PULSED GROWTH OF FUNGAL HYPHAL TIPS

Somatic fungal hyphae are generally assumed to elongate at steady line ar rates when grown under constant environmental conditions with ample nutrients. However, patterns of pulsed hyphal elongation were detecte d during apparent steady growth of hyphal tips in fungi from several m ajor taxonomic groups (Oomycetes, Psythium aphanidermatum and Saproleg nia ferax; Zygomycetes, Gilbertella persicaria; Deuteromycetes, Tricho derma viride; Ascomycetes, Neurospora crassa and Fusarium culmorum; Ba sidiomycetes, Rhizoctonia solani). Growing hyphal tips were recorded w ith video-enhanced phase-contrast microscopy at high magnification, an d digital images were measured at very short time intervals (1-5 s). I n all fungi tested, the hyphal elongation rate was never perfectly ste ady but fluctuated continuously with alternating periods of fast and s low growth at more or less regular intervals. Pulsed growth was observ ed in fungi differing in cell diameter, overall growth rate, taxonomic position, and presence and pattern of Spitzenkorper organization, sug gesting that this is a general phenomenon. Frequency and amplitude of the pulses varied among the test organisms. T. viride and N. crassa sh owed the most frequent pulses (average of 13-14 per min), and F. culmo rum the least frequent (2.7 per min). Average pulse amplitude varied f rom 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 co rrelated with the merger of satellite Spitzenkorper with the main Spit zenkorper. These findings are consistent with a causal relationship be tween fluctuations in the overall rate of secretory vesicle delivery/d ischarge at the hyphal apex and the fluctuations in hyphal elongation rate.