Transglutaminases (TGases), which are widespread cross-linking enzymes in plants, play key roles in photosynthesis and abiotic/biotic stress responses; however, evidence concerning the genetics underlying how TGase improves the capability of photosynthesis and the mechanism of TGase-mediated photosynthesis are not clear in this crop species. In this study, we clarified the function of TGase in the regulation of photosynthesis in tomato by comparing wild-type (WT) plants, tgase mutants generated by the CRISPR/Cas9 system and TGase-overexpressing (TGaseOE) plants. Our results showed that increasing the transcript level of TGase resulted in an enhanced net photosynthetic rate (Pn), whereas the tgase mutants presented significantly inhibited Pns and CO₂ assimilation compared with the WT. Although the total RuBisCO activity was not affected by TGase, the initial and activation status of RuBisCO and the activity of RuBisCO activase (RCA) and fructose-1,6-bisphosphatase (FBPase) in TGaseOE plants were significantly higher than that in WT plants. Except for RuBisCO small subunit (RbcS), the transcription levels of Benson–Calvin cycle-related genes were positively related to the endogenous TGase activity. Furthermore, TGaseOE plants had higher protein levels of RuBisCO large subunit (RbcL) and RCA than did WT plants and showed a reduced redox status by enhancing the activity of dehydroascorbate reductase (DHAR) and glutathione reductase (GR), which was compromised in TGase-deficient plants. Overall, TGase positively regulated photosynthesis by maintaining the activation states of the Benson–Calvin cycle and inducing changes in cellular redox homeostasis in tomato.

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