A phosphodiesterase is responsible for the block of cellulase gene expression in light in Trichoderma reesei

Plant cell wall degradation is of major importance for sustainable production of materials and fuel in the future. The biotechnological workhorse Trichoderma reesei produces the required enzymes as well as heterologous proteins for industrial applications.

We showed previously, that cellulase gene expression is regulated by light in T. reesei and that the photoreceptor ENV1 is involved in this process. Thereby, light tolerance with respect to cellulase production is increased in early high producer mutants such as QM9414. However, in the wild-type QM6a, cellulase levels drop dramatically in light. ENV1 is essential for elevated cAMP levels during growth in light and exerts its function at least in part via the cAMP pathway. It was assumed that ENV1 acts by dampening the function of phosphodiesterases. We tested this hypothesis by investigating mutants in the phosphodiesterase genes pde1 and pde2 in Neurospora crassa, which showed only minor regulatory effects with respect to cellulase formation or gene regulation upon growth on cellulose.

In contrast, in Trichoderma reesei we found that indeed PDE1 dampens transcript levels of the major cellulase gene cbh1 in light on cellulose and this effect is mediated by ENV1. Accordingly, the light dependent growth defect of mutants lacking ENV1 prevails in double mutants with phosphodiesterases. Additionally our first data also suggest an effect on secondary metabolism.

In summary, we found an important contribution of a phosphodiesterase to cellulase gene expression as well as rewiring of the associated light dependent signaling pathway between T. reesei and N. crassa.