3.3 Overexpression of MdASMT9 enhanced photosynthetic capacity and increased stomatal aperture under heat stress
Heat stress has a particularly adverse impact on plant photosynthesis, one of the most vital biochemical processes. A significant decrease in net photosynthesis (Pn) and stomatal conductance (Gs) was determined in both WT and MdASMT9 -OE lines after 8h of exposure to 48°C and 24 h of recovery. However,MdASMT9 -OE lines exhibited higher Pn compared to the WT (Figure 3A). Consistent with the Pn value, Gs showed a similar pattern after the recovery period (Figure 3B). To assess the effect of heat stress on photosynthesis, chlorophyll fluorescence imaging was used to measure the maximum photochemical quantum yield of PSII as theFV/FM ratio (Figure 3C and 3D). Under normal conditions, both WT and MdASMT9 -OE lines showed comparable fluorescence intensities, but after heat stress exposure, theFV/FM value was higher in theMdASMT9 -OE plants compared to the WT.
Exposure to high temperatures can negatively impact the chloroplast ultrastructure, thereby reducing photosynthetic efficiency, as confirmed through TEM (Figure 3E). After exposure to 48°C for 3 h, the ultrastructure of chloroplasts in WT plants was damaged, characterized by an accumulation of larger starch grains. However, MdASMT9 -OE lines exhibited less fragmented and better-organized chloroplast ultrastructure. These results collectively suggest that MdASMT9overexpression protected the photosynthetic apparatus from damage and enhanced photosynthetic capacity under heat stress.
Stomatal movement impacts leaf transpiration and heat dissipation, playing a crucial role in heat stress response. Thus, the stomatal morphology of MdASMT9 -OE and WT plants was compared (Figure 3F). Their stomatal aperture exhibited no significant difference under normal conditions, and both declined significantly when exposed to heat stress. However, OE-3 and OE-4 showed greater stomatal aperture than the WT line after exposure to heat stress (Figure 3F and 3G). ABA plays a crucial part in regulating stomatal movement, and we assessed the ABA levels in WT plants and MdASMT9 -OE lines. Both WT and transgenic lines displayed significantly higher ABA contents under high stress (Figure 3H). However, the MdASMT9 -OE plants had notably lower ABA levels than the WT. These results demonstrated that MdASMT9overexpression significantly reduced ABA synthesis and promoted stomatal opening, conducive to transpiration and heat dissipation in plants under heat stress.