Using fimbrin to quantify the endocytic subapical collar during polarized growth in three filamentous fungi.

Filamentous fungi produce specialized cells called hyphae. These cells grow by polarized extension at their apex, which is maintained by the balance of endocytosis and exocytosis at the apex. Although endocytosis has been well characterized in other organisms, the details of endocytosis and its role in maintaining polarity during hyphal growth in filamentous fungi is comparatively sparsely studied. In recent years, a concentrated region of protein activity that trails the growing apex of hyphal cells has been discovered. This region, dubbed the "endocytic collar" (EC), is a dynamic 3-dimensional region of concentrated endocytic activity, the disruption of which results in the loss of hyphal polarity. Here, fluorescent protein-tagged fimbrin was used as a marker to map the collar during growth of hyphae in three fungi: Aspergillus nidulans, Colletotrichum graminicola, and Neurospora crassa. Advanced microscopy techniques and novel quantification strategies were then utilized to quantify the spatiotemporal localization and recovery rates of fimbrin in the EC during hyphal growth. Correlating these variables with hyphal growth rate revealed that the strongest observed relationship with hyphal growth is the distance by which the EC trails the apex, and that measured endocytic rate does not correlate strongly with hyphal growth rate. This supports the hypothesis that endocytic influence on hyphal growth rate is better explained by spatiotemporal regulation of the EC than by the raw rate of endocytosis.

The conidial coin toss: A polarized conidial adhesive in Colletotrichum graminicola.

Colletotrichum graminicola is an economically significant fungal pathogen of maize. The primary infective conidia of the fungus, falcate conidia, are splash-dispersed during rain events. The adhesion of the falcate conidia triggers germination and is required for the development of infection structures. Falcate conidia are capable of immediate adhesion upon encountering the substrate. We report that rapid adhesion in C. graminicola is polarized, with a single-sided strip of adhesive material running the length of a single side (or face) of the conidium between the tips. This strip of adhesive is co-localized with dynamic transverse actin cables, and both the adhesive strip and actin cables are formed after liberation of the conidium from its conidiogenous cell but prior to adhesion to the infection court. Orientation of conidia upon contact with substrate determines whether they will rapidly adhere, and those which do not initially adhere can be induced to do so by applying force to reorient or "flip" the conidia. We propose that C. graminicola possesses an adhesive mechanism resulting in an adhesion efficiency of approximately 50% upon initial contact with substrata, and that an increase in adhesion efficiency can be induced by disturbance.