3.1 Endogenous MT play a positive role in thermotolerance of apple plants
Endogenous MT content in apple plants increased significantly under high temperature stress (48°C) (Figure 1A). QRT-PCR was used to measure expression of four synthase genes following 8 h high temperature treatment: MdTDC1 , MdT5H4 , MdAANAT2 andMdASMT9 (Figure 1B-E). The gene expression levels ofMdTDC1 , MdAANAT2 , and MdASMT9 were altered by high temperature treatment. MdASMT9 showed the greatest increase, with the expression level 120 times higher than the control after 1h of heat treatment. To further study the role of endogenous MT under heat stress, two previously generated MdASMT9 -OE transgenic apple lines (OE-3 and OE-4) were subjected to high-temperature stress (Zhou et al. , 2022). OE-3 and OE-4 transgenic apples had significantly higher MT content than the wild-type (WT) under both normal and high-temperature conditions (Figure 1F). In addition, WT, OE-3, and OE-4 showed increased MT levels under high temperature compared to normal conditions. These findings indicate that endogenous MT may play a role in apple’s response to heat stress.
The top leaves of WT plants were charred and shriveled, while those of the transgenic lines showed symptoms of slight dehydration after exposure to 48°C for 8 h (Figure 1G). The relative electrolyte leakage (REL) increased significantly because of heat stress damage. However, it was lower in OE lines compared to WT (Figure 1H). As shown in Figure 1I, OE lines and WT plants showed the same trend for the malondialdehyde (MDA) content. These results indicate that both transgenic lines suffered less physiological damage under heat stress, implying a positive role for endogenous MT in apple plants’ response to high-temperature stress.