out of the nine, JAZ genes of lettuce and advanced them to the T₃ generation to uncover the biological functions of each member of the family. We tested our lettuce knockout lines in fields managed by Dr. Ivan Simko in the Salinas Valley and confirmed that they perform better than their parental lines for both growth and defense, while maintaining the nutritional quality (Yang et al. in preparation). The near future holds an exciting prospect to share these new lettuce lines with breeding programs for improved crop performance under stress, considering agricultural microbiome management strategies put forth by Rodrigues and Melotto (2023).