Sucrose utilization during somatic embryo development in black spruce: involvement of apoplastic invertase in the tissue and of extracellular invertase in the medium

The involvement of apoplastic invertase (Ap Inv) and sucrose synthase (SuSy) in the somatic embryo development of black spruce (Picea mariana) was investigated under different maturation conditions. Replacing 6% sucrose with 3% or 1% sucrose in the maturation medium drastically decreased Ap Inv activity and amount in embryogenic tissues. This was accompanied by a decrease in the hexose pool that resulted in a lower starch deposition and protein amount in embryogenic tissues together with a lower embryo production. Conversely, SuSy activity was stable during maturation regardless of the sucrose concentration used in the medium. The presence of an extracellular enzyme responsible for sucrose hydrolysis in the maturation medium was also verified. An immunodetection experiment with anti-acid invertase antibodies revealed the presence of an active 53 kDa polypeptide in the medium, which had a similar molecular mass to that of the Ap Inv polypeptide found in embryogenic tissues. Utilization of sucrose from the medium by the tissues was also studied using labelled 14C-sucrose. Distribution of the radioactivity between tissular sucrose, glucose, and fructose showed that sucrose was diffused into the cell wall of embryogenic tissues and partly hydrolyzed by Ap Inv. These results show that the utilization of sucrose from the medium, the Ap Inv activity in embryogenic tissues, and the release of an active invertase into the medium operate together for the utilization of the carbohydrates during somatic embryo development in black spruce.     

Decreased sucrose content triggers starch breakdown and respiration in stored potato tubers (Solanum tuberosum)

To change the hexose-to-sucrose ratio within phloem cells, yeast-derived cytosolic invertase was expressed in transgenic potato (Solanum tuberosum cv. Desirée) plants under control of the rolC promoter. Vascular tissue specific expression of the transgene was verified by histochemical detection of invertase activity in tuber cross-sections. Vegetative growth and tuber yield of transgenic plants was unaltered as compared to wild-type plants. However, the sprout growth of stored tubers was much delayed, indicating impaired phloem-transport of sucrose towards the developing bud. Biochemical analysis of growing tubers revealed that, in contrast to sucrose levels, which rapidly declined in growing invertase-expressing tubers, hexose and starch levels remained unchanged as compared to wild-type controls. During storage, sucrose and starch content declined in wild-type tubers, whereas glucose and fructose levels remained unchanged. A similar response was found in transgenic tubers with the exception that starch degradation was accelerated and fructose levels increased slightly. Furthermore, changes in carbohydrate metabolism were accompanied by an elevated level of phosphorylated intermediates, and a stimulated rate of respiration. Considering that sucrose breakdown was restricted to phloem cells it is concluded that, in response to phloem-associated sucrose depletion or hexose elevation, starch degradation and respiration is triggered in parenchyma cells. To study further whether elevated hexose and/or hexose-phosphates or decreased sucrose levels are responsible for the metabolic changes observed, sucrose content was decreased by tuber-specific expression of a bacterial sucrose isomerase. Sucrose isomerase catalyses the reversible conversion of sucrose into palatinose, which is not further metabolizable by plant cells. Tubers harvested from these plants were found to accumulate high levels of palatinose at the expense of sucrose. In addition, starch content decreased slightly, while hexose levels remained unaltered, compared with the wild-type controls. Similar to low sucrose-containing invertase tubers, respiration and starch breakdown were found to be accelerated during storage in palatinose-accumulating potato tubers. In contrast to invertase transgenics, however, no accumulation of phosphorylated intermediates was observed. Therefore, it is concluded that sucrose depletion rather than increased hexose metabolism triggers reserve mobilization and respiration in stored potato tubers.     

An Invertase Inhibitory Protein From Pteris deflexa Link Fronds

Plant invertases play important roles in sucrose metabolism. Cell wall invertase was reported to participate in phloem loading and unloading. Soluble invertases would be involved in hexose level regulation in mature tissues and in stored sucrose utilization within vacuoles. Invertase inhibitory proteins were described as one of the possible mechanisms for invertase activity regulation in some plant species; nevertheless, these proteins were found only in sink tissues, suggesting that this mechanism would not be relevant in the sucrose turnover of leaves. This report describes the purification of invertase from Pteris deflexa fronds and the occurrence of an invertase inhibitory protein in this fern organ, as well as its purification and invertase-inhibitor interactions. The Mr of the invertase and of its inhibitory protein were 90,000 and 18,000, respectively. SDS-PAGE in the presence of 2-mercaptoetanol gave two subunits for the enzyme (Mr=66,000 and 30,000) and only one for the inhibitor. The inhibitor protein is a glycoprotein (12% w/w of neutral sugars) that did not show agglutinating activity like some others, and also showed a high heat stability at pH 5.0. The optimum pH of invertase activity is 5.0, while invertase inhibitory protein caused maximal inhibition at the same pH value. Invertase-inhibitor complex formation occurs in an immediate manner and a protease activity was discarded. The inhibition is non-competitive (Ki=1.5 × 10 ^?6 M) without interactions among the binding sites. The complex is slightly dissociable and sucrose was able to partially reduce the inhibitory effect. Up to the present, invertase inhibitory proteins have been found solely in heterotrophic tissues. In this work we demonstrate that this protein is also present in an autotrophic tissue of a lower vascular plant.     

An invertase inhibitory protein from Pteris deflexa link fronds

Plant invertases play important roles in sucrose metabolism. Cell wall invertase was reported to participate in phloem loading and unloading. Soluble invertases would be involved in hexose level regulation in mature tissues and in stored sucrose utilization within vacuoles. Invertase inhibitory proteins were described as one of the possible mechanisms for invertase activity regulation in some plant species; nevertheless, these proteins were found only in sink tissues, suggesting that this mechanism would not be relevant in the sucrose turnover of leaves. This report describes the purification of invertase from Pteris deflexa fronds and the occurrence of an invertase inhibitory protein in this fern organ, as well as its purification and invertase-inhibitor interactions. The Mr of the invertase and of its inhibitory protein were 90,000 and 18,000, respectively. SDS-PAGE in the presence of 2-mercaptoetanol gave two subunits for the enzyme (Mr=66,000 and 30,000) and only one for the inhibitor. The inhibitor protein is a glycoprotein (12% w/w of neutral sugars) that did not show agglutinating activity like some others, and also showed a high heat stability at pH 5.0. The optimum pH of invertase activity is 5.0, while invertase inhibitory protein caused maximal inhibition at the same pH value. Invertase-inhibitor complex formation occurs in an immediate manner and a protease activity was discarded. The inhibition is non-competitive (Ki=1.5 x 10(-6) M) without interactions among the binding sites. The complex is slightly dissociable and sucrose was able to partially reduce the inhibitory effect. Up to the present, invertase inhibitory proteins have been found solely in heterotrophic tissues. In this work we demonstrate that this protein is also present in an autotrophic tissue of a lower vascular plant.     

猕猴桃果实采后成熟过程中糖代谢及其调节

20℃下采后猕猴桃果实中淀粉酶活性快速上升于果实软化启动阶段,随着果实进入快速软化阶段,淀粉迅速水解,葡萄糖和果糖快速积累,SPS活性增加,酸性转化酶活性下降,蔗糖积累;至果实软化后期,SPS活性降低,蔗糖含量下降.AsA和低温可抑制淀粉酶活性、己糖积累、SPS活性上升和酸性转化酶活性下降,延缓蔗糖积累,相反,乙烯则可促进淀粉酶活性,加速淀粉降解和己糖积累进而直接或间接增加SPS活性,促使蔗糖积累.采后猕猴桃果实的SPS活性变化中有己糖激活效应和蔗糖反馈抑制效应.AsA、低温和乙烯等对糖代谢的调节主要是通过对SPS活性的影响而实现的.     

Comparison of sucrose metabolism during the rehydration of desiccation-tolerant and desiccation-sensitve leaf material of Sporobolus stapfianus

Sucrose accumulated during dehydration is a major potential energy source for metabolic activity during rehydration. The objective of the present study was to investigate aspects of leaf sucrose metabolism during the rehydration of desiccation-tolerant Sporobolus stapfianus Gandoger (Poaceae) over a 10-day period. Comparison was then made to sucrose metabolism during the rehydration of both desiccation-tolerant excised leaf material (dehydrated attached to the parent plant) and desiccation-sensitive leaf material (dehydrated detached from the parent plant to prevent the induction of tolerance) over a 48-h period. The pattern of sugar mobilization and glycolytic enzyme activity during the rehydration of the desiccation-tolerant excised leaves was similar to that in leaves attached to the parent plants. Significant breakdown of sucrose was not apparent in the initial phase of rehydration, suggesting the utilization of alternate substrates for respiratory activity. The desiccation-tolerant excised tissues provided a suitable control to compare the metabolism of rehydrating desiccation-sensitive material. In contrast to the tolerant tissues, sucrose breakdown in the sensitive leaves commenced immediately after watering and the accumulation in hexose sugars was inversely proportionate to the decrease in sucrose content. Hexokinase (EC 2.7.1.1), PFK (ATP phosphofructokinase, EC 2.1.7.11), aldolase (EC 4.1.2.13), enolase (EC 4.2.1.11), and PK (pyruvate kinase, EC 2.7.1.40) activity levels were significantly lower in the desiccation-sensitive material during rehydration.     

Function and regulation of plant invertases: sweet sensations

The disaccharide sucrose and the cleavage products glucose and fructose are the central molecules for carbohydrate translocation, metabolism and sensing in higher plants. Invertases mediate the hydrolytic cleavage of sucrose into the hexose monomers. Plants possess three types of invertases, which are located in the apoplast, the cytoplasm and the vacuole, respectively. It has become evident that extracellular and vacuolar invertase isoenzymes are key metabolic enzymes that are involved in various aspects of the plant life cycle and the response of the plant to environmental stimuli because their substrates and reaction products are both nutrients and signal molecules. Invertases, alone or in combination with plant hormones, can regulate many aspects of the growth and development of plants from gene expression to long-distance nutrient allocation and are involved in regulating carbohydrate partitioning, developmental processes, hormone responses and biotic and abiotic interactions.     

Sucrose phosphate synthase activity and the co-ordination of carbon partitioning during sucrose and amino acid accumulation in desiccation-tolerant leaf material of the C4 resurrection plant Sporobolus stapfianus during dehydration

Both sucrose and amino acids accumulate in desiccation-tolerant leaf material of the C(4) resurrection plant, Sporobolus stapfianus Gandoger (Poaceae). The present investigation was aimed at examining sucrose phosphate synthase (SPS) activity and various metabolic checkpoints involved in the co-ordination of carbon partitioning between these competing pathways during dehydration. In the initial phase of dehydration, photosynthesis and starch content declined to immeasurable levels, whilst significant increases in hexose sugars, sucrose, and amino acids were associated with concomitant significant increases in SPS and pyruvate kinase (PK) activities, and maximal activity levels of phosphoenolpyruvate carboxylase (PEPCase), NADP-dependent isocitrate dehydrogenase (NADP-ICDH), and NADH-dependent glutamate synthase (NADH-GOGAT). The next phase of dehydration was characterized by changes in metabolism coinciding with net hexose sugar phosphorylation. This phase was characterized by a further significant increase in sucrose accumulation, with increased rates of net sucrose accumulation and maximum rates of SPS activity measured under both saturating and limiting (inhibitory) conditions. SPS protein was also increased. The stronger competitive edge of SPS for carbon entering glycolysis during hexose phosphorylation was also demonstrated by the further decrease in respiration and the simultaneous, significant decline in both PEPCase and PK activities. A decreased anabolic demand for 2-oxoglutarate (2OG), which remained constant, was shown by the co-ordinated decrease in GOGAT. It is proposed that the further increase in amino acids in this phase of dehydration may be in part attributable to the breakdown of insoluble proteins.     

Glucose as a regulator of insulin-sensitive hexose uptake in 3T3 adipocytes

In the present study we examined the role of glucose in the regulation of its own transport activity in the cultured 3T3 fat cell. A regulatory control of glucose became apparent after these cells were cultured in the absence of glucose. Glucose deprivation of the cells was accompanied by a specific time and protein synthesis-dependent increase in dGlc (2-deoxyglucose) uptake (up to 5-fold), which was due to an increase in the apparent Vmax of the transport system. Concomitantly, the stimulatory effect of insulin on hexose uptake almost completely disappeared. Addition of glucose to the glucose-deprived cells rapidly reversed the deprivation effects. Cycloheximide experiments revealed that the glucose deprivation-induced increase in hexose uptake required protein synthesis as well as a protein synthesis-independent response to glucose deprivation that retarded the turnover of hexose transport activity. Taken together, these data indicate that glucose deprivation is accompanied by retardation of the rate of degradation, internalization, or inactivation of hexose transporters while the increase in dGlc uptake requires at least the continuation of protein synthesis-dependent de novo synthesis, insertion, or activation of hexose transporters. Hexose competitively taken up with dGlc, including the nonmetabolizable glucose analogue 3-O-methylglucose, could replace glucose in the process of prevention and reversal of the deprivation effects, indicating that competitive transport but not the metabolism of hexose is a prerequisite for the regulatory effect of glucose on the activity of its own transport system. In conclusion, our results indicate that in cultured 3T3 fat cells glucose itself is involved in the regulation of the activity of its own transport system by influencing the rate of degradation, internalization, or inactivation of hexose transporters by a protein synthesis-independent mechanism.     

Tissue-specific induction of the mRNA for an extracellular invertase isoenzyme of tomato by brassinosteroids suggests a role for steroid hormones in assimilate partitioning

Brassinosteroids (BRs) induce various growth responses when applied exogenously to plant tissues, and the analysis of biosynthetic mutants reveals an essential role for plant growth and development. Only a few BR-regulated genes have been identified so far, and the corresponding gene products are assumed to be involved in cell elongation. The present study shows that BR growth responses are linked to the regulation of carbohydrate metabolism by induction of the mRNA for the key enzyme of an apoplastic phloem-unloading pathway. Addition of BRs to autotrophic tomato suspension culture cells specifically elevates the activity of cell-wall-bound invertase, whereas the intracellular invertase activities were not affected. This enhanced enzyme activity was shown to correlate with the induction of the mRNA of extracellular invertase Lin6, whereas the mRNA levels of the other three extracellular invertase isoenzymes were not affected. The induction level induced by different BRs correlates with their growth-promoting activity. The physiological significance of this regulation is further supported by the low concentrations and short incubation times required to induce Lin6 mRNA. This regulatory mechanism results in an elevated uptake of sucrose via the hexose monomers, and thus an increased supply of carbohydrates to the BR-treated cells. Experiments with tomato seedlings showed that the localized BR-dependent growth response of the hypocotyl elongation zone was accompanied by a specific induction of Lin6 mRNA that is restricted to the corresponding tissues. This study demonstrates a role of BRs in tissue-specific source/sink regulation.     

Differentiation of legume cotyledons as related to metabolic gradients and assimilate transport into seeds

Legume seed development is closely related to metabolism and nutrient transport. To analyse this relationship, a combination of biochemical, histological and transgenic approaches was used. Sugars within tissue sections have been quantitatively measured by metabolic imaging. During cotyledon differentiation glucose gradients emerge related to a particular cell type, with higher concentrations in non-differentiated premature regions. Sucrose in creases at the beginning of maturation in a layer underneath the outer epidermis expressing a sucrose transporter. Sucrose distribution is initially controlled by uptake activity and the permeability within the parenchyma and, later on, also by differences in growth and starch accumulation. Increased sucrose levels are accompanied by increased levels of sucrose synthase and ADP-Glc pyrophosphorylase mRNAs, but carbon flux into starch is initially still low. Rates increase at a stage when hexose concentrations become low, allowing increased flux through the sucrose synthase pathway. Transfer cell formation represents a regional specification of the cotyledonary epidermis for embryo nutrition characterized by increased transport-active cell surfaces and up-regulated expression of transport-related genes. The E2748 pea seed mutation blocks epidermal differentiation into transfer cells and leads to the loss of epidermal cell identity. Embryos with impaired epidermis cannot tolerate elevated levels of sucrose and respond with disorganized growth. The E2748 gene product is required for transfer cell formation in developing cotyledons with no other function during plant growth. Seed coat permeability provides a hypoxic environment for embryo development. However, at maturity, seed energy supply is not limited indicating fundamental developmental and metabolic adaptations. Results from transgenic seeds show that altered expression of single genes induces complex and unexpected changes. In AGP-antisense seeds the block in starch synthesis leads to pleiotropic effects of water and nitrogen content and induces temporal changes in seed development.     

Infection of maize leaves with Ustilago maydis prevents establishment of C4 photosynthesis

The Basidiomycete fungus Ustilago maydis is the common agent of corn smut and is capable of inducing gall growth on infected tissue of the C4 plant maize (Zea mays). While U. maydis is very well characterized on the genetic level, the physiological changes in the host plant in response to U. maydis infection have not been studied in detail, yet. Therefore, we examined the influence of U. maydis infection on photosynthetic performance and carbon metabolism in maize leaf galls. At all stages of development, U. maydis-induced leaf galls exhibited carbon dioxide response curves, CO2 compensation points and enzymatic activities that are characteristic of C3 photosynthesis, demonstrating that the establishment of C4 metabolism is prevented in infected tissue. Hexose contents and hexose/sucrose ratio of leaf galls remained high at 6 days post infection, while a shift in free sugar metabolism was observed in the uninfected controls at that time point. Concomitantly, transitory starch production and sucrose accumulation during the light period remained low in leaf galls. Given that U. maydis is infectious on young developing tissue, the observed changes in carbohydrate metabolism suggest that the pathogen manipulates the developing leaf tissue to arrest sink-to-source transition in favor of maintaining sink metabolism in the host cells. Furthermore, evidence is presented that carbohydrate supply during the biotrophic phase of the pathogen is assured by a fungal invertase.     

Complex regulation of gene expression, photosynthesis and sugar levels by pathogen infection in tomato

The infection of plants with pathogens results in the induction of defence reactions as well as changes in carbohydrate metabolism. On the one hand, the pathogen attempts to manipulate the carbohydrate metabolism of the plant for its own advantage. On the other, the plant has to reorganize carbon fluxes to ensure fight against the pathogen. In order to further investigate the connection between pathogen infection and carbohydrate metabolism, the effects of two types of pathogen, biotrophic and necrotrophic, on gene expression, endogenous sugar levels and photosynthesis of tomato plants were analysed. Photosynthetic gene expression was downregulated on infection with Pseudomonas syringae and Botrytis cinerea . In contrast, expression of a sink-specific gene encoding a cell wall invertase and of defence genes was induced by both pathogens. These results provide evidence for a co-regulation of defence, sink and photosynthetic gene expression in planta in response to both types of pathogen. The brassinosteroid-containing plant restorative ComCat enhanced resistance against B. cinerea and counter-regulated the repression of photosynthetic gene expression. Endogenous sugar levels decreased and the hexose to sucrose ratio increased on treatment with B. cinerea . The application of chlorophyll fluorescence imaging revealed the spatio-temporal heterogeneity of the pathogen response. At 24 h after infection, inhibition of photosynthetic electron transport was restricted to the direct vicinity of the infection site, which was surrounded by a circle of increased photosynthetic activity. The photosynthesis of the remaining leaf was not affected at this stage. These results show the usefulness of chlorophyll fluorescence imaging for the assessment of the complex spatio-temporal changes and for the definition of the areas relevant for other types of determination, e.g. gene expression.     

Soluble acid invertase activity in leaves is independent of species differences in leaf carbohydrates, diurnal sugar profiles and paths of phloem loading

Leaf sucrose, starch, hexose and maximum extractable soluble acid invertase activity were compared throughout the day in source leaves of 13 plant species chosen for their putative phloem-loading type (apoplastic or symplastic). Four species which represent the different phloem-loading types (tomato, barley, maize and Fuchsia ) were studied in detail. Using this information we wished to determine whether a positive correlation between foliar carbohydrates and acid invertase activity exists in leaves from different species and, furthermore, whether this relationship is determined by phloem-loading type. Acid invertase activity was relatively constant throughout the day in all species. The extent of sucrose, hexose and starch accumulation and the sucrose: starch ratio measured at a given time were species-dependent. No correlations were found between foliar soluble acid invertase activity and the hexose, sucrose or starch content of the leaves in any of the species, regardless of phloem-loading type. The species examined could be divided into three distinct groups: (1) high sucrose, low invertase; (2) low sucrose, low invertase; and (3) low sucrose, high invertase. The absence of an inverse relationship between leaf sucrose, hexose or starch contents and endogenous soluble acid invertase suggests that this enzyme is not directly involved in carbon partitioning in leaves but serves an auxiliary function.     

2‐DE proteome maps for the leaf apoplast of Nicotiana benthamiana

We provide 2‐D gel reference maps for the apoplastic proteome of Nicotiana benthamiana leaves infiltrated or not with the bacterial gene vector Agrobacterium tumefaciens. About 90 proteins were analyzed by LC‐MS/MS for identification and function assignment. We show, overall, an effective response of the plant to agroinfiltration involving a specific, cell wall maintenance‐independent up‐regulation of defense protein secretion. The proteome maps described should be a useful tool for systemic studies on plant–pathogen interactions or cell wall metabolism. They also should prove useful for the monitoring of secreted recombinant proteins and their possible pleiotropic effects along the cell secretory pathway of N. benthamiana leaves used as an expression platform for clinically useful proteins.     

Sucrose metabolism in grape (Vitis vinifera L.) branches under low temperature during overwintering covered with soil

The effect of low temperature on sugar content and activities of key enzymes related to sucrose metabolism in grape (Vitis vinifera L.) branches during overwintering covered with soil was investigated. We measured the contents of soluble sugar and the activities of sucrose-phosphate synthase (SPS), sucrose synthase (SS), acid invertase (AI) and neutral invertase (NI) of three grape varieties with different freezing tolerance, Beta, Vidal and Merlot, in October, 2011, January, 2012 and March, 2012. The result showed that: total soluble sugar had the significant negative correlation, ?0.988, with temperature during overwintering covered with soil. The content of hexose was about twofold content of sucrose in January, while sucrose increased and the hexose decreased to a very low level in March, the ratios between hexose and sucrose declined to 0.26, 0.15 and 0.18. Sucrose was more important than hexose in protecting grape branches from cold injury under low temperature, but non-freezing. The accumulation of sucrose was mostly due to the elevation of the SPS activity, whereas the increase of hexose was due to the enhanced AI activity. Three grape varieties responded to low temperature positively as reflected by the variations of physiological and biochemical characteristics, such as superoxide dismutase, catalase and proline. Besides, by the principal components analysis and combined with cultivation practices, among twelve characteristics, the sugar metabolism mainly contributed to the difference of the cold resistance. The results indicated that sucrose metabolism regulation played an important role during overwintering covered with soil, and it was the key factor to explain the difference of cold resistance.     

Fusarium oxysporum mediates systems metabolic reprogramming of chickpea roots as revealed by a combination of proteomics and metabolomics

Molecular changes elicited by plants in response to fungal attack and how this affects plant–pathogen interaction, including susceptibility or resistance, remain elusive. We studied the dynamics in root metabolism during compatible and incompatible interactions between chickpea and Fusarium oxysporum f. sp. ciceri (Foc), using quantitative label‐free proteomics and NMR‐based metabolomics. Results demonstrated differential expression of proteins and metabolites upon Foc inoculations in the resistant plants compared with the susceptible ones. Additionally, expression analysis of candidate genes supported the proteomic and metabolic variations in the chickpea roots upon Foc inoculation. In particular, we found that the resistant plants revealed significant increase in the carbon and nitrogen metabolism; generation of reactive oxygen species (ROS), lignification and phytoalexins. The levels of some of the pathogenesis‐related proteins were significantly higher upon Foc inoculation in the resistant plant. Interestingly, results also exhibited the crucial role of altered Yang cycle, which contributed in different methylation reactions and unfolded protein response in the chickpea roots against Foc. Overall, the observed modulations in the metabolic flux as outcome of several orchestrated molecular events are determinant of plant's role in chickpea–Foc interactions.     

In situ activities of hexokinase and fructokinase in relation to phosphorylation status of root meristem cells of Vicia faba during reactivation from sugar starvation

The plant cell cycle is equipped with two principal control points: PCP1 in G1 and PCP2 in G2 phase. These checkpoints can arrest the cell cycle in response to carbohydrate starvation, while sugar presence can revive the replication and mitotic activity. The process of cell cycle revival is strongly repressed by okadaic acid (OA) or 6-dimethylaminopurine (6-DMAP), inhibitors of specific protein phosphatases 1 or 2A or kinases (cyclin-dependent kinases), respectively. In the present study, it was investigated whether inhibition of cell cycle revival is performed through interference of the above-mentioned inhibitors with the metabolic pathway of sucrose applied to the cells. Changes of hexokinase (HK) and fructokinase (FK) activities, key enzymes of hexose metabolism, were analyzed in Vicia faba root meristem cells arrested in G1 and G2 phase by carbohydrate starvation as well as in those recovered with glucose or sucrose in the presence of OA or 6-DMAP. It was shown that in the sugar-starved cells, the activity of both enzymes decreased significantly. During cell regeneration with carbohydrates, the activity of HK was induced more by sucrose than by glucose, while FK remained inactive after glucose addition. Moreover, in situ investigation of the activities of HK and FK showed that OA-induced and 6-DMAP-induced repression of the cell cycle revival is connected with the interference of these drugs in the metabolic pathway of sucrose. It was also indicated that stronger OA-induced and 6-DMAP-induced inhibition of the replication and mitosis revival, at the early stages of sucrose regeneration, was correlated with the stronger influence of these inhibitors on HK and FK activities.     

Root elongation and branching is related to local hexose concentration in Arabidopsis thaliana seedlings

Root architecture can be profoundly affected by the carbon availability in the plant. We hypothesized that this effect could be mediated by the carbon status of root cells involved in elongation and branching processes. Arabidopsis thaliana plants were grown at several photosynthetic photon flux densities (PPFD) and were supplied with various sucrose concentrations in the root medium. Hexose and sucrose concentration was estimated in individual roots in the apical growing region of the primary root and of secondary roots as well as in the zone of primordia development. Local sugar concentration was high in fast‐growing and in highly branched roots and robust relationships between root elongation rate or branching and hexose concentration (but not sucrose) were found that were common to all situations experienced. Moreover, these relationships accounted for the plant‐to‐plant variability within a treatment as well as for the variability among individual secondary roots within a plant. These results support the view that local hexose concentration integrates changes in carbon availability from several sources and acts as a signal to induce at least part of the response of the root architecture to the environment.