Possible role of ionic gradients in the apical growth of Neurospora crassa

The effects of the Ca2+/H+ exchanger A23187 and the K+/H+ exchanger nigericin on the growth of Neurospora crassa were analyzed. Both ionophores had the same effects on the fungus. They both inhibited growth in liquid media, apical extension being more affected than protein synthesis. A sudden challenge to either ionophore on solid media rapidly stopped hyphal extension. Additionally, both ionophores induced profuse mycelium branching and upward hyphal growth. Hyphae growing on nigericin-containing media also burst at the apex. Both ionophores caused a rapid inhibition in the apically-occurring synthesis of structural wall polysaccharides, but they did not affect mitochondrial energy conservation. With the use of DiBAC, a membrane-potential sensitive fluorophore, it was excluded that their effects were due to depletion of the plasma membrane potential. Considering that both ionophores exchange H+ for different metallic ions, we concluded that their effect was due to dissipation of a proton gradient, which is directly or indirectly involved in the apical growth of the fungus.