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 the 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 hyphal apices imaged by video-enhanced phase-contrast microscopy. In addition 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 plaques 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, the main Spitzenkorper temporarily increased in size. Satellites were observed in 14 fungi, most of which had relatively large (5-10 mu m diam.), 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, satellites were present only in growing cells. They were transient and remained visible for 3-8s before merging with the main Spitzenkorper. Within the hyphae, satellites travelled up to six times faster than the average cell elongation rate. Multiple satellites some times occurred simultaneously; up to three were seen within a hyphal apex at the same time. 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 apical vesicles surrounding a group of microvesicles located next to the plasma membrane. The identification and behavior of the satellites represent clear evidence of directional mass transport of vesicles toward the hyphal apex. Our observations indicate that satellites are a common phenomenon in growing hyphal apices of septate fungi and that they contribute to growth of the hyphal apex.