investigation of the effects of Ca2+ channel inhibitors on branching and chemotropism in the oomycete Achlya bisexualis: Support for a role for Ca2+ in apical dominance

In an attempt to better understand branching and chemotropism, we describe the effects of Ca2+ channel inhibitors on these processes in Achlya bisexualis, using a branch induction technique and whole plate assays. Branching appears to be a two step process with the initial formation of a bump from which a branch emerges. Verapamil increased numbers of branches in whole plate assays and decreased the distance from the first branch to the tip. In induction assays verapamil increased the number of bumps formed, although in some hyphae it inhibited the transition from an initial bump to a branch. When a branch formed it did not affect the time taken to branch. It had no effect on chemotropism. Lanthanum (La3+) and gadolinium (Gd3+) also increased branching in whole plate assays but their effect was much less marked and they had no effect on bump/branch number in induction assays. Gd3+ decreased the time taken to branch. Both La3+ and Gd3+ increased chemotropism. These data suggest firstly that the respective inhibitors may affect different parts of the branching process and secondly that Ca2+ influx through channels may not be a requirement for branching, indeed such movements may suppress branching. This would fit with elevated Ca2+ at the tip playing a role in apical dominance.