Yeast Thigmotropism Controlled by a Cell-Surface Mucin and a MAP-Kinase Pathway

Budding yeast can switch from growth as ovoid cells to growth in an adhesive and invasive filamentous form. Limiting availability of reduced nitrogen is thought to be the signal that induces filamentous growth. In contrast, we find that filamentation is a thigmotropic response. In the absence of repressive agents, growth on a surface is necessary and sufficient to induce filamentation, whereas nitrogen limitation is neither sufficient nor necessary. The thigmostimulus is transmitted to the genome via a MAP-kinase pathway that has been linked to filamentation. The FLO11 gene, encoding a cell-surface mucin protein, maps genetically at the top and at the bottom of this thigmotropism MAP-kinase pathway. These results suggest that yeast differentiation to the filamentous form is primarily a response to surface contact for the purposes of colonization and invasion, rather than foraging for nutrients. Gene expression was assayed for yeast cells grown in liquid cultures and on solid substrates with various nutrient compositions. Samples were collected from wild-type and strains harboring mutations of a proposed thigmotropism MAP-kinase pathway to identify the genomic response to thigmostimuli (touch).