4.1. Plant Materials and Growth Conditions

Tomato (Solanum lycopersicum cv ‘Micro-Tom’) was used as the WT tomato plants. pBI121#OE (pBI121 overexpressed control plants) and sly-MIR398b#OE (sly-miR398b overexpressing lines) plants were generated in ‘Micro-Tom’ background. Specifically, sly-MIR398b precursor (410 bp) was cloned from tomato and then inserted into vector pBI121 (14,758 BP, a binary Agrobacterium vector with a GUS reporter gene for plant transformation) downstream of CaMV 35S promoter. The exact sequence length of sly-MIR398b precursor and the primer pair used for amplifying sly-MIR398b precursor sequence were included in the previous paper [38]. The resultant vector was then introduced into Agrobacterium tumefaciens strain GV3101 for tomato transformation. Homozygous transgenic plants were generated and identified as described previously [38]. Two sly-miR398b overexpressing lines (MIR398b#OE1 and MIR398b#OE8) were obtained [38]. The amounts of sly-miR398b increased sharply to a similar extent in the two miR398b overexpressing lines (MIR398b#OE1 and MIR398b#OE8) compared with those in control line, and they showed similar phenotypes with regard to the effects of salt stress on plant growth, oxidative damage, antioxidant response, and photosynthesis performance [38]. Therefore, in the present project, we selected one (MIR398b#OE1) of them (MIR398b#OE1 and MIR398b#OE8) for further study. Tomato seedlings were grown in a growth chamber with temperature of 25 °C/20 °C day/night and photoperiod of 16/8 h light/dark (with a white light intensity of 200 µmol m⁻² s⁻¹).