Microbial Volatile Organic Compounds Produced by Bacillus amyloliquefaciens GB03 Ameliorate the Effects of Salt Stress in Mentha piperita Principally Through Acetoin Emission

We investigated the effects of microbial volatile organic compounds (mVOC) emitted by Bacillus amyloliquefaciens GB03 on Mentha piperita growing under different levels of NaCl stress, by evaluating their growth-promoting potential and ability to increase salt tolerance effects. Plants were exposed to bacterial VOCs without having any physical contact with the rhizobacteria. The VOCs emitted by the rhizobacteria (mVOCs) were analyzed using SPME fibers. An increase in the level of salt concentration led to a decrease in plant growth. However, these negative effects of salinity were inhibited in the plants exposed to mVOCs. Plants grown in a saline media and exposed to GB03 VOCs had significantly better morphological characteristics and higher total chlorophyll content compared to controls. The level of endogenous jasmonic acid (JA), salicylic acid, and abscisic acid increased in salt-stressed plants compared to controls. The level of JA did not show any change in plants grown in a saline media either exposed to mVOCs or not. In contrast, the amount of salicylic acid increased remarkably in salt-stressed plants exposed to mVOCs compared to controls (salt-stressed plants not exposed to mVOCs), but the levels of abscisic acid decreased in salt-stressed plants exposed to mVOCs. The chromatographic analyses of the mVOCs produced by salt-stressed GB03 bacteria were similar, regardless of the concentration of salt in the media where the bacteria were grown, although it was observed that the relative percentage of acetoin increased with salt concentration. After determining that acetoin was the main VOCs compound, we exposed plants to acetoin, which demonstrated that acetoin caused similar effects on plants grown under salt stress conditions as those exposed to GB03 mVOCs. Based on these results, the use of mVOCs from PGPR is suggested as a useful technological innovation to facilitate the growth of M. piperita in salt-stressed environments.