Decreased sink/source ratio enhances hexose transport in the fruits of greenhouse tomatoes: integration of gene expression and biochemical analyses

Abstract

To examine the physiological role of hexose transporters in determining the sink strength of individual fruits, the regulation of hexose transporters gene expression was studied when the sink/source ratio was artificially altered under the greenhouse condition; this was done in two cultivars of tomato, i.e. Grandella and Isabella. The sink/source ratio treatments included: saving one fruit per truss (1F), two fruits per truss (2F), three fruits per truss (3F) and no fruit pruning (control). The results showed that fruit thinning could increase starch, sucrose, and hexose contents in the fruits; it could also modulate the activity of the key enzymes and the expression of tomato hexose transporter genes (LeHTs). Based on the relative transcript levels, all examined LeHTs were unregulated at the end of cell division and the cell expansion stage of fruit development, but the strongest expression level observed at the onset of ripening was related to LeHT1 and LeHT2. Given the concomitancy of cell wall invertase (EC 3.2.1.26) activity and the LeHTs relative expression cell wall, invertase activity seemed to be involved in the expression level of LeHTs. The increased trends of the LeHTs expression with the decrease of the sink/source ratio confirmed the role of hexose transporters in determining the sink strength of the tomato fruits.

Hydrogen Sulfide Alleviates Cadmium Stress by Enhancing Photosynthetic Efficiency and Regulating Sugar Metabolism in Wheat Seedlings

“…This mainly occurs in the form of SAInv and A/NInv according to the optimal pH [70]. When plant cells respond to hexoses, these enzymes regulate the intracellular sucrose/hexose ratio [71]. In the present study, the genes (TaSAInv, TaA/NInv, and TaSPS) that catalyzed sucrose decomposition and synthesis were significantly up-regulated under the stress of Cd.…”

Metabolite Fruit Profile Is Altered in Response to Source–Sink Imbalance and Can Be Used as an Early Predictor of Fruit Quality in Nectarine

“…These are the main exportable sugars in P. persica, and they were previously reported as the major soluble sugars in the leaves of this species (Moing et al, 1992). The increased content of exportable sugars together with the increase in the expression of sugar transporters related to phloem loading/unloading in response to thinning (Nuñez et al, 2019;Aslani et al, 2020) probably sustained the high metabolic rates and carbohydrate demand to sustain cell proliferation and endocarp lignification at S1 and S2 phases in fruit from thinned trees. We did not observe accumulation of these sugars in fruits, because once they arrived in the fruit, they were converted to glucose and fructose to sustain respiration and also directed to other biosynthetic pathways such as synthesis of cell wall components, amino acids, and precursors related to lignin synthesis.…”

GABA shunt pathway is stimulated in response to early defoliation-induced carbohydrate limitation in Mandarin fruits

“…In contrast, a decline in sugar content was observed in tomato [ 34 ], muskmelon [ 17 ] and mango [ 21 ] in response to carbohydrate limitation, and may be related to the time at which defoliation is applied during fruit growth. Among the enzymes involved in sucrose metabolism, invertase hydrolyzes sucrose to glucose and fructose, which contributes to the regulation of carbohydrates translocated into the sink tissues [ 35 ]. While acid invertase is mainly found in the vacuole and cell wall, neutral invertase is located in the cytoplasm [ 36 ].…”

“…Resulting glucose and fructose were then catabolized in the cytosol. A previous study also reported that acid invertase activity is enhanced for hexose unloading out of the vacuole, during the second stage of development in tomato fruits subjected to source/sink ratio change via pruning [ 35 ]. Glycolysis is the main pathway catabolizing hexoses in the cytosol.…”