Influence of Oxygen on Tip Growth ofPilobolus CrystallinusHyphae

Abstract

Radial expansion of Pilobolus crystallinus colonies was inhibited by oxygen. The upper limit of p02 for radial growth was estimated at less than 30 mm Hg. At partial pressures of oxygen greater than 30 mm Hg the fungus only grew beneath the substrate surface and under an oxygen atmosphere (p02 = 760 mm Hg) growth ceased until respiration created an area of low p02 (<30 mm Hg) at the hyphal tips. Growth as measured by dry weight changes, however, was favored by aeration. Oxygen inhibits extension of hyphal tips which in turn effects vertical organization of the colony, but this fungus probably is not microaerophilic. Colonies of filamentous fungi normally expand linearly because essentially all growth occurs at the hyphal tips. Contributing to this tip growth is a zone of intense metabolic activity, termed the peripheral growth zone (Trinci, 1971; Trinci and Saunders, 1977). As the peripheral growth zone widens, branches are initiated behind the tip. This tends to maintain a fixed peripheral growth zone width, and thereby linear growth of the leading hyphae. The obvious exception to the rule of linear growth is deceleration of growth due to depletion of nutrients or accu? mulation of inhibitory metabolites. I have noted that radial growth of a Pilobolus crystallinus (Wiggers) Tode colony frequently is not linear but in fact may accelerate as the colony develops. The underlying mechanism of this accelerating expansion is not immediately obvious but the implication is that fungal activity creates an environment that progressively favors radial growth. In this paper I present evidence that depletion of oxygen in the peripheral growth zone is largely responsible for the observed non-linear growth. MATERIALS AND METHODS