Nutrient Uptake in Rust Fungi

Nutrient uptake in obligate parasitism was recently reviewed by Bushnell and Gay (5) for the powdery mildews. In the rust fungi, the experimental approach to solving this question is quite different, because the fungus mycelium grows only in restricted areas within the leaf. The rust pustules act as foci for the accumulation of many metabolites. The fungus derives nutrients by alteration of the direction of normal phloem transport. Also, since sporulation ruptures the host epidermis, increased water loss through the pustules makes more nutrients availabJe by increased transpiration (23,38). Many methods have been proposed to study the uptake of nutrients by the rust fungi; Fuchs and Gärtner (11) hoped that the rust pathogen would grow in axenic culture in the correct nutritional environment. Reisener et al (36) fed a leaf with labeled metabolites and analyzed the uredospores to determine which nutrients had been taken up. Shaw and Samborski (41) and Staples and Ledbetter (42) first tried autoradiographic methods to elucidate nutrient uptake. All methods have pitfalls and the three following approaches will be discussed: analysis of uptake during axenic culture, feeding the host with radiolabeled metabolites and