Isoprene serves as the fundamental C5 hemiterpenoid and most abundant biogenic volatile organic compound (BVOC) emitted by plants, playing critical roles in thermotolerance, atmospheric chemistry, and microbial signaling.
Chemical Properties
Isoprene (2-methyl-1,3-butadiene, C₅H₈, MW 68.12 g/mol) exists as a volatile, colorless liquid (bp 34°C, mp -146°C, density 0.681 g/cm³) with a conjugated diene structure enabling electrophilic addition and Diels-Alder reactivity. Highly flammable (flash point -51°C), it polymerizes exothermically to polyisoprenes (cis: natural rubber; trans: gutta-percha); atmospheric lifetime is approximately 2 hours via OH radical scavenging (k = 1×10⁻¹¹ cm³ molecule⁻¹ s⁻¹).
Biosynthetic Pathway
Plants synthesize isoprene via the MEP pathway in chloroplasts: pyruvate + glyceraldehyde-3-P → DXP → MEP → HMBPP → DMAPP → isoprene, catalyzed by isoprene synthase (IspS, EC 4.2.3.27; Mg²⁺-dependent isomerase). Emission rates peak at 20–50 μg g⁻¹ h⁻¹ dw under heat/light stress (>30°C, >1000 μmol m⁻² s⁻¹ PAR); global flux is ~500–750 Tg C yr⁻¹, primarily from oak and poplar species.
