Summary: The role of Ca in the maintenance of apical dominance in was investigated. In the presence of the calcium-channel blocker verapamil (1 m), 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 such as “frost” and “spray”. In verapamil-treated hyphae, unlike untreated controls, Ca was not observed in hyphal tips by fluorescence microscopy and the exaggerated branching pattern could be corrected by the addition of 10 m-Ca. 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 Ca in hyphal tips. Exogenously added Ca (50–500 m) almost completely corrected the abnormal branching seen in these mutants, converting them to an essentially wild-type appearance. These observations suggest that low Ca levels, induced in the wild-type by verapamil, and constitutive in the mutants, are responsible for the abnormal branching patterns.


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