Comprehensive protection of tomato photosystem under cold stress by Streptomyces sp. TOR3209

The plant growth-promoting rhizobacterium Streptomyces sp. TOR3209 induces plant tolerance in a wide range of stress conditions. However, the protection of photosystem under cold stress has not been fully understood. Here we reported that the photochemistry activity of photosystem II (PSII) was increased in tomato plants receiving TOR3209 treatment, including the maximum quantum efficiency of PSII photochemistry (<i>Fv</i>/<i>Fm</i>), PSII operating efficiency (ΦPSII), PSII maximum efficiency (<i>Fv</i>’/<i>Fm</i>’), and non-photochemical quenching (NPQ). Microscopic study revealed that the integrity of chloroplast structure was greatly improved by TOR3209, which was damaged at low temperature. Moreover, TOR3209 treatment resulted in good protection on leaf stomatal and guard cell size. In response to TOR3209 treatment, the intercellular CO₂ concentration (<i>Ci</i>) and stomatal limitation values (<i>Ls</i>) were decreased while the mesophyll conductance (<i>g_m</i>) and chloroplast CO₂ concentration (Cc) were increased. The carotenoid content in TOR3209-treated tomato was accumulated at a higher level, which was involved in photoprotection and biosynthesis of abscisic acid (ABA), as well as the increased amounts of ABA in the leaves were subsequently verified in the plants treated with TOR3209. These results demonstrated that TOR3209 treatment comprehensively protected tomato photosynthesis at low temperatures.