Ca2+Gradients in Hyphae and Branches ofSaprolegnia ferax

Abstract

Hyphae ofSaprolegnia feraxmaintain a tip-high gradient of cytoplasmic Ca2+which is essential to tip growth. The gradient extends further along the periphery than the center of the hyphae, consistent with an extended region of Ca2+influx. Both cytoplasmic [Ca2+] and the gradient's slope are higher in faster growing hyphae, indicating greater exocytosis or apical extensibility in response to elevated [Ca2+]. Conversely, UV-induced elevation of apical [Ca2+] to levels little above those of faster growing hyphae transiently halted growth, showing that the relationship between growth and apical [Ca2+] is applicable within a narrow range. UV elevated [Ca2+] concentrated in central cytoplasm, a putative storage region, consistent with maintenance of observed gradients formed without exogenous Ca2+from intracellular stores. Branch formation did not show elevated [Ca2+] levels in the subtending hyphae, but a gradient formed as the branch developed, indicating either an unknown initiator or subdetectable [Ca2+] initiation concentrations which are less than those needed for tip maintenance.