Induction of Volatile emission in response to stress and their role in stress adaptation

Plants accumulate a diverse array of natural products, which are thought to be involved in their interactions with the environment. These chemicals function in interactions with microbes, animals, and even other plants, as well as protecting the plant from ultraviolet radiation and oxidants. Some compounds may attract beneficial insects or microbes, whereas others kill or repel herbivorous. Many of these compounds have been referred to as “secondary metabolites” to distinguish them from the “primary metabolites” required for the growth of all plants. These secondary metabolites, however, are likely to be essential for the successful competition or reproduction. Among the secondary metabolites, a relevant role is played by volatile organic compounds (VOCs). The importance of these compounds can be deduced by the considerable amount of photoassimilated carbon released back into the atmosphere as VOCs. Stored VOCs may be volatilized into the atmosphere by a healthy unwounded plant depending on their concentration and physiochemical properties. While many plants contain large amounts of stored VOCs, others do not synthesize and emit them until an environmental stimulus is perceived. Induced VOCs (IVOCs) may be emitted hours or days after a stress, both from the stressed sites as well as systemically from undamaged plant leaves. In contrast to the constitutive VOCs, the novel IVOCs are produced only after biotic and abiotic inductions. The advantage of novel IVOCs is that they are de novo synthesized only when needed and therefore they optimize carbon usage and do not reduce plant fitness. The VOCs are important ifochemicals and their role in shaping the biotic interactions is well known. However, inducible VOCs are also emitted in response to abiotic stress perception and they may play a role in stress adaptation or response. Environmental stress such as physical damage, nutrient deficiency, salinity, draught and ozone exposure hamper the IVOCs emission. Stress in plants could be defined as any change in growth conditions that disrupts metabolic homeostasis and requires an adjustment of metabolic pathways in a process that is usually referred to as acclimation. There is a broad diversity of known IVOCs, including alkenes, alkanes, carboxylic acids and alcohols, but the dominating compounds tend to be terpenes and C6 green leaf volatiles (GLVs). GLVs are produced via the lipoxygenase (LOX) pathway, and they can account for > 50% of the emissions from damaged plant parts. Chemically, GLVs are mostly saturated or monounsaturated aldehydes, alcohols and esters, and they can have different configurational isomers with different sensory properties. GLVs are typically released only from damaged plant organelles within 1–2 seconds of the mechanical damage occurring, but some GLVs are released from younger undamaged leaves of herbivore damaged plants, indicating that the LOX pathway can be activated in intact leaves. The high diversity of IVOCs suggests that plants are capable of disseminating information to their environment by using IVOCs and that plants can actively change the growth conditions using reactive IVOCs. The functional roles of these compounds are broadly acknowledged, thus several challenges presented themselves to scientists interested in their ecological and evolutionary dynamics. Engineered or mutant plants with altered rates of IVOC emission and composition will be the tools of the future to increase our understanding of the ecological impacts of IVOCs. The present chapter will review the current knowledge on the emission and function of IVOCs in response to abiotic stress. The review will also focus on the role of Ethylene, Jasmonic Acid and derivatives (i.e. Methyl Jasmonate), Nitric Oxide, Isoprene and Isoprenoids.