UV microirradiations elicit Ca2+-dependent apex-directed cytoplasmic contractions in hyphae

Summary Polarized tip-ward cytoplasmic contractions were induced in hyphae ofSaprolegnia ferax with ultraviolet microirradiations. These unidirectional contractions were similar in appearance and ionic requirements to those previously induced in hyphae ofBasidiobolus magnus, suggesting that the observed inherent cytoplasmic polarity is a general phenomenon. During growth the cytoplasm is continually moving forward with respect to the lateral cell wall and plasma membrane in order to maintain its position in the tip. These contractions may be an exaggerated form of this cytoplasmic migration. F-actin was most concentrated in the contracted cytoplasm, implying that it may be involved in generating the contraction. Contractions were enhanced by external Ca²⁺ and by irradiating the tip region which is rich in Ca²⁺ sequestering organelles, suggesting that flooding of the cytoplasm with Ca²⁺ caused the contractions. H⁺ did not affect contraction frequency. Neither the change in cytoplasmic consistency that preceded contraction, the contraction itself, nor the F-actin damage induced were confined to the microirradiated zone. This is in keeping with irradiation-induced damage to a network under tension or a flux of diffusible ions causing the response. Thus Ca²⁺ may regulate actin-myosin interactions that generate cytoplasmic migration.Abbreviations EGTA ethyleneglycol-bis-(-amino-ethyl ether) N, N-tetra-acetic acid - F-actin filamentous actin - PIPES piperazine-N N-bis-(2-ethanesulfonic acid) - wavelength