Action mechanisms of small microbial volatile compounds in plants.

Microorganisms communicate with plants by exchanging chemical signals throughout the phytosphere. Before direct contact with plants occurs, beneficial microorganisms emit a plethora of volatile compounds (VCs) that promote plant growth and photosynthesis as well as developmental, metabolic, transcriptional and proteomic changes in plants. These compounds can also induce systemic drought tolerance and improve water and nutrient acquisition. Recent studies have shown that this capacity is not restricted to beneficial microbes; it also extends to phytopathogens. The response of plants to microbial VCs has frequently been associated with volatile organic compounds with molecular masses ranging between ca. 45 Da and 300 Da. However, microorganisms also release a limited number of VCs with molecular masses of less than ca. 45 Da that react with proteins and/or act as signaling molecules. Some of these compounds promote photosynthesis and growth when exogenously applied in low concentrations. Recently, evidence has been compiled that small VCs are important determinants of plant responses to microbial volatile emissions. However, the regulatory mechanisms involved in these responses remain poorly understood. This review summarizes the current knowledge about the biochemical and molecular mechanisms involved in the growth, developmental and metabolic responses of plants to small microbial VCs.