Electron microscope autoradiography of14C photosynthate distribution at the haustorium-host interface in powdery mildew ofPisum sativum

Abstract

Haustorial complexes were isolated from leaves ofPisum sativum infected withErysiphe pisi and exposed to14CO2 for 2 hours. The constituents of the isolated fraction were quantified and ultrastructurally described and the distribution of14C studied by electron microscope autoradiography and statistical treatment. Most (86%) of the isotope in the fraction was associated with haustorial complexes. Three classes of haustorial complexes were distinguished by degree of labelling and ultrastructure. Most of the haustorial complexes were termed healthy (i.e., they showed a normal ultrastructure) and were heavily labelled; necrotic complexes were unlabelled; and a class with intermediate labelling, modified extrahaustorial membrane and usually a normal haustorial cytoplasm was termed pre-necrotic. In healthy haustorial complexes the haustorial lobes and extrahaustorial membrane showed the highest grain densities and the body and extrahaustorial matrix were also significantly labelled. Comparison of the results suggest that ultrastructural modifications leading to necrosis were caused by dehydration which in turn determined reduction in photosynthate transfer. Other factors influencing transport into haustoria and the status of the extrahaustorial matrix are also discussed.