Decoding polyamine-ethylene dynamics: synergies at synthesis and signalling levels during osmotic stress and ripening

Amid climate uncertainty, agriculture faces numerous challenges, including drought, salinity, hypoxia, heat, cold, osmotic, biotic stresses, and, in turn, oxidative stress, in its struggle to secure global food supplies. Plant development is influenced by numerous internal and external factors that collectively regulate the entire life cycle of a plant and, thus, yield. Ethylene (ETH) and polyamines (PAs) are multifunctional regulatory compounds. Their interaction plays a significant role under osmotic stress and fruit ripening, both synergistically and antagonistically. Although both of them are produced from the same precursor, S-adenosylmethionine (SAM), their signalling pathways differ and induce distinct physiological effects that are often antagonistic. Putrescine, spermidine, and spermine are the most abundant PAs that are typically linked to stress resilience, membrane stability, antioxidative responses, and cellular defence. ETH, on the other hand, exhibits two distinct characteristics: at moderate levels, it functions as a ripening hormone stress signal to initiate adaptive responses, but at higher or prolonged stress levels, it promotes senescence and inhibits growth. This review article discusses SAM as the common linking molecule between the overlapping ETH and PA biosynthesis, ETH-PA interaction under osmotic stress, as well as their signalling and regulatory crosstalk in ripening, and how ROS are generated and mitigated by ETH and PA application in a stressful environment. We discuss recent advances in this subject, highlight some unanswered questions, and provide a roadmap for increasing productivity and unleashing climate-smart farming.