Cryptic isoprene emission of soybeans under wounding and high temperature

Isoprene is the most abundant non-methane biogenic hydrocarbon emitted by some plants, mostly trees. It plays a critical role in atmospheric chemistry by contributing to ozone and aerosol formation. Isoprene also benefits plants, particularly under stress, through its signaling roles. Legume crops like soybean were thought to have evolutionarily lost isoprene synthase (ISPS) and are typically considered non-emitters. Here we report that damage to soybean leaves by wounding or burning triggered a burst of isoprene emission from undamaged parts of the leaves. In silico analysis identified intact ISPS genes in the soybean genome, with features similar to known ISPSs. Protein made from these gene sequences catalyzed isoprene production in the presence of dimethylallyl diphosphate. Isoprene emission in soybeans was linked to reduced photosynthesis rates and stomatal conductance. Metabolomic analysis showed that leaf damage caused a surge in glyceraldehyde 3-phosphate and pyruvate levels, leading to an increase of most of the methylerythritol 4-phosphate (MEP) pathway metabolites. Heat stress also led to significant isoprene emission from soybean leaves. We conclude that soybeans possess functional ISPSs and can make isoprene only under some conditions by regulating photosynthesis and MEP pathway.