As in most filamentous fungi, growth of Neurospora crassa begins by highly polarized cell extension that leads to the formation of a hyphal tube of uniform diameter. The continuously extending tip sends off periodic branches, which are themselves, capable of extension and branching. There are few other biological systems that develop in this manner. New branches normally emerge a short distance behind the apex of the extending tip and grow at an angle to the main hypha and are referred to as 'lateral' branches. Under normal conditions the majority of branch points form laterally, but alternate branch morphology is seen at a minority of branch points. In this minority morphology, the two tips emerge at an angle to the previous direction of growth forming a forked structure. These are referred to as 'dichotomous' or 'apical' branch points. In this project, the genetics of control of branch morphology will be examined. The project will focus on the lateral/dichotomous dimorphism observed in Neurospora crassa.
The project is expected to advance along two independent fronts. First, the collection of morphological mutations will be expanded by screening for suppressors of a select group of known morphological mutations. In this phase of the project, the focus will be on mutations that cause a shift in favor of dichotomous branching. This is expected to uncover suppressors which act on specific branching mutants, and hopefully will also recover those which are capable of at least partially suppressing the branching effect on a wider selection of morphological mutations, acting, in effect, as low-branching mutants. Such mutations are largely unknown in Neurospora. Second, the morphological effects of cold shock will be further examined. In Neurospora, temperature shifts (25 to 4) have been shown to cause a dramatic, but temporary branching effect. One phase of the response is characterized by a switch in morphology to all dichotomous branching. By screening for mutants with altered responses to cold shock, genes responsible for the developmental switch between branch morphologies should be identified.
Filamentous fungi play a large role in human affairs. They are major recyclers of biomass and a significant source of losses in agriculture and lumber production as pathogens and agents of spoilage. They affect both producers (loss of product, cost of antifungal actions) and consumers (increased costs, food spoilage, decomposition of wood products) and thus have a significant economic impact. An increase in our understanding of tip growth and branching in fungi would prove useful for more direct control of the growth of the organism. The project should prove useful to batch culturing technologies, control of fungi in material subjected to temperature shifts, and should expand our understanding of the growth of filamentous fungi in the field.
