Behaviour of zoospores and zoospore cysts in relation to root infection by Pythium aphanidermatum

Abstract

The pre-penetration sequence involving zoospore taxis, encystment, cyst germination and germ-tube orientation was studied by video microscopy when roots of wheat, cress or tomato were placed in suspensions of zoospores of Pythium aphanidermatum (Edson) Fitzp. The sequence was also studied in vitro, to identify the responsible factors.Several amino acids, sugars or volatile compounds elicited one of more stages of the sequence, but only glutamic acid and aspartic acid elicited all stages and thus could account for the observed behaviour on roots. Polyuronates of root surface slime also may be needed for encystment and orientation of spores, because germ-tubes emerged towards roots from a fixed point near the former position of the water-expulsion vacuole. However, identical patterns of zoospore accumulation and encystment on untreated roots and roots encased in alginate gel indicated that neither polyuronates nor specific sugar residues of root slime caused localized encystment on roots.Based on these results, a 'simplest case' model of the pre-penetration sequence on roots is proposed.

Role of calcium in adhesion and germination of zoospore cysts of Pythium: a model to explain infection of host plants

“…Pythium aphanidermatum (Edson) Fitzp. (isolate CBS 634.70) was used for most experiments, as in earlier work (Mitchell & Deacon, 1986a, 6, c ;Jones et al, 1991). The cysts germinate in response to L-asparagine, L-glutamic acid or Laspartic acid but not other amino acids.…”

“…Treatments were applied as detailed below ; all supplements were adjusted to pH 7 with NaOH before use. Germination was assessed after 1.5 h, using a Leitz Orthoplan microscope with a video camera attachment (Jones et al, 1991). Cells were scored as germinated when the germ-tube length at least equalled the cyst diameter (approx.…”

“…These findings suggest that Ca2+ could provide the link in a possible cascade initiated by docking. In order to establish this, it is necessary to show a direct relationship between calcium, adhesion and germination, and also, ideally, to account for the roles of other factors such as root-derived amino acids which influence the infection sequence (Jones et al, 1991). We have now done this for Pythiurn spp.…”

“…Several stages of this sequence can be simulated in vitro and some of the potential host-derived triggers or elicitors have been identified (e.g. Hinch & Clarke, 1980;Mitchell & Deacon, 19863, c ;Longman & Callow, 1987;Jones et al, 1991). However, it is unclear whether the stages of the sequence are necessarily under separate control or are parts of a cascade.…”

“…are attracted to roots by chemotaxis and then undergo a developmental sequence leading to infection (Deacon, 1988). They search the root surface for a suitable site, then dock, encyst and adhere with precise orientation, so that the germ-tube emerges from a pre-determined point next to the host and immediately initiates penetration (Mitchell & Deacon 1986a; Jones et al, 1991). Several stages of this sequence can be simulated in vitro and some of the potential host-derived triggers or elicitors have been identified (e.g.…”