Molecular mechanisms of human bacterial pathogens internalization and survival in leafy vegetables

Pathogens that cause foodborne illness pose a challenge to food safety and security. Owing to the health appearance of contaminated crops, these foodborne pathogens can be integrated into marketplaces throughout the world. Previously we have shown that some human pathogens elicit immune responses within the plant; however, Salmonella enterica serovar Typhimurium (STM) can disrupt the plant innate immune system and survive for long time inside the leaf.

Collaborators: Shlomo Sela (The Volcani Center) and Michael McClelland (UC Irvine)

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).

Collaborator: Sheng Yang He (Michigan State University)

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 wellbeing 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 indicators of the effects of the soil environment on the soil microbial community and plants, such as microbial community dynamics (taxonomic and genetic) and plant stress physiology.

Our approach shall lead to a better understanding of the effects of crop management practices on soil microorganisms and plants. In the long-term, this shall increase the inclusion of soil health as a factor when management decisions are made by farmers, land managers and crop advisers.

Collaborators: Jorge Rodrigues (UC Davis) and Daniel Geisseler (UC ANR)