DIFFERENTIAL ENCYSTMENT OF ZOOSPORES OF PYTHIUM SPECIES BY SACCHARIDES IN RELATION TO ESTABLISHMENT ON ROOTS

Localized encystment of fungal zoospores on roots has been reported to be mediated by specific saccharide residues of root surface mucilage. This was investigated by coating wheat roots with calcium alginate gel to block potential recognition and by study of encystment responses to polysaccharides in vitro . Gel coatings did not alter the encystment pattern of Pythium aphanidermatum on wheat roots, and attempts to block encystment on gelled or ungelled roots with α-L-fucosidase or lectins that bind D-glucose, D-mannose, L-fucose, N-acetyl-D-glucosamine or N -acetyl-D-galactosamine were unsuccessful. This supports an alternative hypothesis, that encystment can be mediated by polyuronates at the root surface and is localized by release of zoospore attractants. Consistent with this, encystment on ungelled roots was markedly reduced by treatment with methylene blue or India ink, and somewhat reduced by ruthenium red. In vitro, P. aphanidermatum, P. catenulatum and P. dissotocum were encysted by sodium alginate, poly-D-galacturonic acid, gum arabic and wheat root mucilage, but D-galacturonic acid and D-glucuronic acid were less effective. There was no common response of the fungi to cellulose, chitin, tomato root mucilage or various defined polysaccharides, but most of these encysted one fungus or another. Only P. catenulatum was encysted by fucosylated xyloglucan or fucoidan, only P. dissotocum by a non-fucosylated xyloglucan, and only P. aphanidermatum by an arabinoxylan. Such fungus-specific effects might be related to host-parasite specialization. © 1993 Academic Press, Inc.