Inhibition Of Starch Synthesis Research Articles

Integrated Analysis of Metabolome and Transcriptome Reveals Insights for Low Phosphorus Tolerance in Wheat Seedling.

Low phosphorus (LP) stress leads to a significant reduction in wheat yield, primarily in the reduction of biomass, the number of tillers and spike grains, the delay in heading and flowering, and the inhibition of starch synthesis and grouting. However, the differences in regulatory pathway responses to low phosphorus stress among different wheat genotypes are still largely unknown. In this study, metabolome and transcriptome analyses of G28 (LP-tolerant) and L143 (LP-sensitive) wheat varieties after 72 h of normal phosphorus (CK) and LP stress were performed. A total of 181 and 163 differentially accumulated metabolites (DAMs) were detected for G28CK vs. G28LP and L143CK vs. L143LP, respectively. Notably, the expression of pilocarpine (C07474) in G28CK vs. G28LP was significantly downregulated 4.77-fold, while the expression of neochlorogenic acid (C17147) in L143CK vs. L143LP was significantly upregulated 2.34-fold. A total of 4023 differentially expressed genes (DEGs) were acquired between G28 and L143, of which 1120 DEGs were considered as the core DEGs of LP tolerance of wheat after LP treatment. The integration of metabolomics and transcriptomic data further revealed that the LP tolerance of wheat was closely related to 15 metabolites and 18 key genes in the sugar and amino acid metabolism pathway. The oxidative phosphorylation pathway was enriched to four ATPases, two cytochrome c reductase genes, and fumaric acid under LP treatment. Moreover, PHT1;1, TFs (ARFA, WRKY40, MYB4, MYB85), and IAA20 genes were related to the Pi starvation stress of wheat roots. Therefore, the differences in LP tolerance of different wheat varieties were related to energy metabolism, amino acid metabolism, phytohormones, and PHT proteins, and precisely regulated by the levels of various molecular pathways to adapt to Pi starvation stress. Taken together, this study may help to reveal the complex regulatory process of wheat adaptation to Pi starvation and provide new genetic clues for further study on improving plant Pi utilization efficiency.

Regulation of Photosynthetic Carbohydrate Metabolism by a Raf-Like Kinase in the Liverwort Marchantia polymorpha.

To optimize growth and development, plants monitor photosynthetic activities and appropriately regulate various cellular processes. However, signaling mechanisms that coordinate plant growth with photosynthesis remain poorly understood. To identify factors that are involved in signaling related to photosynthetic stimuli, we performed a phosphoproteomic analysis with Marchantia polymorpha, an extant bryophyte species in the basal lineage of land plants. Among proteins whose phosphorylation status changed differentially between dark-treated plants and those after light irradiation but failed to do so in the presence of a photosynthesis inhibitor, we identified a B4-group Raf-like kinase, named PHOTOSYNTHESIS-RELATED RAF (MpPRAF). Biochemical analyses confirmed photosynthesis-activity-dependent changes in the phosphorylation status of MpPRAF. Mutations in the MpPRAF gene resulted in growth retardation. Measurement of carbohydrates demonstrated both hyper-accumulation of starch and reduction of sucrose in Mppraf mutants. Neither inhibition of starch synthesis nor exogenous supply of sucrose alleviated the growth defect, suggesting serious impairment of Mppraf mutants in both the synthesis of sucrose and the repression of its catabolism. As a result of the compromised photosynthate metabolism, photosynthetic electron transport was downregulated in Mppraf mutants. A mutated MpPRAF with a common amino acid substitution for inactivating kinase activity was unable to rescue the Mppraf mutant defects. Our results provide evidence that MpPRAF is a photosynthesis signaling kinase that regulates sucrose metabolism.

NTRC Plays a Crucial Role in Starch Metabolism, Redox Balance, and Tomato Fruit Growth.

NADPH-thioredoxin reductase C (NTRC) forms a separate thiol-reduction cascade in plastids, combining both NADPH-thioredoxin reductase and thioredoxin activities on a single polypeptide. While NTRC is an important regulator of photosynthetic processes in leaves, its function in heterotrophic tissues remains unclear. Here, we focus on the role of NTRC in developing tomato (Solanum lycopersicum) fruits representing heterotrophic storage organs important for agriculture and human diet. We used a fruit-specific promoter to decrease NTRC expression by RNA interference in developing tomato fruits by 60% to 80% compared to the wild type. This led to a decrease in fruit growth, resulting in smaller and lighter fully ripe fruits containing less dry matter and more water. In immature fruits, NTRC downregulation decreased transient starch accumulation, which led to a subsequent decrease in soluble sugars in ripe fruits. The inhibition of starch synthesis was associated with a decrease in the redox-activation state of ADP-Glc pyrophosphorylase and soluble starch synthase, which catalyze the first committed and final polymerizing steps, respectively, of starch biosynthesis. This was accompanied by a decrease in the level of ADP-Glc. NTRC downregulation also led to a strong increase in the reductive states of NAD(H) and NADP(H) redox systems. Metabolite profiling of NTRC-RNA interference lines revealed increased organic and amino acid levels, but reduced sugar levels, implying that NTRC regulates the osmotic balance of developing fruits. These results indicate that NTRC acts as a central hub in regulating carbon metabolism and redox balance in heterotrophic tomato fruits, affecting fruit development as well as final fruit size and quality.

ITRAQ-based proteome profile analysis of superior and inferior Spikelets at early grain filling stage in japonica Rice

BackgroundLarge-panicle rice varieties often fail to achieve their yield potential due to poor grain filling of late-flowering inferior spikelets (IS). The physiological and molecular mechanisms of poor IS grain filling, and whether an increase in assimilate supply could regulate protein abundance and consequently improve IS grain filling for japonica rice with large panicles is still partially understood.ResultsA field experiment was performed with two spikelet removal treatments at anthesis in the large-panicle japonica rice line W1844, including removal of the top 1/3 of spikelets (T1) and removal of the top 2/3 of spikelets (T2), with no spikelet removal as a control (T0). The size, weight, setting rate, and grain filling rate of IS were significantly increased after spikelet removing. The biological functions of the differentially expressed proteins (DEPs) between superior and inferior spikelets as well as the response of IS to the removal of superior spikelets (SS) were investigated by using iTRAQ at 10 days post anthesis. A total of 159, 87, and 28 DEPs were identified from group A (T0-SS/T0-IS), group B (T0-SS/T2-IS), and group C (T2-IS/T0-IS), respectively. Among these, 104, 63, and 22 proteins were up-regulated, and 55, 24, and 6 proteins were down-regulated, respectively. Approximately half of these DEPs were involved in carbohydrate metabolism (sucrose-to-starch metabolism and energy metabolism) and protein metabolism (protein synthesis, folding, degradation, and storage).ConclusionsReduced endosperm cell division and decreased activities of key enzymes associated with sucrose-starch metabolism and nitrogen metabolism are mainly attributed to the poor sink strength of IS. In addition, due to weakened photosynthesis and respiration, IS are unable to obtain a timely supply of materials and energy after fertilization, which might be resulted in the stagnation of IS development. Finally, an increased abundance of 14–3-3 protein in IS could be involved in the inhibition of starch synthesis. The removal of SS contributed to transfer of assimilates to IS and enhanced enzymatic activities of carbon metabolism (sucrose synthase, starch branching enzyme, soluble starch synthase, and pullulanase) and nitrogen metabolism (aspartate amino transferase and alanine amino transferase), promoting starch and protein synthesis in IS. In addition, improvements in energy metabolism (greater abundance of pyrophosphate-fructose 6-phosphate 1-phosphotransferase) might be played a vital role in inducing the initiation of grain filling. These results collectively demonstrate that carbohydrate supply is the main cause of poor IS grain filling.

Inactivation of thioredoxin f1 leads to decreased light activation of ADP‐glucose pyrophosphorylase and altered diurnal starch turnover in leaves of Arabidopsis plants

Chloroplast thioredoxin f (Trx f) is an important regulator of primary metabolic enzymes. However, genetic evidence for its physiological importance is largely lacking. To test the functional significance of Trx f in vivo, Arabidopsis mutants with insertions in the trx f1 gene were studied, showing a drastic decrease in Trx f leaf content. Knockout of Trx f1 led to strong attenuation in reductive light activation of ADP-glucose pyrophosphorylase (AGPase), the key enzyme of starch synthesis, in leaves during the day and in isolated chloroplasts, while sucrose-dependent redox activation of AGPase in darkened leaves was not affected. The decrease in light-activation of AGPase in leaves was accompanied by a decrease in starch accumulation, an increase in sucrose levels and a decrease in starch-to-sucrose ratio. Analysis of metabolite levels at the end of day shows that inhibition of starch synthesis was unlikely due to shortage of substrates or changes in allosteric effectors. Metabolite profiling by gas chromatography-mass spectrometry pinpoints only a small number of metabolites affected, including sugars, organic acids and ethanolamine. Interestingly, metabolite data indicate carbon shortage in trx f1 mutant leaves at the end of night. Overall, results provide in planta evidence for the role played by Trx f in the light activation of AGPase and photosynthetic carbon partitioning in plants.

The sink-specific plastidic phosphate transporter PHT4;2 influences starch accumulation and leaf size in Arabidopsis.

Nonphotosynthetic plastids are important sites for the biosynthesis of starch, fatty acids, and amino acids. The uptake and subsequent use of cytosolic ATP to fuel these and other anabolic processes would lead to the accumulation of inorganic phosphate (Pi) if not balanced by a Pi export activity. However, the identity of the transporter(s) responsible for Pi export is unclear. The plastid-localized Pi transporter PHT4;2 of Arabidopsis (Arabidopsis thaliana) is expressed in multiple sink organs but is nearly restricted to roots during vegetative growth. We identified and used pht4;2 null mutants to confirm that PHT4;2 contributes to Pi transport in isolated root plastids. Starch accumulation was limited in pht4;2 roots, which is consistent with the inhibition of starch synthesis by excess Pi as a result of a defect in Pi export. Reduced starch accumulation in leaves and altered expression patterns for starch synthesis genes and other plastid transporter genes suggest metabolic adaptation to the defect in roots. Moreover, pht4;2 rosettes, but not roots, were significantly larger than those of the wild type, with 40% greater leaf area and twice the biomass when plants were grown with a short (8-h) photoperiod. Increased cell proliferation accounted for the larger leaf size and biomass, as no changes were detected in mature cell size, specific leaf area, or relative photosynthetic electron transport activity. These data suggest novel signaling between roots and leaves that contributes to the regulation of leaf size.

Nutritive value of cereals grown in Apulian places

The new EU Common Agricultural Policy by the de-coupling measure provides the same subsidy for any crop which respects good economic and environmental conditions; hence the need to identify crop systems which give a better yield in relation to the soil fertility, climatic conditions and species adaptability. The aim of the study was to evaluate the chemical composition and the nutritive value of cereals such as spelt (cultivar Davide), durum wheat (cultivar Portobello), rye (cultivar Askari) and two cultivars of barley, Dasio and Otis. The cereals were grown in 3 Apulian places: Gravina di Puglia (BA), Monopoli (BA) and Monteroni (LE), very different among each other for the climatic conditions and pedological features of soil (fertility, texture and composition). In vitro gas production was assessed by the Menke and Steingass (1988) technique, checked until 72 h and expressed as ml/g DM. The metabolizable energy (ME; MJ/kg DM) was calculated as: 1.06 + 0.157GP + 0.084CP + 0.22CF - 0.081CA, where GP is 24 h net gas production (ml/g DM), CP, CF and CA are crude protein, fat and ash (% DM), respectively. Data were processed by ANOVA according to a 5 (cereals tested) x 3 (cultivation places) experimental model. Student’s t test was used to compare differences between means. The CP content of all the cereals tested was higher in crops grown in Gravina compared to Monopoli and Monteroni, with particular regards to durum wheat (13.60 vs 10.97 and 10.35, P<0.01), rye (14.27 vs 7.83 and 7.75, P<0.01) and spelt (13.25 vs 12.16, P<0.05; vs 11.87, P<0.01). The CP content of both the cultivars of barley was significantly lower in Monopoli (Dasio: 11.40 vs 13.16 and 12.75, respectively for Gravina and Monteroni, P<0.01; Otis: 8.20 vs 12.61 and 11.34, P<0.01).Temperatures above 30 ºC and dry winds during late Spring may have affected final kernel weight by reducing the duration of grain filling, due to the suppression of current photosynthesis and by inhibition of starch synthesis in the endosperm. Thus, kernels remain thin and show a lower hectolitre weight and a higher CP percentage as a consequence of the greater incidence of the pericarp on the total kernel weight. This is a typical situation that occurs in South Italy areas, due also to the features of the marginal lands. Indeed, the occurrence of a reduced production of starch may be hypothesised since a lower amount of GP was found, especially for barley (Dasio) and rye grown in Gravina, characterised by poor soils. The ME of durum wheat and rye did not differ among places, while barley cultivars displayed a different trend: the ME of the Dasio cultivar grown in Gravina was significantly lower compared to Monopoli and Monteroni (12.23 vs 13.33 and 13.22, P<0.01). On the other hand the ME of the Otis cultivar grown in Gravina (13.26) was similar to that of Monteroni (13.23) but significantly greater compared to Monopoli (12.54, P<0.05). For spelt the best ME was recorded in Gravina (12.36 vs 11.52, P<0.05), despite the hostile conditions of this place. Therefore, it may be assumed that the greater rusticity of this cereal allows a better adaptability to marginal areas and its use as a convenient ingredient for the ruminant diet.

Starch biosynthesis: further evidence against the primer nonreducing-end mechanism and evidence for the reducing-end two-site insertion mechanism

Two reactions were studied with three varieties of starch granules from maize, wheat, and rice. In Reaction-I, the granules were reacted with 1 mM ADP-[ 14C]Glc and in Reaction-II, a portion of the granules from Reaction-I was reacted with 1 mM ADP-Glc. The starch granules were solubilized and reacted with the exo-acting glucoamylase and beta-amylase to an extent of 50% or less of the 14C-label. The amounts of 14C-labeled products from glucoamylase and beta-amylase were nearly equal for Reaction-I and Reaction-II. If the addition had been to the nonreducing ends of primers, Reaction-II would not have given any labeled products from the hydrolysis of glucoamylase and beta-amylase. These results indicate that the elongation of the starch chain is the addition of D-glucose to the reducing end by a de novo two-site insertion mechanism and not by the addition of D-glucose to the nonreducing end of a primer. This is in conformity with previous results in which starch granules were pulsed with ADP-[ 14C]Glc and chased with nonlabeled ADP-Glc, giving 14C-labeled D-glucitol from the pulsed starch and a significant decrease in 14C-labeled D-glucitol from the chased starch on reducing with NaBH 4 and hydrolyzing with glucoamylase [ Carbohydr. Res. 2002, 337, 1015–1022]. It also is in conformity with the inhibition of starch synthesis that occurs when putative primers are added to starch granule–ADP-Glc digests, indicating that the elongation is not by the nonreducing-end primer mechanism [ Carbohydr. Res. 2005, 340, 245–255].

Contribution of adenosine 5'-diphosphoglucose pyrophosphorylase to the control of starch synthesis is decreased by water stress in growing potato tubers

Water stress stimulates sucrose synthesis and inhibits starch and cell-wall synthesis in tissue slices of growing potato (Solanum tuberosum L. cv. Desiree) tubers. Based on the analysis of fluxes and metabolites, Geigenberger et al. (1997, Planta 201: 502-518) proposed that water deficits up to -0.72 MPa stimulate sucrose synthesis, leading to decreased starch synthesis as a result of the resulting decline of phosphorylated metabolite levels, whereas more-severe water deficits directly inhibit the use of ADP-glucose. Potato plants with decreased expression of adenosine 5'-diphosphoglucose pyrophosphorylase (AGPase) have been used to test the prediction that the contribution of AGPase to the control of starch synthesis should decrease in severely water-stressed tuber material. Freshly cut slices from wild-type and antisense tubers were incubated at a range of mannitol concentrations (20, 300 and 500 mM) and the metabolism of [(14)C]glucose was analysed. A 86-97% reduction of AGPase activity led to a major but non-stoichiometric inhibition of starch accumulation in intact growing tubers attached to the plant (40-85%), and an inhibition of starch synthesis in non-stressed tuber slices incubated in 20 mM mannitol (60-80%). The inhibition of starch synthesis was accompanied by a 2- to 8-fold increase in the levels of sugars in intact tubers and a 2- to 3-fold stimulation of sucrose synthesis in tuber slices, whereas respiration and cell-wall synthesis were not significantly affected. The strong impact of AGPase on carbon partitioning in non-stressed tubers and tuber slices was retained in slices subjected to moderate water deficit (300 mM mannitol, corresponding to -0.72 MPa). In discs incubated in 500 mM mannitol (corresponding to -1.2 MPa) this response was modified. A 80-97% reduction of AGPase resulted in only a 0-40% inhibition of starch synthesis. Further, the water stress-induced stimulation of sucrose synthesis was abolished in the transformants. The results provide direct evidence that the contribution of AGPase to the control of starch synthesis can be modified by environmental factors, leading to a lower degree of control during severe water deficits. There was also a dramatic decrease in the labelling of cell-wall components in wild-type tuber slices incubated with 300 or 500 mM mannitol. The water stress-induced inhibition of cell-wall synthesis occurred independently of AGPase expression and the accompanying changes in starch and sucrose metabolism, indicating a direct inhibition of cell-wall synthesis in response to water stress.

Decreased expression of sucrose phosphate synthase strongly inhibits the water stress-induced synthesis of sucrose in growing potato tubers.

Water stress stimulates sucrose synthesis and inhibits starch synthesis in wild-type tubers. Antisense and co-suppression potato transformants with decreased expression of sucrose-phosphate synthase (SPS) have been used to analyse the importance of SPS for the regulation of this water-stress induced change in partitioning. (i) In the absence of water stress, a 70-80% decrease in SPS activity led to a 30-50% inhibition of sucrose synthesis and a slight (10-20%) increase of starch synthesis in tuber discs in short-term labelling experiments with low concentrations of labelled glucose. Similar changes were seen in short-term labelling experiments with intact tubers attached to well-watered plants. Provided plants were grown with ample light and water, transformant tubers had a slightly lower water and sucrose content and a similar or even marginally higher starch content than wild-type tubers. (ii) When wild-type tuber slices were incubated with labelled glucose in the presence of mannitol to generate a moderate water deficit (between -0.12 and -0.72 MPa), there was a marked stimulation of sucrose synthesis and inhibition of starch synthesis. A similar stimulation was seen in labelling experiments with wild-type tubers that were attached to water-stressed wild-type plants. These changes were almost completely suppressed in transformants with a 70-80% reduction of SPS activity. (iii) Decreased irrigation led to an increase in the fraction of the dry-matter allocated to tubers in wild-type plants. This shift in allocation was prevented in transformants with reduced expression of SPS. (iv) The results show that operation of SPS and the sucrose cycle in growing potato tubers may lead to a marginal decrease in starch accumulation in non-stressed plants. However, SPS becomes a crucial factor in water-stressed plants because it is required for adaptive changes in tuber metabolism and whole plant allocation.

Starch Phosphorylation in Potato Tubers Proceeds Concurrently with de Novo Biosynthesis of Starch.

The in vivo phosphorylation of starch was studied in Solanum tuberosum cv Dianella and Posmo. Small starch granules contain 25% more ester-bound phosphate per glucose residue than large starch granules. The degree of phosphorylation was found to be almost constant during tuber development. Isolated tuber discs synthesize starch from externally supplied glucose at a significant rate. Tuber discs supplied with glucose and [32P]orthophosphate incorporate radiolabeled phosphorus into the starch. The level of 32P incorporation is proportional to the amount of starch synthesized. The incorporation of 32P from orthophosphate is correlated to de novo synthesis of starch, since the incorporation of 32P is diminished upon inhibition of starch synthesis by fluoride. Based on the amount of [14C]glucose phosphate isolated after hydrolysis of purified starch from tuber discs incubated in the presence of [U-14C]glucose, approximately 0.5% of the glucose residues of the de novo-synthesized starch are phosphorylated. This value is in general agreement with the observed levels of phosphorus in starch accumulated during tuber development. Thus, the enzyme system responsible for starch phosphorylation is fully active in the isolated tuber discs, and the starch phosphorylation proceeds as an integrated part of de novo starch synthesis.

Comparison of the effect of rapidly and gradually developing water‐stress on carbohydrate metabolism in spinach leaves

Abstract. The effect of water‐stress on photosynthetic carbon metabolism in spinach (Spinacia oleracea L.) has been studied in experiments in which water‐stress was induced rapidly by floating leaf discs on sorbitol solutions or wilting detached leaves, and in experiments in which water‐stress was allowed to develop gradually in whole plants as the soil dried out. In both short‐ and long‐term water stress, the rate of photosynthesis in saturating CO2 did not decrease until leaf water potential decreased below ‐1.0 MPa. However, at smaller water deficits there was already an inhibition of starch synthesis, while sucrose synthesis remained constant or increased. This change in partitioning was accompanied by an increase in activation of sucrose‐phosphate synthase (revealed as an increase in activity assayed in the presence of low hexose‐phosphate and inorganic phosphate, while the activity assayed with saturating hexosephosphates remained unaltered). Water‐stressed leaves had a two‐ to three‐fold higher sucrose content at the end of the night, and contained less starch than non‐stressed leaves. When leaves were held in the dark, sucrose was mobilized initially, while starch was not mobilized until the sucrose had decreased to a low level; in water‐stressed leaves, starch mobilization commenced at a two‐fold higher sucrose content. It is concluded that water‐stressed leaves maintain higher sucrose and lower starch levels than non‐stressed leaves. This response is found in rapid and long‐term stress, and represents an inherent response to water deficits.

Fluoride-Induced Inhibition of Starch Biosynthesis in Developing Potato, Solanum tuberosum L., Tubers Is Associated with Pyrophosphate Accumulation

Pretreatment of discs excised from developing tubers of potato (Solanum tuberosum L.) with 10 millimolar sodium fluoride induced a transient increase in 3-phosphoglycerate content. This was followed by increases in triose-phosphate, fructose 1,6-bisphosphate and hexose-phosphate (glucose 6-phosphate + fructose 6-phosphate + glucose 1-phosphate). The effect of fluoride is attributed to an inhibition of glycolysis and a stimulation of triose-phosphate recycling (the latter confirmed by the pattern of (13)C-labeling [NMR] in sucrose when tissue was supplied with [2-(13)C]glucose). Fluoride inhibited the incorporation of [U-(14)C] glucose, [U-(14)C]sucrose, [U-(14)C]glucose 1-phosphate, and [U-(14)C] glycerol into starch. The incorporation of [U-(14)C]ADPglucose was unaffected. Inhibition of starch biosynthesis was accompanied by an almost proportional increase in the incorporation of (14)C into sucrose. The inhibition of starch synthesis was accompanied by a 10-fold increase in tissue pyrophosphate (PPi) content. Although the subcellular localization of PPi was not determined, a hypothesis is presented that argues that the PPi accumulates in the amyloplast due to inhibition of alkaline inorganic pyrophosphatase by fluoride ions.

Control analysis of photosynthate partitioning

Experiments were carried out to investigate the contribution of ADP-glucose pyrophosphorylase and the plastid phosphoglucosemutase to the control of starch synthesis. Mutants ofArabidopsis thaliana (L.) Heyhn. were constructed with 50% and 7% of the wild-type adenosine 5'-diphosphoglucose pyrophosphorylase (ADPGlc-PPase), or 50% and null plastid phosphoglucomutase (PGM). The changes in the steady-state rates of sucrose synthesis, starch synthesis and CO2 fixation were measured in saturating CO2 in low (75 μmol·m(-2)·s(-1)) and high (600 μmol·m(-2)·s(-1)) irradiance. In low irradiance, a 50% decrease of PGM had no significant effect on fluxes, while a 50% and 93% decrease of ADPGlc-PPase led to a 23% and 74% inhibition of starch synthesis. Decreased ADPGlc-PPase led to an increase of hexose phosphates, triose phosphates and fructose-1,6-bisphosphate. Fixation of CO2 was not inhibited because the inhibition of starch synthesis was matched by a stimulation of sucrose synthesis. In high irradiance, a 50% decrease of PGM led to a 20% inhibition of starch synthesis. A 50% and 93% decrease of ADPGlc-PPase led to a 39% and 90% inhibition of starch synthesis. Sucrose synthesis was also inhibited, and the rate of photosynthesis was decreased. Decreased ADPGlc-PPase led to an increase of hexose phosphates but triose phosphates and fructose-1,6-bisphosphate did not increase. These results are used to estimate flux-control coefficients for these enzymes for starch synthesis. Firstly, the flux to starch is only controlled by ADPGlc-PPase in low irradiance, but control is redistributed to other enzymes in the pathway when a rapid flux is imposed, e.g. in high irradiance and CO2. Secondly, reducing the rate of starch synthesis by decreasing the activity of enzymes in this pathway does not always lead to a compensating increase in the rate of sucrose synthesis. Thirdly, decreasing the activity of an enzyme by a factor of two compared to the remainder of the pathway often leads to it exerting very considerable control. Fourthly, each enzyme starts to exert considerable control when only a fraction of its Vmax activity is being utilised in vivo, for example the maximum flux at ADPGlc-PPase never exceeded 20% of the Vmax activity. The summation theory is also applied to check whether additional major control sites are required. In low irradiance, the efficiency of light harvesting will exert considerable control over the rate of starch synthesis.

Low Oxygen Inhibition of Photosynthesis Is Caused by Inhibition of Starch Synthesis

Photosynthesis of C(3) plants is occasionally inhibited upon switching from normal to low partial pressure of O(2). Leaves of Solanum tuberosum exhibited this effect reproducibly under saturating light and 700 microbars of CO(2). We determined the partitioning of recent photosynthate between starch and sucrose and measured the concentration of hexose monophosphates in the stroma and cytosol after nonaqueous fractionation. The reduction in the rate of photosynthesis upon switching to low partial pressure of O(2) was caused by reduced starch synthesis. The concentration of hexose monophosphates in the stroma fell and the glucose 6-phosphate to fructose 6-phosphate to fructose 6-phosphate ratio fell from 2.7 to 1.3, indicating an inhibition of phosphoglucoisomerase as described by K-J Dietz ([1985] Biochim Biophys Acta 839: 240-248). The concentration of hexose monophosphates in the cytosol increased, ruling out a sucrose synthesis limitation by reduced transport from the chloroplast as the explanation for low O(2) inhibition of photosynthesis.

Effect of Water Deficit during Grain Filling on the Pattern of Maize Kernel Growth and Development1

There are distinct phases of grain development in maize (Zea mays L.). Little has been reported on the relative effects of water deficits during the different phases of maize kernel development. Since endosperm cell number is important in determining sink capacity or strength, a water deficit during the lag phase may affect kernel development differently than one during the linear phase dominated by starch deposition. The objective of this research was to determine the effect of short‐ and long‐term water deficit, imposed during different phases of grain filling, on the pattern of maize kernel growth. We conducted two separate experiments. In the first, a short‐term (10 day) water deficit was imposed during endosperm cell division (the lag phase) or during the period of rapid starch deposition (the linear‐filling phase). In the second, a water deficit was initiated during the same phases, but extended until maturity. Plants were grown outdoors in Rabat, Morocco, in large pots arranged to achieve a stand density of 50 000 plants/ha. Short‐term water deficit during the lag phase resulted in a significant but transient delay in the accumulation of kernel dry mass. However, recovery from this delay was such that overall rate and duration of kernel growth were not affected. Similarly, short‐term water deficit during the linear‐filling phase had no effect on the rate or duration of kernel growth. When the long‐term water deficit treatment was initiated during the lag phase, kernel growth rate was not affected for approximately 24 days. Grain filling then terminated prematurely, resulting in a significant decrease in the duration of the grain‐filling period, and a 50% decrease in final kernel weight. When long‐term stress was imposed during the linear‐filling phase, kernel growth was not affected for 20 days, but then terminated abruptly, resulting in an 8‐day decrease in the grain‐filling period. Final kernel weight was not significantly decreased, however. Our results show that kernel growth is more sensitive to water deficits during endosperm cell division than during the period of rapid starch deposition. The higher sensitivity to water deficits during the lag phase was not a consequence of either a lack of carbohydrate availability to the grain, or an inhibition of starch synthesis, but was apparently due to an inhibition of endosperm cell division and the subsequent establishment of kernel sink capacity.

Temperature Effects on in vitro Kernel Development of Maize1

This study was conducted to ascertain the effects of temperature extremes on kernel characteristics, respiration, starch synthesis, soluble sugar, and protein content during grain filling in maize (Zea mays L.). Kernels cultured in vitro on a defined media at 15, 30, and 35 C were compared to kernels from ears developed in the field. Kernels cultured for 7 days at 35 C had higher dry weights than kernels from other treatments; however, their growth ceased by the 14th day. High soluble sugar content of aborted kernels suggests that inhibition of starch synthesis may have caused kernel abortion at 35 C. Respiration was higher for kernels from field‐grown ears. During the linear phase of dry matter accumulation, kernel growth rates were 0.65, 6.26, and 5.26 mg kernel‐1 day ‐1 for 15 C, 30 C, and field control treatments, respectively. The higher rate of kernel growth at 30 C was negated by a pronounced shortening of the duration of the grain filling period. Relative to the 30 C treatment, a three‐fold increase in duration of grain filling at 15 C was not adequate to compensate for the 90% reduction in kernel growth rate that occurred. Cool temperatures that occurred in the field or that were maintained in vitro caused soluble sugar levels to remain high and deposition of starch in the endosperm to lag several days behind that observed at 30 C.Cool temperatures did not affect the synthesis of proteins in general or cause differential synthesis of an individual or specific class of protein in the zein fraction. These data show that kernel abortion and shortening of the grain filling period at higher than optimum temperatures and severe reduction in kernel growth rates at lower than optimum temperatures are possible mechanisms by which temperature may mediate kernel growth and development and ultimately grain yield of maize.

Root regeneration from Leaf Cuttings ofLycopersicon esculentumMill.: Application of the Leaf Plastochron Index and Responses to exogenous Gibberellic Acid

Application of the leaf plastochron index (LPI) allowed the consistent description of root regenerative capacity from tomato leaf cuttings. GA3 inhibited root regeneration regardless of LPI. This inhibition was alleviated by IAA or IBA if applied prior to, during, or after GA3 treatment. However, ABA reversed rooting inhibition only if applied prior to or during GA3 treatment. The GA3 effect on rooting was correlated with an inhibition of localized starch accumulation within the basal petiole regions prior to and during initiation of the root pri mordia. GA3 had no significant effect on the amount or intercellular distribution of amylolytic activity within the petiole tissues during this period. On the basis of the present study, we hypothesize that the mode of GA3 action involves a localized inhibition of starch synthesis rather than increased starch hydrolysis which may, in turn, lead to inhibition of root regenera tion in tomato leaf cuttings.