柑橘果实遮光处理对发育中的果实光合产物分配、糖代谢与积累的影响

研究了柑橘果实膨大初期遮光处理对果皮色素、果实含糖量、光合产物在果实内的分配及果实中蔗糖代谢相关酶活力变化的影响.结果表明遮光处理使果皮中的叶绿素含量迅速降低而类胡萝卜素积累缓慢,蔗糖的相对含量则明显上升.遮光处理还促进了光合产物向果皮运输,相应地降低了汁囊中光合产物分配比率,使果实汁囊中蔗糖含量下降.果皮中SS、SPS和转化酶活力在遮光处理后均有较大的提高,而汁囊中则差异不大.上述结果表明,在果实自身光合作用被抑制的条件下,果皮是通过提高酶活力来增强库强度,从而使其在与汁囊竞争中获得更多的光合产物,造成汁囊含糖量下降.     

套袋对梨果实发育过程中糖组分及其相关酶活性的影响

以翠冠和黄金梨为试材,测定套袋和未套袋(对照)梨果实发育时期果实中蔗糖、葡萄糖、果糖和山梨醇含量以及蔗糖代谢相关酶酸性转化酶(AI)、中性转化酶(NI)、蔗糖合成酶(SS)和蔗糖磷酸合成酶(SPS)的活性,并对果实中糖组分积累与酶活性的关系进行了分析.结果表明:(1)两梨品种套袋果实在发育过程中蔗糖、葡萄糖、果糖、山梨醇和糖代谢相关酶活性变化趋势与对照基本一致,套袋果实糖含量均低于对照但差异不显著,而各相关酶活性在两类果实间差异表现各异.(2)在梨果实发育早期,果实中以分解酶类为主,糖分积累低;发育后期以合成酶类为主,糖分积累多.(3)两品种套袋和对照果实AI活性与葡萄糖含量均呈显著或极显著正相关,SS合成方向活性与蔗糖含量均为极显著正相关,且翠冠对照果SPS活性与蔗糖含量呈极显著正相关.可见,套袋通过提高果实发育早期转化酶(Inv)活性,降低果实后期蔗糖磷酸合成酶(SPS)、蔗糖合成酶(SS)的活性来影响糖分积累,从而影响梨果品质.     

宁夏枸杞果实糖积累和蔗糖代谢相关酶活性的关系

通过对枸杞果实发育过程中果实生长模式、蔗糖、果糖、葡萄糖和淀粉含量及糖代谢相关酶活性的测定,研究了宁夏枸杞果实生长发育过程中糖的代谢积累与相关酶活性的关系.结果表明:(1)宁夏枸杞果实发育呈双S"曲线,果实主要以积累己糖为主.(2)蔗糖磷酸合成酶(SPS)活性在果实发育初期处于下降的趋势,在花后19d开始上升,果实转色后又逐渐下降;蔗糖合成酶(SS)活性总体表现为SS分解方向的活性大于SS合成方向的活性,说明枸杞果实发育过程中,SS的活性主要以分解方向的为主;酸性转化酶(AI)和中性转化酶(NI)的活性随果实发育呈上升趋势,但在果实成熟后期有所下降,且AI和NI活性高于合成酶类的活性,较高的转化酶类活性促进了果实内部己糖的积累.(3)在枸杞果实生长发育中,葡萄糖和果糖含量与AI和NI均呈极显著正相关,而与其它酶不具有相关性.说明AI和NI在宁夏枸杞果实的糖代谢中起着主要的调控作用.     

Carbohydrate availability affects growth and metabolism in peach fruit

Along with sucrose, sorbitol represents the main photosynthetic product and form of translocated carbon in peach. This study aimed at determining whether peach fruit carbohydrate metabolism is affected by changes in source–sink balance , and specifically whether sorbitol or sucrose availability regulates fruit enzyme activities and growth. In various trials, different levels of assimilate availability to growing fruits were induced in vivo by varying crop load of entire trees, leaf : fruit ratio (L:F) of fruiting shoots, or by interrupting the phloem stream (girdling) to individual fruits. In vitro, fruit tissue was incubated in presence/absence of sorbitol and sucrose. Relative growth rate (RGR), enzyme activities and carbohydrates were measured at different fruit growth stages of various peach cultivars in different years. At stage III, high crop load induced higher acid invertase (AI, EC 3.2.1.26) activities and hexose : sucrose ratios. Both sorbitol and sucrose contents were proportional to L:F, while sorbitol dehydrogenase (SDH, EC 1.1.1.14) activity was the only enzyme activity directly related to L:F in both fruit growth stages. Girdling reduced fruit RGR and all major carbohydrates after 4 days and SDH activity already after 48 h, but it did not affect sucrose synthase (SS, EC 2.4.1.13), AI and neutral invertase (NI, EC 3.2.1.27). Fruit incubation in sorbitol for 24 h induced higher SDH activities than in buffer alone. In general, assimilate availability affected both sorbitol and sucrose metabolism in peach fruit, and sorbitol may function as a signal for modulating SDH activity. Under highly competitive conditions, AI activity may be enhanced by assimilate depletion, providing a mechanism to increase fruit sink strength by increasing hexose concentrations.     

Effect of drought on sorbitol and sucrose metabolism in sinks and sources of peach

In peach (Prunus persica [L.] Batsch.), sorbitol and sucrose are the two main forms of photosynthetic and translocated carbon and may have different functions depending on the organ of utilization and its developmental stage. The role and interaction of sorbitol and sucrose metabolism was studied in mature leaves (source) and shoot tips (sinks) of ‘Nemaguard’ peach under drought stress. Plants were irrigated daily at rates of 100, 67, and 33% of evapotranspiration (ET). The relative elongation rate (RER) of growing shoots was measured daily. In mature leaves, water potential (Ψ_w), osmotic potential (Ψ_s), sorbitol‐6‐phosphate dehydrogenase (S6PDH, EC 1.1.1.200), and sucrose‐phosphate synthase (SPS, EC 2.4.1.14) activities were measured weekly. Measurements of Ψ_s, sorbitol dehydrogenase (SDH, 1.1.1.14), sucrose synthase (SS, EC 2.4.1.13), acid invertase (AI, EC 3.2.1.26), and neutral invertase (NI, EC 3.2.1.27) activities were taken weekly in shoot tips. Drought stress reduced RER and Ψ_w of plants in proportion to water supply. Osmotic adjustment was detected by the second week of treatment in mature leaves and by the third week in shoot tips. Both SDH and S6PDH activities were reduced by drought stress within 4 days of treatment and positively correlated with overall Ψ_w levels. However, only SDH activity was correlated with Ψ_s. Among the sucrose enzymes, only SS was affected by drought, being reduced after 3 weeks. Sorbitol accumulation in both mature leaves and shoot tips of stressed plants was observed starting from the second week of treatment and reached up to 80% of total solutes involved in osmotic adjustment. Sucrose content was up to 8‐fold lower than sorbitol content and accumulated only occasionally. We conclude that a loss of SDH activity in sinks leads to osmotic adjustment via sorbitol accumulation in peach. We propose an adaptive role of sorbitol metabolism versus a maintenance role of sucrose metabolism in peach under drought stress.     

‘台农1号’芒果果实发育过程中的糖分积累与相关酶活性研究

以‘台农1号’芒果为材料,测定了果实生长发育过程中淀粉、蔗糖、葡萄糖和果糖含量以及淀粉酶、蔗糖代谢相关酶———酸性转化酶(AI)、中性转化酶(NI)、蔗糖合成酶(SS)和蔗糖磷酸合成酶(SPS)的活性,并对果实中糖组分与酶活性的关系进行了分析.结果显示,(1)台农1号芒果果实属于单S型生长曲线,发育前期主要积累淀粉、葡萄糖和果糖,果实成熟软化时,淀粉酶活性降至最低,淀粉水解,蔗糖快速积累.(2)酸性转化酶活性在果实整个发育过程中维持最高,完熟时略有降低;蔗糖磷酸合成酶在果实发育前期略有降低,完熟时升至最高;蔗糖合成酶和中性转化酶活性在整个发育期一直很低且较稳定.(3)淀粉含量与淀粉酶活性呈显著正相关,与SPS活性呈极显著负相关,蔗糖、葡萄糖含量均与SPS、SS呈显著、极显著的正相关;果糖含量与SS呈极显著的正相关.研究表明,芒果成熟时淀粉分解、酸性转化酶活性的降低,且蔗糖合成酶和蔗糖磷酸合成酶活性的增加是引起果实蔗糖积累的主要因子.     

The role of sucrose-metabolizing enzymes in pear fruit that differ in sucrose accumulation

In the paper, the soluble sugar composition and activities of enzymes metabolizing sucrose: invertase (β-fructosidase, EC 3.2.1.26), sucrose synthase (SS; EC 2.4.1.13) and sucrose-phosphate synthase (SPS; EC 2.4.1.14) were investigated during fruit development of two pear species: Pyrus bretschneideri Rehd. cv. ‘Yali’ and P. pyrifolia Nakai cv. ‘Aikansui’, characterized as low and high sucrose types, respectively. It was found that, at the end of fruit development of ‘Aikansui’, the level of sucrose was five times higher than in ‘Yali’ in the same period. It was coincident with the significantly higher activities of SS (synthesis) and SPS and lower activities of invertase (vacuolar and cell wall-bound acid invertase and neutral invertase). The high correlation was found between sucrose level and SS (synthesis) and SPS activities in ‘Aikansui’ pears.     

Sucrose Phosphate Synthase and Acid Invertase as Determinants of Sucrose Concentration in Developing Muskmelon (Cucumis melo L.) Fruits

Fruits of orange-fleshed and green-fleshed muskmelon (Cucumis melo L.) were harvested at different times throughout development to evaluate changes in metabolism which lead to sucrose accumulation, and to determine the basis of differences in fruit sucrose accumulation among genotypes. Concentrations of sucrose, raffinose saccharides, hexoses and starch, as well as activities of the sucrose metabolizing enzymes sucrose phosphate synthase (SPS) (EC 2.4.1.14), sucrose synthase (EC 2.4.1.13), and acid and neutral invertases (EC 3.2.1.26) were measured. Sucrose synthase and neutral invertase activities were relatively low (1.7 ± 0.3 micromole per hour per gram fresh weight and 2.2 ± 0.2, respectively) and changed little throughout fruit development. Acid invertase activity decreased during fruit development, (from as high as 40 micromoles per hour per gram fresh weight) in unripe fruit, to undetectable activity in mature, ripened fruits, while SPS activity in the fruit increased (from 7 micromoles per hour per gram fresh weight) to as high as 32 micromoles per hour per gram fresh weight. Genotypes which accumulated different amounts of sucrose had similar acid invertase activity but differed in SPS activity. Our results indicate that both acid invertase and SPS are determinants of sucrose accumulation in melon fruit. However, the decline in acid invertase appears to be a normal function of fruit maturation, and is not the primary factor which determines sucrose accumulation. Rather, the capacity for sucrose synthesis, reflected in the activity of SPS, appears to determine sucrose accumulation, which is an important component of fruit quality.     

Roles of gibberellins in increasing sink demand in Japanese pear fruit during rapid fruit growth

Our previous work demonstrated that exogenous gibberellins (GAs) applications during rapid fruit growth significantly increases sink demand and results in a larger fruit in Japanese pear. In an attempt to unravel the mechanism of increased sink demand by applied GAs, the histology, cell wall components of the flesh, and carbon accumulation in the fruit were assessed for Japanese pear (Pyrus pyrifolia, cultivar ‘Kousui’), as were the activities of sucrose- and sorbitol-cleaving enzymes. Our results show that most vascular tissues occurred in core tissue with very little vascular tissue in the flesh. Application of a mixture of GA₃ + GA₄ in lanolin paste significantly increased the amount of ethanol-insoluble solids, e.g., total pectins, hemicellulose, and cellulose in the cell walls. There was a significantly increased sink demand (assessed by ¹³C accumulation in the fruit) by the applied GAs, and this increased sink strength was closely related to increased activities of cell wall-bound invertase in the core, neutral invertase and NAD-dependent sorbitol dehydrogenase in the flesh during rapid fruit growth. As well, concentrations of sorbitol and sucrose in the flesh were decreased by GA application, while glucose concentration increased. Most importantly, the fact that sink activity can be increased by GA application implies that endogenous GAs are likely to be important modulators for sugar metabolism. Hence, selecting for genotypes with elevated GA production in the growing fruit and increased activities of key enzymes for sugar metabolism could result in increased fruit size.     

Dynamic characteristics of sugar accumulation and related enzyme activities in sweet and non-sweet watermelon fruits

Sugar content largely determines watermelon fruit quality. We compared changes in sugar accumulation and activities of carbohydrate enzymes in the flesh (central portion) and mesocarp of elite sweet watermelon line 97103 (Citrullus lanatus subsp. vulgaris) and exotic non-sweet line PI296341-FR (C. lanatus subsp. lanatus) to elucidate the physiological and biochemical mechanisms of sugar accumulation in watermelon fruit. The major translocated sugars, raffinose and stachyose, were more unloaded into sweet watermelon fruit than non-sweet fruit. During the fruit development, acid α-galactosidase activity was much higher in flesh of 97103 than in mesocarp of 97103, in flesh and mesocarp of PI296341-FR fruit. Insoluble acid invertase activity was higher in 97103 flesh than in 97103 mesocarp, PI296341-FR flesh or mesocarp from 18 days after pollination (DAP) to 34 DAP. Changes in soluble acid invertase activity in 97103 flesh were similar to those in PI296341-FR flesh and mesocarp from 18 DAP to full ripening. Sucrose synthase and sucrose phosphate synthase activities in 97103 flesh were significantly higher than those in 97103 mesocarp and PI296341-FR fruits from 18 to 34 DAP. Only insoluble acid invertase and sucrose phosphate synthase activities were significantly positively correlated with sucrose content in 97103 flesh. Therefore, phloem loading, distribution and metabolism of major translocated sugars, which are controlled by key sugar metabolism enzymes, determine fruit sugar accumulation in sweet and non-sweet watermelon and reflect the distribution diversity of translocated sugars between subspecies.     

Cloning and expression of genes related to the sucrose-metabolizing enzymes and carbohydrate changes in peach

To shed light on the relationship between sucrose metabolism and expression of genes related to sucrose-metabolizing enzymes, six genes encoding sucrose-metabolizing enzymes were isolated, and the levels of four main carbohydrates and related enzyme activities as well as the expression of these six genes were determined in fruits, leaves and phloem-enriched fraction throughout peach fruit development. Sucrose content in mature fruit ranked first followed by glucose, fructose and sorbitol in that order, while sorbitol was the highest and sucrose lowest in phloem-enriched fraction and leaves. Glucose and fructose had similar change patterns throughout fruit development. Cloning results reveal that the nucleotide sequences of the six genes have high similarity to corresponding genes isolated from other plants. In addition, the expression of these genes and the levels of related enzyme activities varied with tissue and stage of fruit development, suggesting a complexity in relationships between carbohydrates, enzymes activities and related gene expression. Sucrose phosphate synthase maybe a key enzyme involved in sucrose synthesis while sucrose synthase may mainly be responsible for sucrose synthesis in peach fruits at later stages of development. Further studies are needed to genetically and physiologically characterize these genes and enzymes in peach and to gain a better understanding of their functions and relationship with carbohydrate metabolism.     

Sink Metabolism in Tomato Fruit : III. Analysis of Carbohydrate Assimilation in a Wild Species

Carbohydrate composition and key enzymes involved in carbohydrate metabolism were assayed throughout development of Lycopersicon esculentum and L. chmielewskii fruit. Translocation and assimilation of asymmetric sucrose and total soluble solids content was also determined in both species. The data showed that L. chmielewskii accumulated less starch than L. esculentum, and this was related to a lower level of ADPglucose pyrophosphorylase and a higher level of phosphorylase in L. chmielewskii. L. chmielewskii accumulated sucrose throughout fruit development rather than glucose and fructose which were accumulated by L. esculentum. A low level of invertase and nondetectable levels of sucrose synthase were associated with the high level of sucrose in L. chmielewskii. Translocation and assimilation of asymmetrically labeled sucrose indicated that sucrose accumulated in L. chmielewskii fruit was imported and stored directly in the fruit without intervening metabolism along the translocation path. In contrast, the relatively low level of radioactive sucrose found in L. esculentum fruit appeared to arise from hydrolysis and resynthesis of sucrose. The possible relationship between the level of soluble solids and differences in carbohydrate metabolism in sink tissue of the two species is discussed.     

网纹甜瓜发育果实糖分积累与蔗糖代谢参与酶的关系

随着网纹甜瓜果实的发育,果实中葡萄糖和果糖的含量增加,蔗糖的快速积累发生在果实发育的中后期,高蔗糖积累型果实中蔗糖积累速率明显快于低蔗糖积累型.蔗糖磷酸合成酶活性在果实发育的前期短暂下降, 而后稳步上升,在果实发育的中后期高蔗糖积累型果实中该酶的活性显著高于低蔗糖积累型果实;随着果实发育,蔗糖合成酶的分解活性降低而合成活性升高.酸性和中性转化酶在未成熟果实中活性较高,而在成熟果实中很低; 高蔗糖积累型果实中酸性转化酶活性显著低于同期低蔗糖积累型果实.合成蔗糖的酶活性小于分解蔗糖的酶活性时蔗糖几乎没有积累.根据这些结果推测,转化酶活性的下降、蔗糖磷酸合成酶活性的增加以及蔗糖合成酶分解活性的下降和合成活性的增加,是引起果实蔗糖积累的主要内在因子.     

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.