Osmoregulation is a crucial factor for methyl jasmonate to enhance chilling tolerance of <i>Jatropha curcas</i> L.

Methyl jasmonate (MeJA) is a vital regulator of plant growth and plays a crucial role in chilling tolerance. However, the mechanism through which MeJA enhances chilling tolerance in plants remains unclear. Therefore, this study conducted hydroponic experiments to evaluate the effects of exogenous MeJA (0-125 μmol L-1) on osmoregulation and chilling tolerance of Jatropha curcas L. seedlings under chilling (5 °C) stress. The seedlings under chilling stress were treated with MeJA and morphological changes, physiological traits, osmoprotectants (proline, betaine and trehalose) contents, activities of key enzymes involved in osmoprotectants metabolism and expression of related genes were investigated. The results showed that treatment with 75 μmol L-1 MeJA alleviated leaf wilting and growth inhibition; significantly decreased water potential, electrolyte leakage and malondialdehyde content; and enhanced tissue vitality, water content, total chlorophyll content, net photosynthetic rate, stomatal conductance, intercellular CO2 concentration and transpiration rate in J. curcas seedlings, thereby improving chilling tolerance. Under chilling stress, 4 days of MeJA treatment remarkably increased the contents of proline, betaine and trehalose in the leaves of J. curcas seedlings by activating their biosynthesis pathways and inhibiting the degradation pathway of proline. The substantial accumulation of osmoprotectants reduced the cellular water potential, maintained the cellular water balance and stabilized the cell membrane. Furthermore, 1-4 days of MeJA treatment led to increased levels of jasmonic acid and ethylene and upregulation of JcMYC2 expression in J. curcas seedlings under chilling stress. This suggested that the JA/MeJA-MYC2 signaling pathway, along with ethylene signaling, may contribute to MeJA-induced chilling tolerance in J. curcas. Our findings suggested that exogenous MeJA treatment increases the capacity for osmoregulation and chilling tolerance in J. curcas seedlings under chilling stress and that osmoregulation is a crucial component of MeJA-induced chilling tolerance.