Utilization of beneficial microorganism will overcome food shortage issue by enhancing crop productivity as well as conserving environmental balance. The fungus Piriformospora indica is a beneficial root-colonizing microorganism that can promote plant resistance and growth by producing diffusible and/or volatile organic compounds (VOCs). However, the underlying associated molecular pathway(s) remain obscure. Moreover, different cultures may affect the fungus growth and VOC production. In this study, we aimed to establish culture medium condition for promote the best of plant growth mediated by P. indica VOCs and elucidate the associated molecular pathway(s). We found that P. indica grown on potato dextrose agar (PDA) medium has a prominent effect on plant growth promotion after 10 days of co-cultivation compared to Kafer medium (KF) and poor nutrient medium (PNM). Dry weight and root length of treated plants were doubled in comparison with untreated plants. Additionally, sucrose inhibition was also observed in VOC-mediated growth promotion. VOC-treated plants grown on PNM had up to a 4.37-fold rise compared to those grown on ½ MS with 2.91-fold rise indicating growth retardation on limited-nutrient plant. To analyse the possible molecular pathway, we found that P. indica VOCs could phosphorylate MAPK6 as early as 5 minutes after exposure. Intriguingly, IRT1 and FRO2 genes were suppressed after 7 days while NIA1 and GLB1 were induced after 5 days of co-cultivation, contradicting previous reports on VOC-mediated growth and might act downstream of MAPK6 showed by unaltered expression in the mapk6 mutant background. Taken together, our results demonstrated that fungus cultured on rich medium exhibited a tremendous effect on nutrient-limited plants that the medium-dependent generated signals may triggered MAPK6 phosphorylation to induce several downstream responses.

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