Jorge A. Tognetti

Sucrose signaling in plants: A world yet to be explored

The role of sucrose as a signaling molecule in plants was originally proposed several decades ago. However, recognition of sucrose as a true signal has been largely debated and only recently this role has been fully accepted. The best-studied cases of sucrose signaling involve metabolic processes, such as the induction of fructan or anthocyanin synthesis, but a large volume of scattered information suggests that sucrose signals may control a vast array of developmental processes along the whole life cycle of the plant. Also, wide gaps exist in our current understanding of the intracellular steps that mediate sucrose action. Sucrose concentration in plant tissues tends to be directly related to light intensity, and inversely related to temperature, and accordingly, exogenous sucrose supply often mimics the effect of high light and cold. However, many exceptions to this rule seem to occur due to interactions with other signaling pathways. In conclusion, the sucrose role as a signal molecule in plants is starting to be unveiled and much research is still needed to have a complete map of its significance in plant function.The role of sucrose as a signaling molecule in plants was originally proposed several decades ago. However, recognition of sucrose as a true signal has been largely debated and only recently this role has been fully accepted. The best-studied cases of sucrose signaling involve metabolic processes, such as the induction of fructan or anthocyanin synthesis, but a large volume of scattered information suggests that sucrose signals may control a vast array of developmental processes along the whole life cycle of the plant. Also, wide gaps exist in our current understanding of the intracellular steps that mediate sucrose action. Sucrose concentration in plant tissues tends to be directly related to light intensity, and inversely related to temperature, and accordingly, exogenous sucrose supply often mimics the effect of high light and cold. However, many exceptions to this rule seem to occur due to interactions with other signaling pathways. In conclusion, the sucrose role as a signal molecule in plants is starting to be unveiled and much research is still needed to have a complete map of its significance in plant function.

Fructan Metabolism: Reversal of Cold Acclimation

Summary Wheat seedlings grown at 23 °C were acclimated for 15 days at 4 °C and then transferred back to 23 °C.The activtiy of sucrose synthase (SS), sucrose phosphate synthase (SPS), invertase, sucrose sucrose fructosyl transferase (SST), fructan hydrolase (FH) and UDPase were measured at different intervals after subjecting the plants to the higher temperature. The activities of SS, SPS and SST, which had increased during the cold period, steadily decreased to reach the levels before the cold acclimation, while the invertase and FH activities significantly increased at 23 °C. SS and SPS activities also fluctuate diurnally by having two peaks of activity during the light period. Arrhenius plots of SS and SPS showed no discontinuity between 4 and 30 °C, suggesting no changes inprotein conformation as a result of modifications in temperature.

Calcium is essential for fructan synthesis induction mediated by sucrose in wheat

The role of Ca2+ in the induction of enzymes involved in fructan synthesis (FSS) mediated by sucrose was studied in wheat (Triticum aestivum). Increase of FSS enzyme activity and induction of the expression of their coding genes by sucrose were inhibited in leaf blades treated with chelating agents (EDTA, EGTA and BAPTA). Ca2+ channel blockers (lanthanum chloride and ruthenium red) also inhibited the FSS response to sucrose, suggesting the participation of Ca2+ from both extra- and intra- cellular stores. Sucrose induced a rapid Ca2+ influx into the cytosol in wheat leaf and root tissues, shown with the Ca2+ sensitive fluorescent probe Fluo-3/AM ester. Our results support the hypothesis that calcium is a component of the sucrose signaling pathway that leads to the induction of fructan synthesis.

Differential Induction of Sucrose Metabolizing Enzymes in Wheat (Triticum aestivum cv. San Agustin) Leaf Sections

Summary The effect of carbohydrate supply on sucrose metabolism was studied in wheat leaf sections. Incubationwith glucose, fructose or sucrose resulted in an increase of sucrose level followed by accumulation of fructans. Concomitantly, sucrose phosphate synthase (SPS) activity increases continuously throughout the length of the experiment (48 hours) while sucrose synthase (SS) was unaffected for the first 30 hours. The de novo synthesis of SPS is suggested by Western blots and by inhibitors of protein synthesis. Thedifferent temporal patterns of SPS and SS activation indicate the distinct roles played by these enzymes in leaves.

Sugar signaling of fructan metabolism: New insights on protein phosphatases in sucrose-fed wheat leaves

Protein phosphatase type 2A (PP2A) activity is required for the sucrose induction of fructan metabolism in wheat leaves, as shown in experiments with the addition of the specific inhibitor okadaic acid (OA) together with sucrose. However, a decrease in total PP2A activity has been found along sucrose treatment. Here we analyze the effect of sucrose feeding to wheat leaves on PP2A activity profiles after Deae-Sephacel and Superose separation, in comparison with those of control leaves. The results show no evidence of changes in PP2A activity profiles as a consequence of sucrose feeding. In all, our data suggest that constitutive levels of PP2A activity may be sufficient for the sucrose-mediated induction of fructan metabolism and that general decrease of PP2A activity produced by long-term treatment with sucrose may be due to a negative feedback regulation.

Sunflower: a potential fructan-bearing crop?

Grain filling in sunflower (Helianthus annuus L.) mainly depends on actual photosynthesis, being the contribution of stored reserves in stems (sucrose, hexoses, and starch) rather low. Drought periods during grain filling often reduce yield. Increasing the capacity of stem to store reserves could help to increase grain filling and yield stability in dry years. Fructans improve water uptake in soils at low water potential, and allow the storage of large amount of assimilates per unit tissue volume that can be readily remobilized to grains. Sunflower is a close relative to Jerusalem artichoke (H. tuberosus L.), which accumulates large amounts of fructan (inulin) in tubers and true stems. The reason why sunflower does not accumulate fructans is obscure. Through a bioinformatics analysis of a sunflower transcriptome database, we found sequences that are homologous to dicotyledon and monocotyledon fructan synthesis genes. A HPLC analysis of stem sugar composition revealed the presence of low amounts of 1-kestose, while a drastic enhancement of endogenous sucrose levels by capitulum removal did not promote 1-kestose accumulation. This suggests that the regulation of fructan synthesis in this species may differ from the currently best known model, mainly derived from research on Poaceae, where sucrose acts as both a signaling molecule and substrate, in the induction of fructan synthesis. Thus, sunflower might potentially constitute a fructan-bearing species, which could result in an improvement of its performance as a grain crop. However, a large effort is needed to elucidate how this up to now unsuspected potential could be effectively expressed.

Sugar Signaling Under Abiotic Stress in Plants

Abstract In plants, abiotic stresses are associated with imbalances between carbon fixation and utilization, resulting in altered sugar concentration. Plants have developed the ability to sense changes in levels of glucose, fructose, sucrose, and trehalose-6-P, among others, and to respond accordingly through a very complex signaling network to modify gene expression and protein activity to cope with the environmental challenge. Two large signaling networks have been described, the sucrose nonfermenting related kinase (SnRK) and the target of rapamycin kinases, which essentially correspond to opposite situations: low or high carbon availability, respectively. The specificity of plant response to a particular stress may be given, in part, by interactions with other signaling pathways, particularly hormones. In-depth knowledge of sugar signaling networks will facilitate the use of either genetic modification or chemical intervention tools to improve the performance of crops under abiotic stress.