At Right: Figure adapted from Lee and Shaw 2008 showing conidium germination of wild type (top row) and the arfB::Tn developmental mutant. This mutant is defective in endocytosis and is characterized by extended isotropic growth and multiple simultaneous germ tube emergence upon germination.  The white arrows indicate septation sites, the black arrow head indicates a septum, the black arrows indicates simultaneous septation sites for arfB::Tn.  Scale bar = 20 µm.

At Right. Figure adapted from Upadhyay and Shaw, 2008 describing the Apical Recycling Model for hyphal growth.

Model for role of endocytosis in hyphal growth.  a) Initial isotropic growth of the conidium upon breaking dormancy is the result of randomly distributed exocytotic vesicles (red circles). b) A hypothesized polarity marker (light blue oval) is placed at the incipient germination site.  c) Upon reaching a critical concentration, the polarity marker organizes exocytosis to the incipient germ tube emergence site.  The exocytotic vesicles organize by an as yet unknown mechanism into the spitzenkorper (organized red circles).  d) Growth is now directional (arrow) as exocytosis is directed toward the polarity marker.  e) As new membrane and polarity marker is deposited at the tip, older marker (progressively darker ovals) matures away from the tip.  If not controlled, this would result in the loss of hyphal directionality.  f)  Endocytosis, mediated by FimA assembled cortical actin patches (green ovals), occurs in a zone 0.5 um subapical to the growing tip.  This harvests membrane and polarity marker from mature membrane and recycles it through the spitzenkorper back to the growing hyphal apex.  Therefore endocytosis precisely maintains the deposition of the cortical marker to the growing tip. g-h) Initial isotropic growth of the conidium upon breaking dormancy follows a pattern similar to wild type. Inefficient or absent endocytosis leads to random deposition of the cortical marker as continued isotropic growth displaces the marker from a controlled growth site (older polarity marker is progressively darker blue to black).  j,k)  The polarity marker randomly accumulates to sufficient levels to support germ tube emergence from random sites.  Hyphal growth is maintained, though hyphae are lobed and swollen, through low levels of endocytosis maintained by the leaky fimA::Tn mutant or by redundancy with other actin binding proteins.