Azotobacter chroococcum, Piriformospora indica, rhizosphere,
The importance of the rhizospheric microorganisms Piriformospora indica and Azotobacter chroococcum as individuals, beneficially interacting with plants, is well known. However, looking at the plant as a holobiont requires analysing the partners as a functionally inseparable unit. Thus, understanding the genesis of this association will be advantageous. Here, we describe the effect of secretory factors from A. chroococcum on chlamydospore germination of P. indica. Pronounced strain dependency of the bacterial partner was observed for chlamydospore germination. A. chroococcum WR5 stimulates spore germination, whereas strain M4 suppresses it. Gel electrophoresis followed by mass spectrometry of A. chroococcum and P. indica, either separately grown or in co-culture with WR5 and M4, reveals differential expression of secretory proteins, among them the flagellar domain protein from WR5, the FliC protein from M4, as well as glutamate dehydrogenase and α-glucosidase b related proteins from P. indica. Taken together, the results suggest that the flagellar domain protein from WR5 possibly improves spore germination and growth by mediating the nutritional switch in P. indica. Subsequently, glutamate dehydrogenase induces nitrogen assimilation in the fungus. High expression levels of FliC in both, individual M4 cultures and in co-cultures with P. indica, suggest that this protein might act as type III secretory protein, possibly involved in secretion of toxins or secondary metabolites suppressing fungal growth. Additionally, downregulation of the α-glucosidase b like protein, presumably involved in plant cell wall degradation, suggests that associations with A. chroococcum WR5 improve the symbiotic stability between the plant and P. indica.