The Colletotrichum trifolii – Medicago sativa interface, in culture: a cytological analysis

Abstract

Light and electron microscopy were used to investigate the interactions between the fungus Colletotrichum trifolii Bain and Essary and callus cultures derived from anthracnose resistant or susceptible alfalfa plants (Medicago sativa L.). Race 1 and race 2 of the pathogen completely overgrew both callus types at the same rate of colonization. Comparisons with the responses observed in planta showed some similarities. In both cases the plant cells proximal to the approaching pathogen underwent vacuolar fragmentation and secreted extracellular materials that were histochemically similar when stained with Toluidine blue O and Sudan black B and that appeared to be lignin- or suberin-like in composition. However, in cultures these responses took place in many cells and began while the hyphal front was 0.5 to 1.0 cm distant from the callus edge. In the intact plants the responses were localized initially to one or two epidermal cells and did not begin until the pathogen had penetrated into the cuticle. Further differences were found in the failure of callus cells to form papillae and in their premature death, prior to completion of the defense response and prior to fungal contact. The observations suggested that perhaps the absence of a cuticle in callus tissue contributes to the development of an uncontrolled defence response in which subsequent permeability barriers are incompletely formed. Such factors could contribute to the observed failure of race-specific resistance. Key words: Colletotrichum trifolii, Medicago sativa, callus cultures, cytology.