The functional characterization of a root knot nematode effector Mi131 and an investigation of the role of jasmonic acid during the Arabidopsis-root knot nematode interaction

Root knot nematodes (Meloidogyne spp.) are devastating pests to agriculture worldwide. The best control measure for these small, microscopic roundworms is to fumigate the field with nematicides prior to planting. However nematicides are problematic because they have non-target toxicity and are being phased-out of use. Therefore, new control measures are urgently required. My thesis looks at both sides of the plant-nematode interaction in order to understand nematode virulence determinants and the plant responses to nematodes. We hope that with this new knowledge, we can one day engineer novel ways to combat nematodes. During plant-RKN interactions, the nematode is presumably secreting molecules into the plant that are crucial for successful infection. These proteins are called “effectors,” and they are postulated to be involved in both plant defense suppression and RKN feeding site generation. In Gleason lab, we are trying to characterize the function of several RKN effectors. One of the potential effectors from Meloidogyne incognita is called Mi131. Mi131 is a protein with a profilin domain, which is typically involved in actin binding. When Mi131 was expressed in plants, the plants were more susceptible to nematodes. To elucidate Mi131 function, I performed yeast two hybrid screens to find Mi131 interaction partner(s) in Arabidopsis. I found that Mi131 can interact with both vegetative and reproductive isoforms of plant actin. In vitro actin polymerization assays indicated that Mi131 inhibits actin polymerization. Further investigations using protoplasts with a GFP-labelled actin cytoskeleton showed that when Mi131 was expressed in these cells, the actin cytoskeleton appeared fragmented. Plants which overexpress AtActin1 have a mutant, dwarf phenotype. Co-expression of Mi131 in these plants could rescue the dwarf phenotype. This indicated that Mi131 can act as an actin-binding profilin in plants and titer out the toxic levels of AtActin1. Previous cell biology studies of the root-knot nematode giant cells had shown that actin re-organization is crucial for RKN feeding site development and expansion. The nematode may secrete Mi131 to interfere with the actin dynamics in the cell and thereby promote RKN feeding site establishment. The Gleason lab is also interested in how the plant responds to RKNs during the compatible interaction, with a focus on the phytohormone jasmonic acid (JA). Some publications had indicated that JA is involved in promoting defence against nematodes. Other publications suggested that JA is required for nematode susceptibility. Therefore, I investigated the role of JA using the model plant Arabidopsis thaliana. From my studies, I found that exogenous methyl jasmonate application on Arabidopsis significantly reduced the number of galls caused by RKNs. Interestingly, I found that MeJA induced resistance was independent of COI1, the JA receptor. The work carried out by myself and others in the lab shows that Arabidopsis mutants in trienoic fatty acid (fad3/7/8) or the octadecanoid pathway (aos) were more susceptible to nematodes. Meanwhile, plants inhibited in JA signaling (coi1-t) showed normal, wildtype levels of infection. Importantly, mutants in which the conversion of 12-oxophytodienoic acid (OPDA) to JA is inhibited (opr3 and acx1/5) also showed wild-type levels of nematode disease. Overall, the data suggests that the JA precursor, OPDA, has a role in plant defence against nematodes.