The vacuole as central element of the lytic system and sink for lipid droplets in maturing appressoria of Magnaporthe grisea

Abstract

Histochemical and ultrastructural studies were carried out on a wild-type strain (Guy11) and a melanin-deficient mutant (buf1) of the rice-blast pathogen, Magnaporthe grisea (= Pyricularia oryzae), in order to investigate the destination of lipid storage reserves during appressorium development. Lipid droplets were abundant in conidia and were mobilised upon germination, accumulating in the appressorial hook which developed at the tip of each germ tube. Following the formation of a septum at the base of the nascent appressorium, one or a few closely appressed central vacuoles became established and were observed to enlarge in the course of appressorium maturation. On unyielding artificial surfaces such as glass or plastic, appressoria matured to completion within 36-48 h, by which time the enlarged vacuole filled most of the inside volume of the appressorium. Light and transmission electron microscopical observations revealed that the lipid droplets entered the vacuole by autophagocytosis and were degraded therein. Histochemical approaches confirmed the vacuole as the key lytic element in maturing appressoria. Endocytosis of a vital dye, Neutral Red, progressed via endosomes which migrated into the vacuole and lysed there, releasing their dye content into the vacuolar lumen. Furthermore, activity of the lysosomal marker enzyme, acid phosphomonoesterase, was strongly localised in the vacuole at all stages of appressorium maturation. It is therefore envisaged that vacuoles are involved in the degradation of lipid storage reserves which may act as sources of energy and/or osmotically active metabolites such as glycerol, which generate the very high turgor pressure known to be crucial for penetration of hard surfaces. On softer surfaces such as onion epidermis, appressoria of M. grisea were able to penetrate before degradation of lipid droplets had been completed.