Leaf Isoprene emission, cellular signaling and coordination of the adaptive responses of plants to climate stress and growth-defense trait tradeoffs

Isoprene, a volatile hemiterpene, is synthesized in the chloroplasts of many plants, including numerous species of forest trees. Isoprene emissions from global forests are comparable in magnitude to methane emissions from global wetlands. In the atmosphere, isoprene exerts a central photochemical control over the oxidative capacity of the troposphere and has a role in determining the lifetime of methane and the concentration of ozone. As a plant trait, the role of isoprene emission has been much more uncertain. It's the latter issue that forms the focus of this lecture.

As a chloroplast metabolite that scales positively with photosynthetic rate, isoprene has the potential to take on a central role in the connection of primary anabolic metabolism to growth and secondary metabolite biosynthesis. Recent research has revealed that the removal or addition of chloroplast isoprene biosynthesis, through engineered genetic modification, causes large effects on leaf transcriptomes, proteomes and metabolomes. Isoprene biosynthesis influences the expression of numerous proteins and the activity of several transcription factors and coordinates the expression of both constitutive and induced cellular signal cascades, including those in the gibberellin, salicylic-acid and jasmonic-acid pathways. Isoprene emission in leaves appears to orchestrate a complex interaction among pathways that determines the channeling of substrate to growth versus defense – in some cases, even uncoupling the well-accepted tradeoff between these functions.

The recent observations of isoprene's role in orchestrating metabolite flow has brought it to the forefront as a key element controlling growth-defense tradeoffs and adaptive responses of plants to climate stress. However, major questions remain to be answered before the entire scope of isoprene's role can be settled. For example, we still do not know why some species emit isoprene, while others do not, or how the high degree of variance in isoprene emission rate is interpreted within the context of the stasis required for plant signaling. Research into isoprene biosynthesis in plants offers an example of how the emerging availability of multi-omic, big data sets and manipulations using genetically-modified organisms, can be integrated with the discipline of plant ecophysiology, and together, inform us about foundational controls over plant adaptation to the environment.