Import of sucrose and its transformation to starch in the developing sorghum caryopsis

Levels of soluble and bound invertases and amylases were studied in relation to the changes in the free sugars and the accumulation of starch in the developing sorghum [Sorghum bicolor (L.) Moench, cv. spv. 351] caryopsis and its associated bractspedicel. Besides sucrose, glucose and fructose as the principal sugars, small amounts of sugars of the raffinose series were detected in the developing caryopsis. Through out the period of caryopsis development, the amount of reducing sugars was higher than that of sucrose. With the advancement in the development of the caryopsis, the contents and levels of sucrose rose with a concomitant fall in the activity of soluble acid (pH 4.8) invertase (EC 3.2.1.26) in the endosperm. In the pericarp-aleurone layer, the activity of soluble acid invertase predominated over soluble neutral (pH 7.5) invertase (EC 3.2.1.27). The activity of bound acid invertase declined with the ageing of the caryopsis. In bracts-pedicel, the activity of bound invertase and the levels of reducing sugars peaked around 18 days post anthesis. In these organs, the level of starch gradually decreased concomitantly with an increase in its level in the developing caryopsis. Amylases (EC 3.2.1.1 and 3.2.1.2) are distributed in the endosperm as well as in the pericarp-aleurone layer. On culturing detached ears in [U-¹⁴C]-sucrose solution for 6 h in the dark at 25°C, 80–90% of the ¹⁴C of extracted major sugars (i.e. sucrose + glucose + fructose) of the caryopsis appeared in sucrose alone. In comparison with the effects of glucose or fructose, transport into the caryopsis of ¹⁴C from [U-¹⁴C]-sucrose supplied to detached ears was promoted by the addition to the radiolabelled sucrose solution of 1% unlabelled sucrose. Addition to the [U-¹⁴C]-sucrose solution fed to the detached ears of 20 mM NaN₃ or HgCl₂ or galactose, lowered the amount of ¹⁴C in the free sugars and starch of the earyopsis.     

Interconversion of free sugars in relation to activities of enzymes catalyzing synthesis and cleavage of sucrose in growing stem tissues of sorghum

Free sugar interconversion and activities of soluble acidic (pH 4.8) and neutral (pH 7.5) invertases, sucrose synthase (synthesis) and sucrose phosphate synthase were investigated in the growing nodes and internodes of sorghum (Sorghum vulgare). The results were substantiated with incorporation of 14C from supplied sucrose and hexoses into endogenous sugars of these stem tissues. With the advancement in plant growth, the content of total free sugars in apical nodes and internodes increased till 70 DAS (flowering stage) followed by a decline. In the corresponding basal tissues, the sugar build-up continued even beyond this stage of plant growth. Compared with basal stem tissues, the apical ones contained high activities of soluble invertases and a low proportion amongst free sugars of sucrose. The activities of sucrose-hydrolyzing enzymes were higher as compared with those of sucrose-synthesizing ones in both nodes and internodes and with the growth of plant, the activity of neutral invertase increased in these tissues. More 14C from supplied sucrose and hexoses appeared in extracted sugars from cut discs of apical nodes and internodes in comparison with their basal counterparts. 14C from supplied sucrose appeared in glucose, fructose and from supplied hexoses appeared in sucrose. The results suggest that in apical nodes and internodes, where a rapid cell division and cell expansion occur, sucrose is obligatorily inverted to meet the increased requirement of hexoses and there is a compartmentalized synthesis and cleavage of sucrose in the nodes and internodes of growing sorghum plant.     

Short-term effects of experimental warming and enhanced ultraviolet-B radiation on photosynthesis and antioxidant defense of <Emphasis Type="Italic">Picea asperata</Emphasis> seedlings

It is generally accepted that sucrose synthase (SuSy), ADP-glucose pyrophosphorylase (AGPase), soluble starch synthase (SSS), granule-bound starch synthases (GBSS) and starch branching enzyme (SBE) play a key role in starch synthesis in wheat grains. Starch synthesis in wheat grains is influenced by genotype and environment. However, what is not known is the degree of variation in enzyme activity during starch accumulation of wheat cultivars differing in kernel types. The present study was carried out to characterize the changing activities of key enzymes during grain filling in two kernel type winter wheat cultivars. Results showed that starch accumulation rate (SAR) and activities of SuSy, AGPase, SSS, GBSS and SBE in large kernel types were significantly higher than those in small kernel types. The soil water deficit experienced during the course of the experiment led to an increase at early grain-filling period and decrease during late grain-filling, respectively, in SAR and activities of key enzymes involved in starch synthesis, especially SuSy, AGPase, SSS, and SBE. Water deficit enhanced grain starch accumulation in small kernel types. It suggests that rainfed treatment increase physiological activities during early grain-filling and promote starch accumulation in small kernel types. The simulation with Richards’ equation showed that it was accumulation duration and SAR that determined the starch accumulation in large kernel types. Compared with small kernel types, plants of large kernel types maintained longer filling duration, higher SAR and greater activities of related enzymes during mid and late grain-filling. These observations suggest stronger sink activities in large kernel types at a later stage of development. Consequently, large kernel types have advantages over the small kernel types in terms of the amount of starch accumulation at mid and late stage, but are sensitive to water deficit.     

Activities of key enzymes in sucrose-to-starch conversion in wheat grains subjected to water deficit during grain filling

This study tested the hypothesis that a controlled water deficit during grain filling of wheat (Triticum aestivum) could accelerate grain-filling rate through regulating the key enzymes involved in Suc-to-starch pathway in the grains. Two high lodging-resistant wheat cultivars were field grown. Well-watered and water-deficit (WD) treatments were imposed from 9 DPA until maturity. The WD promoted the reallocation of prefixed 14C from the stems to grains, shortened the grain-filling period, and increased grain-filling rate or starch accumulation rate (SAR) in the grains. Activities of Suc synthase (SuSase), soluble starch synthase (SSS), and starch branching enzyme (SBE) in the grains were substantially enhanced by WD and positively correlated with the SAR. ADP Glc pyrophosphorylase activity was also enhanced in WD grains initially and correlated with SAR with a smaller coefficient. Activities of granule-bound starch synthase and soluble and insoluble acid invertase in the grains were less affected by WD. Abscisic acid (ABA) content in the grains was remarkably enhanced by WD and very significantly correlated with activities of SuSase, SSS, and SBE. Application of ABA on well-watered plants showed similar results as those by WD. Spraying with fluridone, an ABA synthesis inhibitor, had the opposite effect. The results suggest that increased grain-filling rate is mainly attributed to the enhanced sink activity by regulating key enzymes involved in Suc-to-starch conversion, especially SuSase, SSS, and SBE, in wheat grains when subjected to a mild water deficit during grain filling, and ABA plays a vital role in the regulation of this process.     

Carbohydrate metabolism during fruit development in sweet pepper (Capsicum annuum) plants

Growth, accumulation of sugars and starch, and the activity of enzymes involved in sucrose mobilization were determined throughout the development of sweet pepper fruits. Fruit development was roughly divided into three phases: (1) an initial phase with high relative growth rate and hexose accumulation, (2) a phase with declining growth rate and accumulation of sucrose and starch, and (3) a ripening phase with no further fresh weight increase and with accumulation of hexoses, while sucrose and starch were degraded. Acid and neutral invertase (EC 3.2.1.26) were closely correlated to relative growth rate until ripening and inversly correlated to the accumulation of sucrose. Acid invertase specifically increased during ripening, concurrently with the accumulation of hexoses. Sucrose synthase (EC 2.4.1.13) showed little correlation to fruit development, and in periods of rapid growth the activity of sucrose synthase was low compared to the invertases. However, during late fruit growth sucose synthase was more active than the invertases. We conclude that invertase activities determine the accumulation of assimilates in the very young fruits, and a reactivation of acid invertase is responsible for the accumulation of hexoses during ripening. During late fruit growth, before ripening, sucrose synthase is transiently responsible for the sucrose breakdown in the fruit tissue. Results also indicate that pyrophosphate-dependent phosphofructokinase (EC 2.7.1.90) and its activator fructose-2,6-bisphosphate (Fru2,6bisP) are involved in the regulation of the sink metabolism of the fruit tissue.     

Carbohydrate metabolism in growing rice seedlings under arsenic toxicity

We studied in the seedlings of two rice cultivars (Malviya-36 and Pant-12) the effect of increasing levels of arsenic in situ on the content of sugars and the activity of several enzymes of starch and sucrose metabolism: alpha-amylase (EC 3.2.1.1), beta-amylase (EC 3.2.1.2), starch phosphorylase (EC 2.4.1.1), acid invertase (EC 3.2.1.26), sucrose synthase (EC 2.4.1.13) and sucrose phosphate synthase (EC 2.4.1.14). During a growth period of 10-20 d As2O3 at 25 and 50 microM in the growth medium caused an increase in reducing, non-reducing and total soluble sugars. An increased conversion of non-reducing to reducing sugars was observed concomitant with As toxicity. The activities of alpha-amylase, beta-amylase and sucrose phosphate synthase declined, whereas starch phosphorylase, acid invertase and sucrose synthase were found to be elevated. Results indicate that in rice seedlings arsenic toxicity causes perturbations in carbohydrate metabolism leading to the accumulation of soluble sugars by altering enzyme activity. Sucrose synthase possibly plays a positive role in synthesis of sucrose under As-toxicity.     

Enzymes of carbohydrate metabolism in the developing rice grain

The levels of reducing and nonreducing sugars, starch, soluble protein, and selected enzymes involved in the metabolism of sucrose, glucose-1-P, and glucose nucleotides were assayed in dehulled developing rice grains (Oryza sativa L. line IR1541-76-3) during the first 3 weeks after flowering. The level of reducing sugars in the grain was highest 5 to 6 days after flowering. The level of nonreducing sugars and the rate of starch accumulation were maximum 11 to 12 days after flowering, when the level of soluble protein was also the highest. The activities of bound and free invertase, sucrose-UDP and sucrose-ADP glucosyltransferases, hexokinase, phosphoglucomutase, nucleoside diphosphokinase, and UDP-glucose and ADP-glucose pyrophosphorylases were high throughout starch deposition, and were maximum, except for nucleoside diphosphokinase which did not increase in activity, between 8 and 18 days after flowering. Soluble primed phosphorylase and ADP glucose-α-glucosyltransferase (starch synthetase) were both present during starch accumulation. Phosphorylase activity was at least 2-fold that of soluble starch synthetase but the synthetase followed more closely the rate of starch accumulation in the grain. The activity of starch synthetase bound to the starch granule also increased progressively with increased starch content of the grain.     

Invertase-mediated Interconversion of Sucrose and Hexoses During Their Translocation in Growing Pearl Millet Plant

Metabolism of free sugars, particularly sucrose, in various plant tissues enroute from leaf sheaths to grains in growing pearl millet was studied. With the enhancement in growth, the levels of both reducing and non-reducing sugars declined in middle and basal leaf sheaths but increased in flag leaf sheath towards plant maturity. In sheath, wall-bound invertase was more active than soluble invertases and the activities of all these enzymes rose towards maturity. Besides hexoses and sucrose, some fructose polymers were also detected in the internodes. In contrast with internodes, where the levels of total free sugars declined till around anthesis, in penultimate node their levels continuously increased, but attained peak values at 65 CAS in middle- and basal nodes. In both these tissues, arriving sucrose encounters invertases but in nodes wall-bound invertase appears to be the pivotal one. On feeding (U-¹⁴C)-sucrose to the detached ear-heads a large proportion of ¹⁴C was incorporated into hexoses alone in peduncle and rachis. PCMBS and HgCl₂ inhibited the metabolism of sucrose supplied to peduncle and rachis pointing to the involvement of invertases in sucrose cleavage in these organs. Through the regulated operation of invertase(s), the nodes seem to maintain a controlled flow of free sugars from source to sink tissue.     

Enzyme activities of starch and sucrose pathways and growth of apical and Basal maize kernels

Apical kernels of maize (Zea mays L.) ears have smaller size and lower growth rates than basal kernels. To improve our understanding of this difference, the developmental patterns of starch-synthesis-pathway enzyme activities and accumulation of sugars and starch was determined in apical- and basal-kernel endosperm of greenhouse-grown maize (cultivar Cornell 175) plants. Plants were synchronously pollinated, kernels were sampled from apical and basal ear positions throughout kernel development, and enzyme activities were measured in crude preparations. Several factors were correlated with the higher dry matter accumulation rate and larger mature kernel size of basal-kernel endosperm. During the period of cell expansion (7 to 19 days after pollination), the activity of insoluble (acid) invertase and sucose concentration in endosperm of basal kernels exceeded that in apical kernels. Soluble (alkaline) invertase was also high during this stage but was the same in endosperm of basal and apical kernels, while glucose concentration was higher in apical-kernel endosperm. During the period of maximal starch synthesis, the activities of sucrose synthase, ADP-Glc-pyrophosphorylase, and insoluble (granule-bound) ADP-Glc-starch synthase were higher in endosperm of basal than apical kernels. Soluble ADP-Glc-starch synthase, which was maximal during the early stage before starch accumulated, was the same in endosperm from apical and basal kernels. It appeared that differences in metabolic potential between apical and basal kernels were established at an early stage in kernel development.     

Biochemical and genomic analysis of sucrose metabolism during coffee (Coffea arabica) fruit development

Sucrose metabolism and the role of sucrose synthase were investigated in the fruit tissues (pericarp, perisperm, and endosperm) of Coffea arabica during development. Acid invertase, sucrose phosphate synthase, and sucrose synthase activities were monitored and compared with the levels of sucrose and reducing sugars. Among these enzymes, sucrose synthase showed the highest activities during the last stage of endosperm and pericarp development and this activity paralleled closely the accumulation of sucrose in these tissues at this stage. Carbon partitioning in fruits was studied by pulse-chase experiments with (14)C-sugars and revealed high rates of sucrose turnover in perisperm and endosperm tissues. Additional feeding experiments with (14)CO(2) showed that leaf photosynthesis contributed more to seed development than the pericarp in terms of photosynthate supply to the endosperm. Sugar analysis, feeding experiments, and histological studies indicated that the perisperm plays an important role in this downloading process. It was observed that the perisperm presents a transient accumulation of starch which is degraded as the seed develops. Two full-length cDNAs (CaSUS1 and CaSUS2) and the complete gene sequence of the latter were also isolated. They encode sucrose synthase isoforms that are phylogenetically distinct, indicating their involvement in different physiological functions during cherry development. Contrasting expression patterns were observed for CaSUS1 and CaSUS2 in perisperm, endosperm, and pericarp tissues: CaSUS1 mRNAs accumulated mainly during the early development of perisperm and endosperm, as well as during pericarp growing phases, whereas those of CaSUS2 paralleled sucrose synthase activity in the last weeks of pericarp and endosperm development. Taken together, these results indicate that sucrose synthase plays an important role in sugar metabolism during sucrose accumulation in the coffee fruit.     

Sucrose and starch metabolism during Fargesia yunnanensis shoot growth

Bamboo is one of the fastest growing plants in the world, but their shoot buds develop very slowly. Information about the sugar storage and metabolism during the shoot growth is lacking. In the present study, we determined the activity of sucrose and starch metabolizing enzymes during the developmental period of Fargesia yunnanensis from shoot buds to the young culms that have achieved their full height. The soluble sugars and starch contents were also determined and analyzed in shoot buds and shoots at different developmental stages. The results showed that there were higher sucrose contents in shoot buds than shoots, which coincides with the sweeter taste of shoot buds. As the shoot buds sprouted out of the ground, the starch and sucrose were depleted sharply. Coupled with this, the activity of soluble acid invertase (SAI), cell wall-bound invertase (CWI), sucrose synthase at cleavage direction (SUSYC) and starch phosphorylase (STP) increased significantly in the rapidly elongating internodes. These enzymes dominated the rapid elongation of internodes. The activities of SAI, CWI, SUSYC and STP and adenosine diphosphate-glucose pyrophosphorylase were higher as compared to other enzymes in the shoot buds, but were far lower than those in the developing shoots. The slow growth of shoot buds was correlated with the low activity of these enzymes. These results complement our understanding of the physiological differences between shoot buds and elongating shoots and ascertain the physiological mechanism for the rapid growth of bamboo shoots.     

结实期土壤水分亏缺影响水稻籽粒灌浆的生理原因

通过分析结实期土壤水分亏缺对水稻(Oryza sativa)籽粒中蔗糖向淀粉合成的生理代谢中关键酶活性及籽粒灌浆的调节作用, 探讨土壤水分亏缺影响水稻籽粒灌浆的生理机制。结果表明, 适度土壤水分亏缺诱导了灌浆高峰期(花后15-20天)水稻籽粒中蔗糖合成酶、腺苷二磷酶葡萄糖焦磷酸化酶、可溶性淀粉合成酶及淀粉分支酶活性的增加, 提高了籽粒灌浆中前期(花后10-20天)籽粒中淀粉积累速率和籽粒灌浆速率。但在灌浆后期(花后20-30天)籽粒中, 上述关键酶活性下降较快, 籽粒活跃灌浆期明显缩短, 灌浆前中期灌浆速率的增加不能完全补偿灌浆期缩短带来的同化物积累损失, 导致水分亏缺处理水稻籽粒充实不良, 结实率、籽粒重和产量显著降低。研究认为, 灌浆期土壤水分亏缺引起的灌浆后期籽粒中蔗糖向淀粉合成代谢中一些关键酶活性快速下降和籽粒内容物的供应不足是籽粒淀粉积累总量减少、粒重降低的主要生理原因。     

Metabolism of carbohydrates during the development of seeds of the brazilian rubber tree [Hevea brasiliensis (Willd. Ex Adr. de Juss) Muell.-Arg.]

This work aimed at the assessment of the metabolism of carbohydrate during the development of the seeds of Brazilian rubber trees. The enzymatic activity of the acid invertase, neutral invertase and sucrose synthase (SuSy) and the levels of total soluble sugars (TSS), reducing sugars (RS) and sucrose were evaluated separately in each part of the fruit and seed—pericarp, seed coat, embryo and endosperm—on different days after the pollination (DAP). Based on the results obtained in this study, it is possible to conclude that in the beginning of the development of the rubber tree seeds, until 95 DAP, the endosperm presents high concentration of RS and low concentration of sucrose. After this period, the endosperm of the seed initiates starch accumulation and the concentration of RS decreases followed by the increase in the concentration of sucrose, presenting, after 120 DAP, an inversion of concentration of these two sugars. In the embryo, the levels of TSS, RS and sucrose show significant increase with the progress of the seed development. In the endosperm, the transition of the division phase and cell expansion for the storage of reserve material seem to occur around 120 DAP and is to be controlled mainly by the enzymes acid invertase and SuSy, while in the embryo, such transition seems to occur around 135 DAP and is to be controlled mainly by the enzymes acid and neutral invertases.     

Starch-synthesizing Enzymes in the Endosperm and Pollen of Maize

Two mutations, amylose-extender and waxy, which affect the proportion of amylose and amylopectin of starch synthesized in the endosperm of maize (Zea mays L.) seeds, are also expressed in the pollen. However, most mutations that affect starch synthesis in the maize endosperm are not expressed in the pollen. In an attempt to understand the nonconcordance between the endosperm and pollen, extracts of mature pollen grains were assayed for a number of the enzymes possibly implicated in starch synthesis in the endosperm. Sucrose synthetase (sucrose-UDP glucosyl transferase, EC 2.4.1.13) activity was not detectable in either mature or immature pollen grains of nonmutant maize, but both bound and soluble invertase (EC 3.2.1.26) exhibited much greater specific activity (per milligram protein) in pollen extracts than in 22-day-old endosperm extracts. Phosphorylase (EC 2.4.1.1) activity was also higher in pollen than in endosperm extracts. ADP-Glucose pyrophosphorylase (EC 2.7.7.27) activity was much lower in pollen than endosperm extracts, but mutations that drastically reduced ADP-glucose pyrophosphorylase activity in the endosperm (brittle-2 and shrunken-2) did not markedly affect enzymic activity in the pollen. Specific activities of other enzymes implicated in starch synthesis were similar in endosperm and pollen extracts.     

Sucrose synthase activity in developing wheat endosperms differing in maximum weight

Past research on kernel growth in wheat (Triticum aestivum) has shown that the kernel itself largely regulates the influx of sucrose for consequent starch synthesis in the endosperm of the grain. The first step in the conversion of sucrose to starch is catalyzed by sucrose synthase (EC 2.4.13). Sucrose synthase activity was assayed in developing endosperms from kernels differing in growth rate and in maximum dry weight accumulation. From 10 to 22 days after anthesis, sucrose synthase activity per wheat endosperm remained constant with respect to time in all grains. However, kernels which had higher rates of kernel growth and which achieved greatest maximum weight had consistently and significantly higher sucrose synthase activities at any point in time than did kernels with slower rates of dry matter accumulation and lower maximum weight. In addition, larger kernels had a significantly greater amount of water in which this activity could be expressed. Although the results do not implicate sucrose synthase as the “rate limiting” enzyme in wheat kernel growth, they do emphasize the importance of sucrose synthase activity in larger or more rapidly growing kernels, as compared to smaller slower growing kernels.     

Fluoride-induced alteration of carbon and nitrogen metabolism in developing wheat grains

Effect of fluoride (10 and 50 mM) on the activities of sucrose metabolizing enzymes, alkaline inorganic pyrophosphatase, and transaminases in relation to the accumulation of free sugars, starch, and soluble protein was studied in detached ears of wheat cultured in a liquid medium. Culturing for 5 d in the presence of fluoride reduced the amount of grain starch whereas contents of total free sugars, particularly sucrose, and soluble protein increased. Fluoride inhibited the activities of soluble acid and neutral invertases, as well as sucrose synthase acting in the cleavage direction. Uptake of uniformly labelled ¹⁴C-sucrose or fructose was also drastically reduced by fluoride. Glutamate-oxaloacetate transaminase (GOT) and glutamate-pyruvate transaminase (GPT) activities also increased with fluoride addition in correspondence with an increase in soluble protein. Apparently, the wheat grain responds to fluoride-mediated disruption of carbon metabolism by a compensatory effect on nitrogen metabolism. This revised version was published online in July 2006 with corrections to the Cover Date.     

花后高温对持绿型小麦叶片衰老及籽粒淀粉合成相关酶的影响

试验选用持绿型冬小麦(Triticum aestivum) ‘豫麦66’ (‘Ym66’)和‘潍麦8号’ (‘Wm8’)为研究材料, 以当地生产上起主导作用的冬小麦品种‘小偃22’ (‘XY22’)和‘小偃6号’ (‘XY6’)为对照。花后用塑料薄膜搭建成增温棚进行高温处理, 测定各品种绿叶数目、叶绿素和丙二醛(MDA)含量及叶片细胞膜透性, 并研究籽粒灌浆成熟期高温对持绿型小麦籽粒淀粉合成相关酶及粒重的影响。结果表明, 高温处理后, 各品种的绿叶数目和叶绿素含量都减少, MDA含量和膜透性都增加, 说明高温加速了小麦叶片衰老。同时, 各品种籽粒中与淀粉合成相关的酶(蔗糖合成酶(SS)和腺苷二磷酸葡萄糖焦磷酸化酶(AGPP)、可溶性淀粉合酶(SSS))活性都低于正常生长下的籽粒中的酶活性, 其中高温对籽粒SS和AGPP活性的影响不显著,而对籽粒SSS活性的影响显著(p = 0.015)。品种间比较, 持绿型小麦在两种处理下, 都表现出较多的绿叶数目和较高的叶绿素含量; 且3种与淀粉合成相关的酶活性也都高于非持绿型小麦, 说明持绿型小麦酶活性受高温抑制程度较小。相关性分析表明, 所有品种籽粒SS、AGPP、SSS活性都与籽粒灌浆速率成极显著的正相关(相关系数r分别为0.905、0.419和0.801)。因而, 持绿型小麦不仅具有较好的持绿特性, 而且籽粒中与淀粉合成相关的3种酶活性都较高, 这有利于其籽粒淀粉的合成, 从而增加籽粒产量。