In filamentous fungi such as Ashbya gossypii, the growth of hyphae can be remarkably rapid, making these organisms a good model system for the study of polarised cell growth. Similar to budding in yeast, hyphal expansion in A. gossypii is driven by polarised exocytosis at the hyphal tip, but how is growth rate controlled? On , Peter Philippsen and colleagues use light and electron microscopy to compare the exocytic machinery in fast- and slow-growing hyphae. The authors show that exocyst components and polarity proteins form a cortical cap at the tips of slow-growing hyphae, and that this defines the zone of vesicle secretion. In fast-growing hyphae, the exocytic zone is only slightly expanded; notably, however, these hyphae contain a Spitzenkörper – a complex multivesicular structure that is found at hyphal tips in several fungi. Moreover, the Spitzenkörper colocalises with several exocyst and polarisome components. The authors next show that actin patches – thought to be sites of endocytosis – are excluded from the exocytic zone, and that the area of exclusion is greater in faster-growing hyphae. On the basis of these data, the authors identify several requirements for fast hyphal extension. Their findings enhance our understanding of polarised exocytosis and cell growth.
