The Bursting Tendency of Hyphal Tips of Fungi: Presumptive Evidence for a Delicate Balance between Wall Synthesis and Wall Lysis in Apical Growth

Abstract

tips of Mucor rouxii. Most hyphal tips from colonies grown on full-strength agar medium burst readily when flooded with distilled water. In contrast, hyphal tips from colonies grown on diluted medium survived flooding with distilled water but succumbed to dilute aqueous solutions (particularly acids, but also neutral salts, EDTA, alcohols, acetone, detergents). Apex bursting was generally inhibited by alkaline solutions but took place at certain concentrations of ethanolamine or NH,OH. Mg2+, Mn2+ or Ca2f caused pronounced swelling of the hyphal apex fol- lowed by an occasional burst. Sharp temperature increases also brought about apical disintegration regardless of nutrient concentration. Colonies of Aspergillus fiiniigatiis, Fusariuiar sp., Neurospora crassa (wild-type A wide diversity of treatments can cause rapid and extensive bursting of hyphal 487 and two morphological mutants), Penicilliuna clavifornze, Rhizoctonia solani and Rhizopus arrhizus also underwent apical bursting when flooded with water or rapidly heated. Schizophyllunz cow?iune, several Phytophthora spp., and some morphological mutants of N. crassn were refractory. The bursting of hyphal tips by flooding colonies with water is not simply an osmotic phenomenon; its temperature coefficient (Qlo 1-3 to 2.0) suggests the additional participation of biochemical reaction(s) as a rate-limiting step in the bursting process. Probably, enzymatic degradation and hence weakening of the wall is a prerequisite for bursting. These observations are offered as circunistantial evi- dence supporting the following conclusions: (i) the growing tips of fungi have a large wall lytic potential; (ii) the release of this activity during growth must be gradual and delicately co-ordinated with wall synthesis; (iii) this balance can be easily disturbed by a wide variety of external stimuli and the ensuing surge of lytic activity results in the violent disintegration of the hyphal apex.