Differential Physiological Prerequisites and Gene Expression Profiles of Conidial Anastomosis Tube and Germ Tube Formation in Colletotrichum gloeosporioides

Abstract

The conidia of a hemibiotrophic fungus, Colletotrichum gloeosporioides, can conventionally form a germ tube (GT) and develop into a fungal colony. Under certain conditions, they tend to get connected through a conidial anastomosis tube (CAT) to share the nutrients. CAT fusion is believed to be responsible for the generation of genetic variations in few asexual fungi, which appears problematic for effective fungal disease management. The physiological and molecular requirements underlying the GT formation versus CAT fusion remained underexplored. In the present study, we have deciphered the physiological prerequisites for GT formation versus CAT fusion in C. gloeosporioides. GT formation occurred at a high frequency in the presence of nutrients, while CAT fusion was found to be higher in the absence of nutrients. Younger conidia were found to form GT efficiently, while older conidia preferentially formed CAT. Whole transcriptome analysis of GT and CAT revealed highly differential gene expression profiles, wherein 11,050 and 9786 genes were differentially expressed during GT formation and CAT fusion, respectively. A total of 1567 effector candidates were identified; out of them, 102 and 100 were uniquely expressed during GT formation and CAT fusion, respectively. Genes coding for cell wall degrading enzymes, germination, hyphal growth, host-fungus interaction, and virulence were highly upregulated during GT formation. Meanwhile, genes involved in stress response, cell wall remodeling, membrane transport, cytoskeleton, cell cycle, and cell rescue were highly upregulated during CAT fusion. To conclude, the GT formation and CAT fusion were found to be mutually exclusive processes, requiring differential physiological conditions and sets of DEGs in C. gloeosporioides. This study will help in understanding the basic CAT biology in emerging fungal model species of the genus Colletotrichum.

Conidia Fusion: A Mechanism for Fungal Adaptation to Nutrient-Poor Habitats

“…Our study has revealed that the conidia fusion rate (CFR) decreases as the nutrient concentrations in the habitat increases (Figures 3 and 4). This result is consistent with previous research and supports the idea that the formation of CF is closely related to the nutrient content of the habitat [21,23].…”

“…Although the CFR of different species or strains had different responses to the nutrient content and composition of the habitat, the rule that the CFR was inversely proportional to the nutrient concentration of the habitat was confirmed for almost all species. Based on the above research, scholars speculated that another significance of CF might be an adaptation to nutrient deficiency in the habitat [23]. However, this hypothesis has not been supported by direct experimental data, and the way CF adapts to nutrient-poor environments is unclear.…”

Hyphal Fusions Enable Efficient Nutrient Distribution in Colletotrichum graminicola Conidiation and Symptom Development on Maize

“…Falcate conidia secrete mycosporines as germination inhibitors when present in high spore numbers, as in acervuli [14,15]. The formation of germling fusions is also dependent on spore densities; this has been documented for C. graminicola as well as several other fungi [15,26,[44][45][46]. In an earlier study, we observed a positive correlation between the formation of penetrating hyphopodia by oval conidia and the germling fusion process on leaves, indicating that the pathogenicity program of oval conidia is also dependent on the spore concentration [15].…”

The NADPH Oxidase A of Verticillium dahliae is Essential for Pathogenicity, Normal Development, and Stress Tolerance, and it Interacts with Yap1 to Regulate Redox Homeostasis

“…Fusion of vegetatively incompatible strains of fungi, including V. dahliae , usually triggers an incompatibility reaction that can cause cell death of the anastomosed compartments [54]. Based on the observations that this reaction in N. crassa is characterized by induction of ROS [57], and that genes involved in the OSR are up-regulated during CAT formation in Colletotrichum gloeosporioides [58], we used NBT staining to study O 2 •− accumulation in incompatible V. dahliae fusions (pairing Ls.17 H1-sgfp × 123V). One-third of such fusions (n = 30) exhibited increased formazan precipitation (Figure 7F), indicating that ROS accumulation is indeed associated with the onset of the incompatibility reaction, similarly to what was observed in N. crassa [57].…”