Soluble sugars, especially sucrose, glucose, and fructose, play an obviously central role in plant structure and metabolism at the cellular and whole-organism levels. They are involved in the responses to a number of stresses, and they act as nutrient and metabolite signalling molecules that activate specific or hormone-crosstalk transduction pathways, thus resulting in important modifications of gene expression and proteomic patterns. Various metabolic reactions and regulations directly link soluble sugars with the production rates of reactive oxygen species, such as mitochondrial respiration or photosynthesis regulation, and, conversely, with anti-oxidative processes, such as the oxidative pentose-phosphate pathway and carotenoid biosynthesis. Moreover, stress situations where soluble sugars are involved, such as chilling, herbicide injury, or pathogen attack, are related to important changes in reactive oxygen species balance. These converging or antagonistic relationships between soluble sugars, reactive oxygen species production, and anti-oxidant processes are generally confirmed by current transcriptome analyses, and suggest that sugar signalling and sugar-modulated gene expression are related to the control of oxidative stress. All these links place soluble carbohydrates in a pivotal role in the pro-oxidant and antioxidant balance, and must have constrained the selection of adaptive mechanisms involving soluble sugars and preventing de-regulation of reactive oxygen species production. Finally, in line with the specific role of sucrose in oxygenic photosynthetic organisms, this role of soluble sugars in oxidative stress regulation seems to entail differential effects of glucose and sucrose, which emphasizes the unresolved issue of characterizing sucrose-specific signalling pathways.

Background: Besides being essential for plant structure and metabolism, soluble carbohydrates play important roles in stress responses. Sucrose has been shown to confer to Arabidopsis seedlings a high level of tolerance to the herbicide atrazine, which causes reactive oxygen species (ROS) production and oxidative stress. The effects of atrazine and of exogenous sucrose on ROS patterns and ROS-scavenging systems were studied. Simultaneous analysis of ROS contents, expression of ROS-related genes and activities of ROS-scavenging enzymes gave an integrative view of physiological state and detoxifying potential under conditions of sensitivity or tolerance.

), we have investigated the involvement of this molecule in the osmotic adaptation of Rhizobium meliloti. Ectoine appeared almost as effective as glycine betaine in improving the growth of R. meliboti under adverse osmotic conditions (0.5 M NaCI). Moreover, improvement of growth of rhizobial strains insensitive to glycine betaine was also observed. Ectoine transport proved inducible, periplasmic protein dependent, and, as shown by competition experiments, distinct from the transport of glycine betaine. Medium osmolarity had little effect on the uptake characteristics, since the rate of influx increased from 12 to only 20 nmol min-' mg of proteinwhen NaCI concentrations were raised from 0 to 0.3 or 0.5 M, with a constant of transport of 80 FM. Natural-abundance "C-nuclear magnetic resonance and radiolabelling assays showed that ectoine, unlike glycine betaine, is not intracellularly accumulated and, as a consequence, does not repress the synthesis of endogenous compatible solutes (glutamate, N-acetylglutaminylglutamine amide, and trehalose). Furthermore, the strong rise in glutamate content in cells osmotically stressed in the presence of ectoine suggests that, instead of being involved in osmotic balance restoration, ectoine should play a key role in triggering the synthesis of endogenous osmolytes. Hence, we believe that there are at least two distinct classes of osmoprotectants: those such as glycine betaine or glutamate, which act as genuine osmolytes, and those such as ectoine, which act as chemical mediators.Rhizobia are soil bacteria which display symbiotic interactions with specific legume hosts. Most of these bacteria are very sensitive to a soil water deficit, which adversely affects their dinitrogen fixation capacity and hence the productivity of the whole legume plant (1,8,19,38). Much attention has been focused during the last decade on the mechanisms of osmotic adaptation in several rhizobium species (18,41,42). Rhizobium meliloti, the microsymbiont in alfalfa root nodules, has been extensively investigated (5,6,22,36), and the mechanisms governing turgor restoration of bacterial cells growing under hyperosmotic conditions have been partially elucidated. Specifically, concomitant accumulations of potassium and glutamate ions were pointed out as the primary response in R meliloti (6), and the same mechanism has been found in most of the bacterial species investigated thus far (7, 27). Trehalose and the dipeptide N-acetylglutaminylglutamine amide (NAGGN) (37) are also accumulated, probably to help the bacterial cells in balancing the high osmotic pressure of the medium. Among a series of exogenously supplied osmoprotectants, glycine betaine and proline betaine proved the most effective (3, 14).Their effect is quite similar to that found in members of the family Enterobacteriaceae (23) except that lowering medium osmolality triggers catabolism of betaines in R meliloti (3, 14, 36). A transport system involving a periplasmic glycine betainebinding protein was identified previously (24, 40). Glycine beta...

Background: Soluble sugars, which play a central role in plant structure and metabolism, are also involved in the responses to a number of stresses, and act as metabolite signalling molecules that activate specific or hormone-crosstalk transduction pathways. The different roles of exogenous sucrose in the tolerance of Arabidopsis thaliana plantlets to the herbicide atrazine and oxidative stress were studied by a transcriptomic approach using CATMA arrays.