Jasmonic Acid-Induced Vacuolar Cation/Proton Exchanger 3 (CAX3) Expression Drives Tolerance to Manganese Toxicity in Mulberry (Morus alba L.).

The transport of manganese (Mn) from the cytosol into the vacuole is a crucial mechanism for metal and ion tolerance in plants. Mulberry exhibits potential to detoxify Mn via phytoremediation by activating the calcium/proton exchanger 3 (MaCAX3); however, its precise molecular mechanism remains uncharacterised. This study elucidates the molecular mechanisms by which MaCAX3 confers tolerance to Mn toxicity in mulberry, demonstrating its activation via induction by methyl jasmonate (MeJA). Our findings indicate that exposure to Mn toxicity severely impaired mulberry growth and biomass, correlating with a significant decline in the maximal photochemical efficiency of PSII (Fv/Fm) and a reduction in chlorophyll content. Exogenous JA application, however, effectively rescued these growth and physiological parameters by enhancing Mn tolerance. The results further establish that MeJA concentrations up to 200 μM released from the MeJA@ZIF-8 composite are pivotal in alleviating Mn toxicity, significantly boosting the activities of superoxide dismutase, peroxidase and catalase, reducing malondialdehyde content and inducing MaCAX3 expression. Moreover, silencing MaCAX3 compromised mulberry's Mn tolerance, whereas its overexpression in Arabidopsis enhanced Mn tolerance by diminishing reactive oxygen species through the activation of antioxidative enzymes. Collectively, these results underscore MeJA-associated regulation of MaCAX3 and the functional role of MaCAX3 in regulating Mn and ion homoeostasis, presenting a promising target for engineering Mn phytoremediation and for developing Mn-stress-tolerant mulberry varieties.