We are studying the interaction of a soil-borne fungal pathogen and its host. Performing genetic analysis with either fungus or plant, our goal is to describe pathogen virulence and host resistance in term of molecular interactions between fungal genes and plant genes. Most molecular research on plant infectious disease focuses on foliar pathogens. Needless to say, soil-borne disease is of equal or greater concern for many agricultural and horticultural commodities. Here in California, for instance, Fusarium wilt disease threatens the cultivation of date palms and cotton. We are working to better understand the genetic basis for natural variation in resistance to Fusarium oxysporum. One resistance locus in the Columbia-0 ecotype has been cloned. RESISTANCE TO FUSARIUM OXYSPORUM 1 (RFO1) was isolated on the basis of strong but quantitative resistance to Fusarium oxysporum f. sp. matthioli. Subsequently, RFO1 proved to be critical for resistance to soil-borne, vascular pathogens in general. Thus, loss-of-function rfo1 is more susceptible, and transgenic RFO1 is more resistant, to other F. oxysporum lineages, f.sp. conglutinans and f. sp. raphani. Moreover, another research group has reported that rfo1 also confers enhanced susceptibility to Verticillium wilt (V. longisporum).
Presently, we are working to identify additional RFO genes, namely RFO2 and RFO3. Genetic analysis tells us that RFO1 is epistatic toRFO2. The strong resistance provided by RFO2-Col-0 requires the presence of RFO1-Col-0. In the absence of RFO1-Col-0, no resistance can be attributed to the RFO2 locus. RFO3 is a weaker quantitative resistance gene and indepedent of RFO1 expression.
Among wilts, unilateral disease symptoms are a telltale sign of Fusarium wilt
Fusarium wilt of Arabidopsis
Rare pathogenic forms (or formae speciales) of Fusarium oxysporum are responsible for soil-borne vascular diseases variously known as wilts, root rot or yellows. Fusarium wilt disease has been reported to afflict numerous agricultural and horticultural commodities, most notably banana plants. A F. oxysporum forma specialis (f sp) classifies one or more pathogenic lineages exhibiting a specific genus or species host range. For instance, F. oxysporum f sp lycopersici promotes Fusarium wilt in Lycopersicon (tomato) species.
Three F. oxysporum formae speciales instigate wilt disease in Arabidopsis. In the field, F. oxysporum f. sp. conglutinans, raphani and matthioli are isolated from diseased plants in the Mustard family (Brassicaceae) Brassica species (e.g., cabbage), Raphanus sativus (radish) and Mathiola incana (stock). In the laboratory, these formae speciales, which are distinct lineages, produce symptoms that are remarkably similar to the disease observed in the natural host.
We are studying Fusarium wilt of Arabidopsis to (1) provide a molecular explanation for quantitative and oligogenic nature of resistance that is seen among Arabidopsis wild accessions; (2) reveal the role of previously described genetic pathways in resistance and susceptibility; and, (3) identify the molecular basis for host-specific virulence as well as general features of virulence in the pathogen F. oxysporum f sp conglutinans.
