Microbial volatile compounds in plant health

Secondary metabolites are produced during the stationary phase when essential nutrients required for bacterial growth get exhausted and bacteria shift toward less-preferred nutrient sources leading to short growth phase but efficient production phase. Rhizobacteria secrete a large number of secondary metabolites that are broadly classified as volatile compounds and water-soluble compounds. These secondary metabolites have potential applications in suppressing the growth of plant pathogens, cell to cell communication, plant growth regulation, biocontrol, symbiosis between microbes and plants, and hormone production. Volatile compounds are the low molecular weight components that can easily vaporize and diffuse through soil pores over large distances due to the low boiling point and high vapor pressures. Volatile compounds include molecules such as hydrogen cyanide, aldehydes, alcohols, ketones, terpenes, and pyrazines that help in enhancing plant growth by the suppression of various fungal diseases. Water-soluble secondary metabolites on the other hand are polar compounds that include broad range of antibacterial and antifungal components such as polyketide, pyoluteorin, mupirocin, diacetyl phloroglucinol and cyclic lipopeptides, multidomain enzyme complexes, and iron quenching siderophores. This chapter provides an explicit description of different classes of volatile and water-soluble secondary metabolites and their role in enhancing the plant growth through various mechanisms.