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Abstract
Hexose released from sucrose by cell wall bound acid invertases is considered to be a major carbon source for powdery mildews. This aspect of pathogenesis was investigated in diploid wheat (Triticum monococcum) infected with powdery mildew [Blumeria graminis f. sp. tritici]. Quantitative assays revealed an increase in acid invertase activity following infection, and that soluble invertases, of vacuolar and (or) extracellular origin, accounted for over 80% of the overall increase. In situ detection showed activity of cell wall bound invertase in both uninfected and infected leaf tissues, but no increase in activity following infection and no activity at the infection sites. Two cDNAs of the putative genes Tm-exin1 and Tm-vin1 coding for a soluble extracellular invertase and a vacuolar invertase, respectively, were identified in a collection of expressed sequence tags derived from mildewed wheat leaf epidermis. Tm-exin1 was induced in infected leaves, and the enhanced expression coincided with conidiation and colonization of wheat leaf by the fungus. Conversely, Tm-vin1 expression remained relatively constant throughout infection. Collectively, these results support the view that invertases mediate hexose production during infection by the fungus causing powdery mildew and, furthermore, suggest that soluble extracellular invertase is responsible for providing the increased hexose required by the fungus.