Molecular and genetic mechanisms underlying the colonization of leafy vegetables by human bacterial pathogens

Unlike foods of animal origin, fresh or ready-to-eat produce, such as leafy vegetables, cannot undergo thermal processes to inactivate human pathogens. Decontamination treatments rely mainly on the use of wash water disinfectant treatments, usually hypochlorite. Although suspensions of foodborne pathogens, such as Escherichia coli and Salmonella enterica are highly susceptible to commercial disinfectants at the recommended doses and exposure times, disinfectants fail to completely eradicate pathogens from fresh produce. Consequently, the lack of an efficient kill-step is a risk factor and one of the great challenges facing the fresh produce industry. There are several plausible explanations for tolerance of plant-associated bacteria to sanitation procedures, including (a) formation of biofilms on the plant surface, (b) residing on surface-protected niches and wounds, and (c) endophytic localization. Thus, post-harvest sanitation of fresh produce is not sufficient for the complete elimination of foodborne pathogens from produce. Prevention of pre-harvest contamination on the farm is one of the most important steps in reducing human health risk and improving food safety. 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. This research addresses the 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 and the bacterial pathogen.

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

Cell-type immune response in Arabidopsis

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 my research is to improve the understanding of the molecular mechanisms underlying this process (Melotto et al. 2017). Environmental conditions play crucial roles in modulating immunity and disease in plants and animals. For instance, many bacterial plant disease outbreaks occur after periods of high humidity and rain. We found that high air relative humidity could effectively compromise Pseudomonas syringae-triggered stomatal closure in both common bean and Arabidopsis, which is accompanied by early up-regulation of the jasmonic acid (JA) pathway and simultaneous down-regulation of salicylic acid (SA) pathway in guard cells. Furthermore, SA-dependent response, but not JA-dependent response, is faster in guard cells than in whole leaves suggesting that the SA signaling in guard cells may be independent from other cell types. Thus, we conclude that high humidity, a well-known disease-promoting environmental condition, acts in part by suppressing stomatal defense and is linked to hormone signaling in guard cells (Panchal et al. 2016a).

In many land plants, the stomatal pore opens during the day and closes during the night. Thus, periods of darkness could be effective in decreasing pathogen penetration into leaves through stomata, the primary sites for infection by many pathogens. Pseudomonas syringae pv. tomato (Pst) DC3000 produces coronatine and opens stomata, raising an intriguing question as to whether this is a virulence strategy to facilitate bacterial infection at night. In fact, we found that: (1) biological concentration of coronatine is effective in opening dark-closed stomata of Arabidopsis leaves; (2) the coronatine defective mutant Pst DC3118 is less effective in infecting Arabidopsis in the dark than under light and this difference in infection is reduced with the wild type bacterium Pst DC3000; and (3) cma, a coronatine biosynthesis gene, is induced only when the bacterium is in contact with the leaf surface independent of the light conditions. These findings suggest that Pst DC3000 activates virulence factors at the pre-invasive phase of its life cycle to infect plants even when environmental conditions (such as darkness) favor stomatal defense. This functional attribute of coronatine may provide epidemiological advantages for coronatine-producing bacteria on the leaf surface (Panchal et al. 2016b).

Genetic resistance in crop plants 

Plant resource allocation to growth and defense is important to optimizing its fitness. Notably, the induction of plant immune responses is often associated with reduction in plant growth. Receptor-like kinases (RLKs) play an important role in this context as they share common signaling components that can fine-tune different type of responses and therefore regulating the balance between growth and defense. Among the RLKs, the Arabidopsis FERONIA (FER) plays a role in this balance by acting as a scaffold in the formation of receptor complexes involved in diverse biological processes. We have found that COK-4, a putative kinase encoded in the common bean anthracnose resistance locus Co-4, is highly similar to the kinase domain of FER and is transcriptionally regulated during immune response. To assess whether COK-4 is functional ortholog of FER, we genetically complemented the Arabidopsis fer-5 mutant with COK-4 and evaluated FER-associated traits. The enhanced apoplastic and stomatal immunity observed in fer-5 plants were rescued in the complemented lines, suggesting that COK-4 and FER may be negative regulators of plant immunity. We also observed that the fer-5 mutant has developmental defects as it accumulates a lower level of dry weight, has fewer leaves, and transition to reproductive stage faster than the wild type plants. These developmental phenotypes were also rescued in the COK-4-expressing fer-5 lines. Altogether, these data provide strong evidence that COK-4 of common bean is involved in the control of growth-defense balance in plants (Azevedo et al. 2018). These finding has major implications to breeding programs that employ the Co-4 locus to control anthracnose in the field.

Plant and microbial indicators of soil health

Soils are one of our most valuable resources. In agricultural systems, management practices that ensure high yields often have negative effects on soil biology. Many current soil health assessments do not allow for a nuanced assessment of soil biological health. However, to ensure the long-term sustainability of cropping systems, both, the well-being of soil microorganisms and crops need to be taken into account. The goal of this project is to develop a holistic approach that combines traditional soil health assessments with sensitive biological indicators of the effects of the soil environment on the soil microbial community, such as microbial community dynamics (taxonomic and genetic), and plant health. Our approach will lead to a better understanding of the effects of crop management practices on soil microorganisms and plants. In the long-term, this comprehensive measurements of soil health may be included in management decisions made by farmers, land managers, and crop advisers.

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