SATELLITE SPITZENKORPER IN GROWING HYPHAL TIPS

Growing hyphal tips of higher fungi contain an organized assemblage of secretory vesicles and other cell components collectively known as th e Spitzenkorper. Until now, the Spitzenkorper has been portrayed as a single spheroid complex located near the apical cell wall. This study demonstrates the occurrence of multiple Spitzenkorper in growing hypha l apices imaged by video-enhanced phase-contrast microscopy. In additi on to the main Spitzenkorper, smaller satellite Spitzenkorper arise a few micrometers behind the apical pole. Four developmental stages were identified: (a) the satellites first appeared as faint phase-dark pla ques next to the plasma membrane, (b) gradually increased in size and assumed an ovoid profile, (c) they migrated to the hyphal apex, and (d ) finally they merged with the main Spitzenkorper. After the merger, t he main Spitzenkorper temporarily increased in size. Satellites were o bserved in 14 fungi, most of which had relatively large (5-10 mu m dia m.), fast-growing hyphae (2-33 mu m/min elongation rate). The average frequency of in-focus satellites was 7+/min for Fusarium culmorum and 11+/min for Trichoderma viride. As with the main Spitzenkorper, satell ites were present only in growing cells. They were transient and remai ned visible for 3-8s before merging with the main Spitzenkorper. Withi n the hyphae, satellites travelled up to six times faster than the ave rage cell elongation rate. Multiple satellites some times occurred sim ultaneously; up to three were seen within a hyphal apex at the same ti me. Localized cell enlargement occurred next to stationary satellites, suggesting that satellite Spitzenkorper are functional as sources of new cell surface before they reach the main Spitzenkorper; therefore, they account for some variations in the profiles of the growing hyphae . By electron microscopy, satellites consisted of small clusters of ap ical vesicles surrounding a group of microvesicles located next to the plasma membrane. The identification and behavior of the satellites re present clear evidence of directional mass transport of vesicles towar d the hyphal apex. Our observations indicate that satellites are a com mon phenomenon in growing hyphal apices of septate fungi and that they contribute to growth of the hyphal apex.