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 location) of zoosporic fungi are reviewed, with emphasis on the molecular signals 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 germ-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 synergistically or antagonistically, giving the potential for ecological variation, including host- and substrate adaptation. Induction of encystment is a key stage in the homing sequence. It can lead to adhesion, germination and host penetration by an autonomous calcium-mediated cascade. Oomycete zoospores orientate during encystment, apparently by interaction of flagella with host surface components, so that the fixed site of germ-tube outgrowth lies next to the host. Adhesive glycoprotein is released and interacts with Ca2+, also released during encystment. Then reabsorption of Ca2+ apparently triggers germination, synergized by specific organic compounds in host exudates. Calcium also mediates the motility pattern of oomycete zoospores and induction of encystment. Other elements of the signal transduction pathway that links the homing sequence to molecular recognition by surface receptors are beginning to be identified.