ENGINEERING MOLECULAR IMMUNITY FOR ROOT-KNOT NEMATODE RESISTANCE IN POTATO

Itsuhiro H. Ko

Root-knot nematodes (RKN, Meloidogyne spp.) are soil-borne pathogens that infect plant roots worldwide. In the Pacific Northwest region, RKNs are a major problem in the potato industry, where potato tuber damage by RKNs can significantly affect market value. Although chemical nematicides can be effective in controlling RKNs, producers and consumers are concerned about the toxicity to mammals, impacts on the environment, and rising costs. In addition, there is no commercially available potato cultivar that is genetically resistant to RKNs in the United States; therefore, novel control strategies to combat RKNs are needed. In my dissertation research, I proposed three ideas to develop such strategies: 1, introducing microbial allies to enhance plant immunity against RKNs; 2, developing a RKN resistant plant; and 3, sabotaging RKN parasitism in the infected tissue.A recent study demonstrated that, under controlled environmental conditions, watering potatoes with a solution of B. subtilis that secretes the potato peptide elicitor 1 (StPep1) decreased root galling by RKNs. However, handling of liquid bacterial cultures for field application can be challenging, especially during transportation. To create an applicable solution for growers, I enhanced the ability of B. subtilis to produce StPep1 and then encapsulated the bacteria into alginate capsules, which are a better solution for transport, storage, and application of the bacteria. Following an application of these capsules into the soil, the capsules slowly degraded over time, releasing B. subtilis that secretes StPep1 near the potato roots. Treating potatoes with these “probiotic capsules” resulted in up to 70% reduction in the formation of root galls. Our long-term goal is to provide growers with a sustainable alternative for pathogen biocontrol in potatoes and beyond through this probiotic capsule treatment. Recent advancements in genetic modification have fostered new ideas in how we can enhance RKN resistance in susceptible plants by mirroring the molecular immunity found in naturally resistant plants. The critical first step is to identify a genetic source of RKN resistance. Solanum sisymbriifolium, or litchi tomato, is a wild Solanum plant that displays resistance to nematodes. To understand the mechanism of RKN resistance in litchi tomato, I utilized genomic and transcriptomic approaches and have identified putative RKN resistant genes in litchi tomato, which encode NLR proteins. Expressing one of these genes into a susceptible potato root reduced RKN galling. Future research into these genes may eventually provide breeders with resistance candidates that they can use for marker assisted breeding in domesticated Solanum and the development of RKN resistant potato cultivars. Previous studies have suggested that resistant Solanum crops can sabotage RKN infection by accumulating reactive oxygen species in infected tissue. However, some races of RKNs (i.e. M. chitwoodi ‘Roza’ population) can break such resistance. I hypothesized that to survive, RKNs have developed strategies to evade host immunity. The traditional approach of analyzing gene expression from nematode-infected tissue overlooks the cell heterogeneity, and it is impossible to study the precise cells involved in the molecular plant-nematode interaction. Therefore, I performed laser capture microdissection of potato root infected with M. chitwoodi Roza population and isolated the following three cell types: 1, nematode and infected giant cells; 2, by-stander cells; and 3, non-infected nearby root cells as a control tissue. Through cell type specific transcriptome analysis, I identified susceptibility genes in plants (e.g. host genes involved in feeding site formation)and effector genes in pathogens (e.g. nematode genes facilitating parasitism). In the future, these genes could be manipulated through RNA interference treatment, thereby sabotaging RKNs parasitism in potato. Collectively, three chapters of my thesis research propose the feasibility of these three ideas, which can be promoted into new strategies to control RKNs in agriculture.

ENGINEERING MOLECULAR IMMUNITY FOR ROOT-KNOT NEMATODE RESISTANCE IN POTATO

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