果实中糖的运输、代谢与积累及其调控

叶片光合产物向果实运输的主要形态是蔗糖,但在木本蔷薇科果树中,光合产物的主要运输形态为山梨醇.糖从质外体空间跨膜运入共质体的过程由糖运输蛋白介导,而糖运输蛋白的基因表达伴随着果实糖的积累而增强.蔗糖代谢酶参与了细胞内外4个与糖运输有关的无效循环.己糖代谢抑制是果实糖快速积累的前提.在木本蔷薇科果实中,蔗糖代谢酶活力仍非常活跃,表明蔗糖可能与山梨醇在果实生长发育中都起重要的作用.糖作为信号分子,调节了承担糖运输与代谢的基因的表达.自然环境因子和栽培措施能有效调控糖运输、代谢与积累.反义抑制Ivr基因表达能提高番茄果实含糖量的实验结果表明遗传工程调控糖积累的潜力.阐明糖信号与其它信号互作对糖运输与代谢的调控机制是今后研究的重点.

Sugar Transport,Metabolism,Accumulation and Their Regulation in Fruits

Photosynthates transported into fruits aremainly in the form of sucrose in most fruit treespecies; but sorbitol takes the place of sucrose inwoody Rosaceae plants. The transport of sugarsacross the plasma membrane from apoplastic spaceinto cells is mediated by sugar transporters. Thefact that gene expression of sugar transporters isupregulated just before and during sugar accumu-lation suggests the participation of sugar transport-ers in sugar accumulation of fruit. The sucrose-me-tabolizing enzymes participate in four futile cyclesthat involve sugar transport between cytosol,vacuole, amyloplast and apoplast. The increase inSS (sucrose synthase) and SPS (sucrose phosphatesynthase) activities and mRNA levels during matu-ration parallels the increase in sugar accumulationindicates that the sucrose-metabolizing enzymeshave important roles on sugar accumulation in fruits.The prerequisite for rapid accumulation of sugar infruit is restriction of hexose catabolism and promo-tion of its synthesis. In woody Rosaceae plants,the fact that sucrose metabolism is also quite activein fruit suggests that sorbitol and sucrose probablyplay similar roles in fruit development. Sugars assignal molecules regulate the expression of genesinvolved in sugar transport and metabolism. Sugartransport, metabolism and accumulation are alsoregulated by natural environmental factors and cul-tural practices. The increase in sugar content of to-mato fruit in acid invertase gene antisense-inhibitedplants provides promising prospect of genetic en-gineering as a potential effective technique in regu-lation of sugar accumulation in fruits. Thus, the sugarcontent of fruit is determined by both intrinsic andextrinsic factors. The future research works will befocused on elucidating the mechanism of sugar sig-nal and other intrinsic signals as well as extrinsicsignals including nutrients, plant hormones andphysical factors on sugar transport, metabolism andaccumulation and the interrelationship among them.