Pseudohyphal growth is induced in Saccharomyces cerevisiae by a combination of stress and cAMP signalling

In Saccharomyces cerevisiae pseudohyphae formation may be triggered by nitr ogen deprivation and is stimulated by cAMP. It was observed that even in a medium with an adequate nitrogen supply, cAMP can induce pseudohyphal growt h when S. cerevisiae uses ethanol as carbon source. This led us to investig ate the effects of the carbon source and of a variety of stresses on yeast morphology. Pseudohyphae formation and invasive growth were observed in a r ich medium (YP) with poor carbon sources such as lactate or ethanol. Extern al cAMP was required for the morphogenetic transition in one genetic backgr ound, but was dispensable in strain Sigma 1278b which has been shown to hav e an overactive Ras2/cAMP pathway. Pseudohyphal growth and invasiveness als o took place in YPD plates when the yeast was subjected to different stress es: a mild heat-stress (37 degreesC), an osmotic stress (1 m NACl), or addi tion of compounds which affect the lipid bilayer organization of the cell m embrane (aliphatic alcohols at 2%) or alter the glucan structure of the cel l wall (Congo red). We conclude that pseudohyphal growth is a physiological response not only to starvation but also to a stressful environment; it ap pears to require the coordinate action of a MAP kinase cascade and a cAMP-d ependent pathway.