MOLECULAR RECOGNITION IN THE HOMING RESPONSES OF ZOOSPORIC FUNGI, WITH SPECIAL REFERENCE TO PYTHIUM AND PHYTOPHTHORA

Recent studies on the homing responses (host location, substrate locat ion) of zoosporic fungi are reviewed, with emphasis on the molecular s ignals that influence root-infecting Pythium and Phytophthora spp. but including other zoosporic parasites and saprobes. The homing sequence involves zoospore taxis, encystment, cyst adhesion, germination and g erm-tube tropism. It is mediated by recognition of chemical diffusates and surface components of the host or substrate. The elicitors differ between fungi and at different stages of the sequence, and can act sy nergistically or antagonistically, giving the potential for ecological variation, including host- and substrate adaptation. Induction of enc ystment is a key stage in the homing sequence. It can lead to adhesion , germination and host penetration by an autonomous calcium-mediated c ascade. Oomycete zoospores orientate during encystment, apparently by interaction of flagella with host surface components, so that the fixe d site of germ-tube outgrowth lies next to the host. Adhesive glycopro tein is released and interacts with Ca2+, also released during encystm ent. Then reabsorption of Ca2+ apparently triggers germination, synerg ized by specific organic compounds in host exudates. Calcium also medi ates the motility pattern of oomycete zoospores and induction of encys tment. Other elements of the signal transduction pathway that links th e homing sequence to molecular recognition by surface receptors are be ginning to be identified.