Do cytokinins, volatile isoprenoids and carotenoids synergically delay leaf senescence?

We propose that relationships between the synthesis rate and in vivo concentration of these metabolites go beyond being mere ontogenic and phenological coincidences; rather, they are directly or indirectly (i.e. by quenching presence of ROS) involved in regulating senescence. We propose that leaves do not enter senescence as long as their total cytokinin content (total = synthesized de novo+ imported from extra-chloroplastic and extra-foliar sources) remains high, and volatile isoprenoid emission remains high. Isoprene emission is a feature of chloroplasts of some plants, especially trees, whereas cytokinins and monoterpenes are synthesized in all types of plastids, in all plants. Co-occurrence of isoprene emission with deciduousness in trees (short life of leaves, Dani et al. 2014b; Loreto & Fineschi 2015) may suggest that isoprene emission is a drain on the MEP pathway, essentially opposing the positive effect of cytokinins and carotenoids in delaying senescence. However, considering available experimental proofs showing antioxidant and thermal protectant activity of isoprene and non-stored monoterpenes, we maintain possible that volatile isoprenoids supplement carotenoids in quenching ROS and help maintain plastome and thylakoid membrane integrity to delay senescence.