Global environmental change is becoming a serious problem worldwide, requires novel eco-friendly products that allow sustainable growth and commercial production following the needs of farmers and consumers. Hence, there are many studies have been done to discover eco-friendly alternatives, and volatile organic compounds emitted from microorganisms have emerged as effective, cheaper, and friendly with environment. Here, we studied the functional microbe-emitted volatiles in mediating plant growth promotion under stress and non-stress conditions, also the insight of molecular mechanisms related to growth promotion. Volatile compounds used in this study were produced by Fusarium verticillioides and Klebsiella aerogene. Shoot elongation in seedlings exposed to Fusarium and Klebsiella were increased but differ in root growth. There is adjusting expression of genes involved in hormone production resulted in the increased shoot growth. CKX1, CKX9 are responsible for degradation of cytokinin, was downregulated in exposed shoots result in the induction of cytokinin levels in shoot. High content of cytokinin may cause systemic acquired resistance process via induction of salicylic acid and PR1b protein activities. Interestingly, RR9, a cytokinin regulator, was induced the transcript levels in exposed plants meaning that there is negative feedback of cytokinin production within plant. Additionally, gibberellin 1 (GA1) production was triggered by down-expression levels of SAP11 (OsDOG), a negative regulator of GA. Fusarium not only induced the elongation of shoot but also enhanced the absorption of the nutrients by elevating the transcriptional levels of NAS2, one of genes in NAS family involve in metal transport. Furthermore, Fusarium and Klebsiella have been shown to increase the tolerance of plant to salt and copper stress, showed by reduction of ROS accumulation and cell death area on root tips. There are different impacts on antioxidant system between salt and copper stress led to the difference in antioxidant systems. Whereas, only SOD and POD-e,f isozymes still increase function in copper-stressed plant exposed to volatiles, almost isozyme of CAT, APX, POD and SOD were inducted activities in exposed plant stress by salt. Proline, member of ROS-scavengers, increased in exposed roots under both copper and salt stress.

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