How and why plants came to smell green: the origins, biosynthesis, and roles of green leaf volatiles

Most terrestrial plants smell green because they produce a family of volatile organic compounds, green leaf volatiles (GLVs). The widespread occurrence of GLVs suggests that these compounds have been evolutionarily acquired to fulfill a common function across land plants. What, then, are their roles? GLVs are hardly detectable in undamaged plant tissues yet are rapidly synthesized from damaged cells within seconds of injury. This suggests that GLVs may be produced to defend against organisms that cause tissue damage or in response to prospective stresses that could result from tissue damage. Plants, unlike animals, have evolved to adopt sessile autotrophy, maximizing growth and completing reproductive cycle as well as possible. Thus, even if parts of the tissues are lost owing to stress, the remaining portions can still complete generational cycles. The ability to generate GLVs is akin to a last-ditch effort by dying cells to protect the remaining tissue and the entire plant body. During plant evolution, this ability was partly developed in cyanobacteria, established in lycophytes, and subsequently inherited by nearly all terrestrial plants. Furthermore, this capability involves plant cells quickly sensing severe stresses that could rupture them, necessitating the rapid activation of the GLV biosynthetic pathway. This review discusses the molecular mechanisms underlying these processes. Additionally, some components of GLVs are released into the environment, contributing to the ecological framework surrounding the plants. Briefly touching on the ecological aspects of GLVs, this review also discusses how plants perceive GLVs emitted by neighboring plants.