*2.2. Plant Material, Growth Condition, and Heat Treatment*

Ailsa Craig (AC) of tomato from Tomato Genetics Resource Center (TGRC) was used as a wild type (WT). Peat and vermiculite were mixed in a suitable ratio (7:3, v:v) for seedling growth. Hoagland nutrient solutions were used twice a week to supplement the tomato with nutrients. The growing conditions of plants were ensured according to the following criteria: photoperiod was performed by 14 h/10 h (day/night), the ambient air temperature was kept by 25 ◦C/20 ◦C (day/night), and photosynthetic photon flux density was arranged to 400 μmol m−<sup>2</sup> s−1. Plants at the five-week seedling stage were used for the following experiments. Two groups of AC, OE-*BAG9*, and *bag9* plants were separated. The control group and the heat stress group were treated for 10 h at 25 ◦C and 45 ◦C, respectively, in growth chambers (Qiushi, Hangzhou, China). Except for the temperature in the growth chambers, other environmental parameters remained the same as previously described. Leaf samples were collected at different times from heated or unheated tomato plants, then frozen rapidly in liquid nitrogen and stored at −80 ◦C before analysis for gene expression, malondialdehyde (MDA), antioxidant, enzyme activity, and immunoblotting. While after being treated for 7 h, leaf samples were collected from the control group and the heat stress group and then immediately analyzed for a maximum quantum yield of PSII (*Fv*/*Fm*) and 3,3 -diaminobenzidine (DAB) and nitroblue tetrazolium (NBT) staining.
