Exogenous dopamine delays the senescence of detached Malus hupehensis leaves by reducing phytohormone signalling and sugar degradation

Leaf senescence is the ultimate stage in leaf development, accompanied by the degradation and redistribution of nutrients and biological macromolecules. Dopamine (DA), a type of catecholamine, plays a role in responding to various plant stresses through its strong antioxidant capacity. However, the relationship between DA and leaf senescence is poorly understood. Thus, leaves isolated from Malus hupehensis were treated with DA, and a dark-induced senescence experiment was conducted. We first found that 100 μmol L−1 DA was the optimal treatment for delaying leaf senescence. The leaves treated with 100 μmol L−1 DA showed higher chlorophyll content and Fv/Fm, and lower relative electrolyte leakage and malondialdehyde content during dark-induced senescence. DA was able to markedly decrease the expression of senescence associated genes (MdSAG12, MdSAG13, and MdSAG29) and a chlorophyll degradation gene (MdPAO). Compared with untreated leaves, the activity of antioxidant enzymes of leaves under DA treatment was significantly higher, and the level of reactive oxygen species (ROS) was significantly lower. Moreover, DA significantly reduced the content of phytohormones (1-aminocyclopropane-1-carboxylate acid, abscisic acid, salicylic acid, and jasmonic acid) that accelerate senescence during dark treatment and changed the expression of genes involved in phytohormone pathways to inhibit the transmission of signals. In addition, DA treatment significantly reduced the expression of genes related to the degradation of soluble sugar (fructose, glucose, sucrose, and sorbitol), thus significantly increasing the soluble sugar content relative to untreated leaves. Overall, our findings revealed that exogenous dopamine could decrease the expression of SAGs, improve the activity of antioxidant enzymes, reduce ROS levels, and the content of phytohormones that accelerate senescence and their signalling pathways, and decrease the degradation rate of soluble sugar to delay the senescence of apple leaves.