What determines growth direction in fungal hyphae?

We used high-resolution video microscopy and image analysis to map the trajectory of the Spitzenkorper in growing hyphae of Neurospora crassa and to correlate it with growth directionality. The Spitzenkorper followed a tortuous trajectory produced by a dominant forward motion accompanied by frequent, transverse oscillations. In hyphae with a fixed growth direction, the regression line of the Spitzenkorper trajectory corresponded to the longitudinal axis of the hypha, A permanent change in growth direction, i.e., the establishment of a new growth axis, was correlated with a sustained shift in Spitzenkorper trajectory away from the existing cell axis. In meandering hyphae, changes in growth directionality occurred somewhat erratically but there was a strong compensatory tendency reversing directional shifts and maintaining an overall fixed direction of growth, Although external factors greatly affect hyphal growth direction (tropisms), they are probably not the primary determinants of growth directionality. Inhibitors of microtubules, but not of actin microfilaments, caused hyphae to lose their growth directionality-providing support for the idea that Spitzenkorper trajectory is determined internally by a growing scaffolding of cytoplasmic microtubules, The meandering morphology of N, crassa hyphae was duplicated by computer simulation in support of the idea that hyphal morphogenesis is controlled by the position of the Spitzenkorper functioning as a vesicle supply center. (C) 1998 Academic Press.