CALCIUM DEFICIENCIES AND APICAL HYPERBRANCHING IN WILD-TYPE AND THE FROST AND SPRAY MORPHOLOGICAL MUTANTS OF NEUROSPORA-CRASSA

The role of Ca2+ in the maintenance of apical dominance in Neurospora crassa was investigated. In the presence of the calcium-channel blocker verapamil (1 mM), wild-type hyphal tips demonstrated enhanced branching which led to a fan-like pattern of growth, similar to that seen in certain of the morphological mutants of N. crassa such as 'frost' and 'spray'. In verapamil-treated hyphae, unlike untreated controls, Ca2+ was not observed in hyphal tips by fluorescence microscopy and the exaggerated branching pattern could be corrected by the addition of 10 mM-Ca2+. Studies using the morphological mutants 'frost' and 'spray', which grow typically on minimal medium with a branching pattern quite similar to verapamil-treated wild-type, also failed to demonstrate Ca2+ in hyphal tips. Exogenously added Ca2+ (50-500 mM) almost completely corrected the abnormal branching seen in these mutants, converting them to an essentially wild-type appearance. These observations suggest that low Ca2+ levels, induced in the wild-type by verapamil, and constitutive in the mutants, are responsible for the abnormal branching patterns.