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.

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A growth chamber experiment was conducted to assess the effect of salinity on emergence, growth, water status, photosynthetic pigments, osmolyte accumulation, and ionic content of quinoa seedlings (Chenopodium quinoa). The aim was to test the hypothesis that quinoa seedlings are well adapted to grow under salinity due to their ability to adjust the metabolic functionality of their cotyledons. Seedlings were grown for 21 days at 250 mM NaCl from the start of the germination. Germination percentage and cotyledon area were not affected by salt whereas seedling height decreased 15%. FW increased in both control and salt-treated cotyledons, but the increase was higher under salinity. DW only increased in salttreated cotyledons. The DW/FW ratio did not show significant differences between treatments. Relative water content, chlorophyll, carotenoids, lipids, and proteins were significantly lower under salinity. Total soluble sugars, sucrose and glucose concentrations were higher in salt-treated than in control cotyledons. Ion concentration showed a different distribution pattern. Na + and Cl − concentrations were higher under salinity, while an inverse result was observed for K + concentration. Proline and glycinebetaine concentrations increased under salinity, but the increase was higher in the former than the latter. The osmoprotective role of proline, glycinebetaine, and soluble sugars is discussed.

The Polylepis tarapacana forests found in Bolivia are unique with respect to their altitudinal distribution (4200-5200 m). Given the extreme environmental conditions that characterize these altitudes, this species has to rely on distinct mechanisms to survive stressful temperatures. The purpose of this study was to determine lowtemperature resistance mechanisms in P. tarapacana. Tissue was sampled for carbohydrate and proline contents and micro-climatic measurements were made at two altitudes, 4300 and 4850 m, during both the dry cold and wet warm seasons. Supercooling capacity (-3 to -6°C for the cold dry and -7 to -9°C for the wet warm season) and injury temperatures (-18 to -23°C for both seasons), determined in the laboratory, indicate that P. tarapacana is a frost-tolerant species. On the other hand, an increase in supercooling capacity, as the result of significant increase in total soluble sugar and proline contents, occurs during the wet warm season as a consequence of higher metabolic activity. Hence, P. tarapacana, a frost-tolerant species during the colder unfavourable season, is able to avoid freezing during the more favourable season when minimum night-time temperatures are not as extreme.

We conducted a coordinated biochemical and morphometric analysis of the effect of saline conditions on the differentiation zone of developing soybean (Glycine max L.) roots. Between d 3 and d 14 for seedlings grown in control or NaCl-supplemented medium, we studied (a) the temporal evolution of the respiratory alternative oxidase (AOX) capacity in correlation with the expression and localization of AOX protein analyzed by tissue-print immunoblotting; (b) the temporal evolution and tissue localization of a peroxidase activity involved in lignification; and (c) the structural changes, visualized by light microscopy and quantified by image digitization. The results revealed that saline stress retards primary xylem differentiation. There is a corresponding delay in the temporal pattern of AOX expression, which is consistent with the xylemspecific localization of AOX protein and the idea that this enzyme is linked to xylem development. An NaCl-induced acceleration of the development of secondary xylem was also observed. However, the temporal pattern of a peroxidase activity localized in the primary and secondary xylem was unaltered by NaCl treatment. Thus, the NaCl-stressed root was specifically affected in the temporal patterns of AOX expression and xylem development.

UV-B radiation (280-320 nm) is harmful to living organisms and has detrimental effects on plant growth, development and physiology. In this work we examined some mechanisms involved in plant responses to UV-B radiation. Seedlings of quinoa (Chenopodium quinoa Willd.) were exposed to variable numbers of UV-B radiation doses, and the effect on cotyledons was studied. We analyzed (1) cotyledons anatomy and chloroplasts ultrastructure; (2) peroxidase activity involved in the lignification processes; and (3) content of photosynthetic pigments, phenolic compounds and carbohydrates. Exposure to two UV-B doses induced an increase in the wall thickness of epidermal cells, which was associated with lignin deposition and higher activity of the peroxidase. The chloroplast ultrastructure showed an appearance typical of plants under shade conditions, likely in response to reduced light penetration into the mesophyll cells due to the screening effect of epidermal lignin deposition. Exposure to UV-B radiation also led to (1) enhancement in the level of phenolics, which may serve a protective function; (2) strong increase in the fructose content, a fact that might be related to higher requirement of erythrose-4P as a substrate for the synthesis of lignin and phenolics; and (3) reduction in the chlorophyll concentration, evidencing alteration in the photosynthetic system. We propose that the observed lignin deposition in epidermal tissues of quinoa is a resistance mechanism against UV-B radiation, which allows growing of this species in Andean highlands.

UV‐B radiation (280–320 nm) is harmful to living organisms and has detrimental effects on plant growth, development and physiology. In this work we examined some mechanisms involved in plant responses to UV‐B radiation. Seedlings of quinoa (Chenopodium quinoa Willd.) were exposed to variable numbers of UV‐B radiation doses, and the effect on cotyledons was studied. We analyzed (1) cotyledons anatomy and chloroplasts ultrastructure; (2) peroxidase activity involved in the lignification processes; and (3) content of photosynthetic pigments, phenolic compounds and carbohydrates. Exposure to two UV‐B doses induced an increase in the wall thickness of epidermal cells, which was associated with lignin deposition and higher activity of the peroxidase. The chloroplast ultrastructure showed an appearance typical of plants under shade conditions, likely in response to reduced light penetration into the mesophyll cells due to the screening effect of epidermal lignin deposition. Exposure to UV‐B radiation also led to (1) enhancement in the level of phenolics, which may serve a protective function; (2) strong increase in the fructose content, a fact that might be related to higher requirement of erythrose‐4P as a substrate for the synthesis of lignin and phenolics; and (3) reduction in the chlorophyll concentration, evidencing alteration in the photosynthetic system. We propose that the observed lignin deposition in epidermal tissues of quinoa is a resistance mechanism against UV‐B radiation, which allows growing of this species in Andean highlands.

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