Molecular biology of the tomato Ve gene family
Resistance to Verticillium wilt disease in tomato has been associated with the Ve gene family comprising two genes, Ve1 and Ve2. This thesis sought to establish the genetic regulation of the Ve1 and Ve2 genes and their functional role in Verticillium resistance. Transcript analyses revealed that Ve1 was induced in plants infected with Verticillium dahliae race 1 (Vd1). In contrast, Ve2 behaved constitutively. This differential expression between Ve1 and Ve2 also was observed in plants infected with other isolates of Verticillium and even Xanthomonas. More importantly, Ve1 transcript levels were induced even in the absence of pathogen by various stresses, including leaf infiltration, stem cutting and exposure to MgCl2. To investigate how these stresses regulated Ve gene expression, two approaches were used. At the physiological level, hormone treatments and signalling mutant analyses showed that auxin, jasmonic acid and salicylic acid could influence Ve1 transcription. At the molecular level, regulatory sequence elements responsible for Ve gene regulation were examined using a promoter-reporter assay. In this assay, systematic deletion mutations of the upstream sequences for the Ve1 and Ve2 genes were fused to an EYFP reporter gene. Reporter mRNA analyses found that the two Ve core promoters possess similar transcriptional activity, suggesting that endogenous Ve1 transcripts are normally repressed. Additionally, neither Vd1 infection nor stem cutting influenced the expression of the EYFP reporter, suggesting that more distant regulatory influences beyond the intergenic region modify the promoter activities and/or endogenous mRNA levels. Overexpression of Ve1 and Ve2 in transgenic susceptible tomato confirmed the role of Ve1 in Vd1 resistance in some plants but not others, implying the influence of a yet undefined factor in disease resistance. Taken together, the results suggest that Ve1 does not function as a classic resistance gene; rather, it is controlled by a range of physiological changes influencing resistance to Vd1 but apparently also participating in a plant’s response to other stresses. The significance of these observations is discussed.