Light restores sporulation in cured Rhizopus microsporus of its endosymbionts unveilling their role in virulence
Rhizopus microsporus is one of the most common agents of mucormycosis, a severe and emerging infection caused by Mucorales fungi that poses a significant clinical challenge, particularly due to the growing population of immunocompromised individuals. An substantial proportion of clinical isolates harbor bacterial endosymbionts, which regulate key fungal functions, such asexual sporulation. The strict dependence on endosymbionts for spore formation has limited a comprehensive understanding of endosymbiosis in R. microsporus biology. Here, we demonstrate that sporulation in endosymbiont-cured strains of this fungus, which previously harbored Mycetohabitans bacteria, can be induced by light. Interestingly, both light and endosymbionts regulate sporulation through the same regulatory pathway, and a high proportion of the genes upregulated by these signals are known to be involved in asexual sporulation in other fungi, including Mucorales. Light-induced sporulation enabled comparative assays to assess the impact of symbiosis on fungal traits. In addition to previously known effects on fungal fitness, this study reveals that germination is independent of endosymbionts, although their presence accelerates the process. Furthermore, it shows that asexual spores lacking endosymbionts exhibit a reduced virulence in a mouse model of mucormycosis. The discovery of light-induced sporulation in endosymbiont-free strains of R. microsporus paves the way for future comparative studies using genetically identical backgrounds, advancing our understanding of fungal-bacterial symbiosis and its role in fungal biology and human pathogenesis. Overall design: RNA-seq experiment to analyze the differences in gene expression caused in cured and infected (Mycetohabitans) R. microsporus ATCC 52814 exposed to light and in the dark Negative: cured, Positive: infected.