PULSED GROWTH OF FUNGAL HYPHAL TIPS

Somatic fungal hyphae are generally assumed to elongate at steady linear rates when grown under constant environmental conditions with ample nutrients. However, patterns of pulsed hyphal elongation were detected during apparent steady growth of hyphal tips in fungi from several major taxonomic groups (Oomycetes, Psythium aphanidermatum and Saprolegnia ferax; Zygomycetes, Gilbertella persicaria; Deuteromycetes, Trichoderma viride; Ascomycetes, Neurospora crassa and Fusarium culmorum; Basidiomycetes, Rhizoctonia solani). Growing hyphal tips were recorded with video-enhanced phase-contrast microscopy at high magnification, and digital images were measured at very short time intervals (1-5 s). In all fungi tested, the hyphal elongation rate was never perfectly steady but fluctuated continuously with alternating periods of fast and slow growth at more or less regular intervals. Pulsed growth was observed in fungi differing in cell diameter, overall growth rate, taxonomic position, and presence and pattern of Spitzenkorper organization, suggesting that this is a general phenomenon. Frequency and amplitude of the pulses varied among the test organisms. T. viride and N. crassa showed the most frequent pulses (average of 13-14 per min), and F. culmorum the least frequent (2.7 per min). Average pulse amplitude varied from 0.012 mu m/s for F. culmorum to 0.068 mu m/s for G. persicaria. In F. culmorum and T. viride, the fast phase of the growth pulses was correlated with the merger of satellite Spitzenkorper with the main Spitzenkorper. These findings are consistent with a causal relationship between fluctuations in the overall rate of secretory vesicle delivery/discharge at the hyphal apex and the fluctuations in hyphal elongation rate.