<div><h3>Background</h3><p>The model cellulolytic fungus <em>Trichoderma reesei</em> (teleomorph <em>Hypocrea jecorina</em>) is capable of responding to environmental cues to compete for nutrients in its natural saprophytic habitat despite its genome encodes fewer degradative enzymes. Efficient signalling pathways in perception and interpretation of environmental signals are indispensable in this process. Ras GTPases represent a kind of critical signal proteins involved in signal transduction and regulation of gene expression. In <em>T. reesei</em> the genome contains two Ras subfamily small GTPases TrRas1 and TrRas2 homologous to Ras1 and Ras2 from <em>S. cerevisiae</em>, but their functions remain unknown.</p> <h3>Methodology/Principal Findings</h3><p>Here, we have investigated the roles of GTPases TrRas1 and TrRas2 during fungal morphogenesis and cellulase gene expression. We show that both TrRas1 and TrRas2 play important roles in some cellular processes such as polarized apical growth, hyphal branch formation, sporulation and cAMP level adjustment, while TrRas1 is more dominant in these processes. Strikingly, we find that TrRas2 is involved in modulation of cellulase gene expression. Deletion of <em>TrRas2</em> results in considerably decreased transcription of cellulolytic genes upon growth on cellulose. Although the strain carrying a constitutively activated <em>TrRas2<sup>G16V</sup></em> allele exhibits increased cellulase gene transcription, the <em>cbh1</em> and <em>cbh2</em> expression in this mutant still strictly depends on cellulose, indicating TrRas2 does not directly mediate the transmission of the cellulose signal. In addition, our data suggest that the effect of TrRas2 on cellulase gene is exerted through regulation of transcript abundance of cellulase transcription factors such as Xyr1, but the influence is independent of cAMP signalling pathway.</p> <h3>Conclusions/Significance</h3><p>Together, these findings elucidate the functions for Ras signalling of <em>T. reesei</em> in cellular morphogenesis, especially in cellulase gene expression, which contribute to deciphering the powerful competitive ability of plant cell wall degrading fungi in nature.</p> </div
