Isoprene enhances leaf cytokinin metabolism and induces early senescence.

●Isoprene, a major biogenic volatile hydrocarbon of climate-relevance, indisputably mitigates abiotic stresses in emitting plants. However functional relevance of constitutive isoprene emission in unstressed plants remains contested. Isoprene and cytokinins (CKs) are synthesised from a common substrate and pathway in chloroplasts. It was postulated that isoprene emission may affect CK-metabolism. ●Using transgenic isoprene-emitting (IE) Arabidopsis and isoprene non-emitting (NE) RNAi grey poplars (paired with respective NE and IE genotypes), the life of individual IE and NE leaves from emergence to abscission was followed under stress-free conditions. We monitored plant growth rate, above-ground developmental phenotype, modelled leaf photosynthetic energy status, quantified the abundance of leaf CKs, analyzed Arabidopsis and poplar leaf transcriptomes by RNA-sequencing in presence and absence of isoprene during leaf senescence. ●Isoprene emission by unstressed leaves enhanced the abundance of CKs (isopentenyl adenine and its precursor) by >200%, significantly upregulated genes coding for CK-synthesis, CK-signaling and CK-degradation, hastened plant development, increased chloroplast metabolic rate, altered photosynthetic energy status, induced early leaf senescence in both Arabidopsis and poplar. IE leaves senesced sooner even in decapitated poplars where source-sink relationships and hormone homeostasis were perturbed. ●Constitutive isoprene emission significantly accelerates CK-led leaf and organismal development and induces early senescence independent of growth constraints. Isoprene emission provides an early-riser evolutionary advantage and shortens lifecycle duration to assist rapid diversification in unstressed emitters.