Control of Branching in Neurospora crassa

The growth and vegetative morphology of filamentous fungi is characterized by two seemingly related activities: Tip growth - the highly polarized extension of hyphal tips, and branching - the process by which new hyphal tips emerge. Tip growth and branching are crucial elements in the colonization and utilization of the organic substrate, keys to the ecological role of this fungus. There appear to be at least two distinct forms of branch formation. One (lateral branching) which results in the new tips which emerge from the side of the tip region of an existing hyphal tube near, but not at, the apex of the tip, and the second (apical branching) in which a growing tip splits to form a pair of tips emerging from what was the apex of the hyphal tip. This review presents a brief description of the tip growth and branching process with a focus on genetic and environmental influences on the control of branching. Evidence that lateral and apical branches are controlled by separate mechanisms will be discussed. An additional focus will be on evidence for a proposed homeostatic system which regulates branch density, including a description of mutants which appear to disrupt the proposed system