Direct evidence for Ca2+ regulation of hyphal branch induction

Irradiation of growing hyphae of Saprolegnia ferax with microbeams of UV (300-380 and 385-450 nm) light induced an increase in cytoplasmic [Ca2+] followed by precocious formation of one or more branches within about 4 min, The distribution of branches was strongly skewed toward the subapical side of the irradiation site, but otherwise was apparantly random. Apical (10-mu m) irradiations were more effective than subapical (50-mu m) ones in that they induced branches at comparable frequencies but with lower doses, consistent with higher concentrations of putative target intracellular Ca2+ storage structures in this region, Once formed, induced adjacent branches seem to compete for ''resources,'' with those closer than similar to 50 mu m inhibiting each other, The results are most consistent with Ca2+-induced accumulation of branch initiating factors being the cause, not the consequence, of branch formation, thus supporting a primary role for Ca2+ in regulation of hyphal tip growth. (C) 1997 Academic Press.