Hyphal Fusion and Autophagy Enable Proper Conidiation and Symptom Development on Maize Leaves by Colletotrichum graminicola

Hyphal and germling fusion is a common phenomenon in ascomycetous fungi. Due to the formed hyphal network, this process enables coordinated development, interaction with plant hosts and efficient nutrient distribution. Recently, our lab showed a positive correlation of germling fusion with the formation of penetrating hyphopodia on maize leaves outgoing from Colletotrichum graminicola oval conidia. To investigate the probable interconnectivity of these processes, we have generated a deletion mutant in Cgso, which homologs are essential for cellular fusion in other fungal species. Indeed, plant infection studies combined with microscopy revealed a significant decrease in symptom development of the ∆Cgso mutant on maize leaves. However, hyphopodia development was not affected, indicating that both processes are not directly connected. Instead, we were able to link the decreased symptom development to a reduced formation of acervuli, asexual fruiting bodies of C. graminicola, which give rise to falcate conidia. Monitoring of a fluorescent labelled autophagy marker eGFP-CgAtg8 revealed a high autophagy activity in hyphae surrounding acervuli. Since the ∆Cgso mutant shows no hyphal fusions at these sites, we conclude that efficient nutrient transport of degraded cellular material by hyphal fusions enables proper acervuli maturation and symptom development on leaves.