Soluble sugars--metabolism, sensing and abiotic stress: a complex network in the life of plants.

Plants are autotrophic and photosynthetic organisms that both produce and consume sugars. Soluble sugars are highly sensitive to environmental stresses, which act on the supply of carbohydrates from source organs to sink ones. Sucrose and hexoses both play dual functions in gene regulation as exemplified by the upregulation of growth-related genes and downregulation of stress-related genes. Although coordinately regulated by sugars, these growth- and stress-related genes are upregulated or downregulated through HXK-dependent and/or HXK-independent pathways. Sucrose-non-fermenting-1- (SNF1-) related protein pathway, analogue to the protein kinase (SNF-) yeast-signalling pathway, seems also involved in sugar sensing and transduction in plants. However, even if plants share with yeast some elements involved in sugar sensing, several aspects of sugar perception are likely to be peculiar to higher plants. In this paper, we have reviewed recent evidences how plants sense and respond to environmental factors through sugar-sensing mechanisms. However, we think that forward and reverse genetic analysis in combination with expression profiling must be continued to uncover many signalling components, and a full biochemical characterization of the signalling complexes will be required to determine specificity and cross-talk in abiotic stress signalling pathways.

Solar and supplemental UV-B radiation effects in lemon peel UV-B-absorbing compound content-seasonal variations.

Effects of solar and supplemental UV-B radiation on UV-B-absorbing compounds and malondialdehyde (MDA) accumulations in the peel of lemons collected in summer and winter were analyzed. UV-B-absorbing compounds were higher in flavedo than in albedo tissue in both seasons; however, the highest values were observed in summer. These compounds were also higher in outer than in inner flavedo surface. Lemons were categorized as sun-, semisun- and shaded-lemon according to localization inside the tree canopy. Depending on-tree localization UV-B-absorbing compounds were higher in flavedo of sun-lemon than in semisun- and shaded-lemon. Supplementary UV-B radiation (22 kJ m(-2) day(-1) UV-BBE) induced UV-B-absorbing compound synthesis in on-tree and postharvest lemons. Two minutes of supplemental UV-B irradiation in summer lemons produced a strong increment (300%) of UV-B-absorbing compound content, whereas in winter lemons a slight increase (30%) was observed only after 3 min of irradiation. By contrast, UV-B-absorbing compound accumulation was not observed in albedo. MDA accumulation showed approximately a similar trend of UV-B-absorbing compounds. According to our results, solar UV-B was not required for UV-B-absorbing compound accumulation in lemon peel. Relationships between UV-B-absorbing compounds, MDA, reactive oxygen species and pathogen protection are also discussed.