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 crass a and to correlate it with growth directionality. The Spitzenkorper fo llowed a tortuous trajectory produced by a dominant forward motion acc ompanied 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, wa s correlated with a sustained shift in Spitzenkorper trajectory away f rom the existing cell axis. In meandering hyphae, changes in growth di rectionality occurred somewhat erratically but there was a strong comp ensatory tendency reversing directional shifts and maintaining an over all fixed direction of growth, Although external factors greatly affec t hyphal growth direction (tropisms), they are probably not the primar y determinants of growth directionality. Inhibitors of microtubules, b ut not of actin microfilaments, caused hyphae to lose their growth dir ectionality-providing support for the idea that Spitzenkorper trajecto ry is determined internally by a growing scaffolding of cytoplasmic mi crotubules, The meandering morphology of N, crassa hyphae was duplicat ed by computer simulation in support of the idea that hyphal morphogen esis is controlled by the position of the Spitzenkorper functioning as a vesicle supply center. (C) 1998 Academic Press.