No abstract

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 presence of sucrose (Suc) in plastids was questioned for several decades. Although it was reported some decades ago, neither Suc transporters nor Suc metabolizing enzymes were demonstrated to be active in those organelles. By biochemical, immunological, molecular and genetic approaches we show that alkaline/neutral invertases (A/N-Invs) are also localized in chloroplasts of spinach and Arabidopsis. A/N-Inv activity and polypeptide content were shown in protein extracts from intact chloroplasts. Moreover, we functionally characterized the Arabidopsis At-A/N-InvE gene coding for a chloroplast-targeted A/N-Inv. The At-A/N-InvE knockout plants displayed a lower total A/N-Inv activity in comparison with wild-type plants. Furthermore, neither A/N-Inv activity nor A/N-Inv polypeptides were detected in protein extracts prepared from chloroplasts of mutant plants. Also, the measurement of carbohydrate content, in leaves harvested either at the end of the day or at the end of the night period, revealed that the knockout plants showed a decrease in starch accumulation but no alteration in Suc levels. These are the first results demonstrating the presence of a functional A/N-Inv inside chloroplasts and its relation with carbon storage in Arabidopsis leaves. Taken together our data and recent reports, we conclude that the participation of A/N-Invs in the carbon flux between the cytosol and the plastids may be a general phenomenon in plants.

1990. Suerose and fruetan metabolism of different wheat cultivars at chilling temperatures. -Physiol. Plant. 78; 554-559.Four wheat (Tritieum aestivum L.) varieties cultivated in different climates from subtropics to North Patagonia were used to study sucrose and fructan metabolism in plants when submitted to a cold period. Higher levels of sugars were found in the more cold tolerant cultivars. Sucrose synthase (EC 2.4.1.13) and sucrose phosphate synthase (EC 2.4.1.14) activities showed a 2-3 fold increase when plants were grown at 4°C for 10 days. The more cold-tolerant wheat cultivars also showed the higher levels of enzyme activities. These metabolieal changes were not due to anatomical or morphological differences produced during growth at 4°C. Materials and methods:. .;:•*•,;: Plant materialFour wheat (Triticum aestivum L.) eultivars adapted to different clitnates were used. Cultivar Trigal 800 and cv. Buck Patacon seeds were kindly provided by Ing. J.