3.2 Overexpression of MdASMT9 enhanced low-N tolerance in apple plants
The expression of MdASMT9 , which encodes the enzyme involved in MT synthesis, was measured in apple plants under low-N stress. The results showed that MdASMT9 expression was significantly upregulated under N-deficiency treatment, first increasing on the 8th day and peaking on the 16th day (Figure 1d).
To further explore the function of endogenous MT in low-N responses, transgenic ‘Royal Gala’ apple seedlings overexpressing MdASMT9(MdASMT9 -OE) were generated and analyzed. LC-MS analysis revealed a significant increase in MT content in the leaves and roots ofMdASMT9 -OE plants compared to WT plants. Moreover, endogenous MT levels in both leaves and roots increased under low-N stress (Figure 1f). Under control conditions, there was no significant difference in phenotype between WT and MdASMT9 -OE plants (Figure 1e). However, after 35 d of low-N treatment, the WT plants exhibited reduced plant height while the damage suffered by OE lines was less severe (Figure 1g). The biomass of the transgenic plants was also less affected by N deficiency compared to WT plants (Figure 1h). These findings suggest that apple plants overexpressing MdASMT9 have enhanced low-N tolerance.