RacA-Mediated ROS Signaling Is Required for Polarized Cell Differentiation in Conidiogenesis of Aspergillus fumigatus.

Conidiophore development of fungi belonging to the genus Aspergillus involves dynamic changes in cellular polarity and morphogenesis. Synchronized differentiation of phialidesfrom the subtending conidiophore vesicle is a good example of the transition from isotropic to multi-directional polarized growth. Here we report a small GTPase, RacA, which is essential for reactive oxygen species (ROS) production in the vesicle as well as differentiation of phialidesin Aspergillus fumigatus. We found that wild type A. fumigatus accumulates ROS in these conidiophore vesicles and that null mutants of racA did not, resulting in the termination of conidiophore development in this early vesicle stage. Further, we found that stress conditions resulting in atypical ROS accumulation coincide with partial recovery of phialideemergence but not subsequent apical dominanceof the phialidesin the racA null mutant, suggesting alternative means of ROS generation for the former process that are lacking in the latter. Elongation of phialideswas also suppressed by inhibition of NADPH-oxidaseactivity. Our findings provide not only insights into role of ROS in fungal cell polarityand morphogenesisbut also an improved model for the developmental regulatory pathway of conidiogenesis in A. fumigatus.