Understanding of exogenous auxin in regulating sucrose metabolism during postharvest tomato fruit ripening

Auxin is involved in regulating the ripening process of tomato fruit, and sucrose metabolism is a vital part of the quality formation during tomato ripening. In comparison, the comprehensive effect of auxin on regulating sucrose metabolism during postharvest tomato fruit ripening remains to be clarified. In this research, tomato fruit at the mature green stage were treated with 2,4-dichlorophenoxyacetic acid (0.45 mM) or deionized water under vacuum respectively. The ethylene production, a* value, firmness, and the contents of sucrose, fructose and glucose, enzymatic activities, also the expressions of major genes related to sucrose metabolism, were measured every 3 d after treatment during the 22 d of storage. Results showed exogenous auxin inhibited tomato fruit ripening, along with the lower a* value and the higher firmness. Meanwhile, exogenous auxin inhibited ethylene production, activities of sucrose phosphate synthase (SPS) and sucrose synthase (SUS), whereas enhanced sucrose hydrolysis and acid invertase (AI) activity. These activities above caused the lower sucrose content during tomato fruit ripening. Furthermore, exogenous auxin changed the expressions of major genes in response to sucrose metabolism, including the induction of sucrose-phosphate synthase3 (SPS3), vacuolar invertase1 (VIN1; TIV1), and cell wall invertase1-3 (CWIN1-3; Lin5-7) (log2 fold change ranged from 1.10 to 7.98), the inhibition of SPS1, SPS4 and sucrosesynthase3 (SUS3) (-3.60 <= log2 fold change <= -1.67), and the fluctuation of transcript levels of cytoplasmic invertase6 (CIN6), SUS6, SUS2 and vacuolar invertase2 (VIN2; Lin9) (-8.94 <= log2 fold change <= 3.45). Results indicated auxin might affect sucrose metabolism in the ripening process of postharvest tomato fruit by means of controlling the corresponding enzymatic activities and transcription profiles of key genes to a certain extent, suggesting auxin is probably not the main key regulator of sucrose accumulation during tomato fruit ripening. This study provides an illuminating insight into the regulatory mechanism of auxin in tomato fruit sucrose metabolism.