CHOLINE - ITS ROLE IN THE GROWTH OF FILAMENTOUS FUNGI AND THE REGULATION OF MYCELIAL MORPHOLOGY

Choline is an essential metabolite for the growth of filamentous fungi . It occurs most notably as a component of the major membrane phosphol ipid, phosphatidyl choline (lecithin), and fulfills a major role in su lphate metabolism in the form of choline-o-sulphate in many species. C holine is usually synthesised endogenously, but exogenous choline can also be taken up, either to compensate for metabolic deficiencies in c holine-requiring mutants such as those of Aspergillus nidulans and Neu rospora crassa, or as a normal function by species such as Fusarium gr aminearum which do not require added choline for growth. E graminearum has a highly specific constitutive uptake system for this purpose. Re cent studies have begun to indicate that choline also plays an importa nt role in hyphal and mycelial morphology. Over a wide range of concen trations, choline influences mycelial morphology, apparently by contro lling branch initiation. At high concentrations of added choline, bran ching is inhibited but specific growth rate is unaffected, leading to the production of rapidly extending, sparsely branched mycelia. Reduct ion of choline concentration allows a progressive increase in branchin g. Additionally, in choline-requiring mutants which have a very reduce d content of choline, multiple tip-formation and apical branching occu rs. Just prior to cessation of growth in choline-starved cultures of A . nidulans choline-requiring mutants, hyphal morphology changes due to a brief phase of unpolarised growth to produce spherical swellings ca lled balloons, at or near hyphal apices. The precise mechanism by whic h choline affects fungal morphology is not yet known, although in A. n idulans it appears to be at least partially due to the influence of me mbrane composition on the synthesis of the hyphal wall polymer chitin. Several hypotheses for the possible mode of action of choline in affe cting fungal morphology are discussed here.