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Exchange rates of CO2 and H2O and metabolism of hydrogen peroxide have been measured in leaves of alfalfa (Medicago sativa L. cv.. Aragón) under drought stress. The inhibitory effect of drought upon photosynthesis depended on the severity of the stress treatment. Leaf water potential (Ψleaf) down to -28 MPa reduced CO2 availability due to! stomata! closure and inhibited the rate of photosynthesis.. Leaf water potential lower than -2.8 MPa directly affected CO2 fixation, although CO2 was not limiting. Transpiration was more affected by stomata! closure than photosynthesis, which led to an apparent improvement in WUE (water use efficiency). Alfalfa leaves with Ψleaf lower than -.2.0 MPa had an increased quantum requirement, probably due to the severe stress. effect on photoenergetic reactions. Ethylene evolution from alfalfa leaves increased when they were subjected to Ψleaf of -1.6 MPa. Under more severe stress, the leaves showed low or almost no ethylene production. In parallel with the increase in ethylene production, alfalfa leaves exhibited an increased membrane lipid peroxidation index (malondialdehyd.e content) and an increased peroxide content. Superoxide .disniutase activity (SOD; EC 1.15.1.1) was not affected by drought stress. Catalase (EC 1.11.1.6) was inhibited at slight stress but significantly increased at a Ψleaf of -2.0 MPa. Peroxidase (EC 1.11.1.7) was progressively inhibited as drought stress developed. The possible implication of reactive O2 intermediates in drought stress-induced senescence of alfalfa leaves is discussed in the light of the pattern of enzymatic scavenging systems. [ABSTRACT FROM AUTHOR]

The effect of drought upon phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31), malate dehydrogenase (MDH; EC 1.1.1.37), alcohol dehydrogenase (ADH; EC 1.1.1.1) and β-hydroxybutyrate dehydrogenase (β-OH-BDH; EC 1.1.130) enzyme activities as well as the leghemoglobin (Lb), malate and ethanol contents of alfalfa nodules (Medicago sativa L. cv Aragón) were examined. Both the leghemoglobin (Lb) content and the Lb/soluble protein ratio were significantly reduced at a nodule water potential (ψmod ) of -1.3 MPa. At lower ψmod,Lb content decreased further, but the ratio remained unchanged. Slight stress (-1.3 MPa) drastically affected acetylene reduction activity (ARA; 60% reduction)' whereas in vitro PEPC activity was maintained at relatively constant values. As stress progressed (-2.0 MPa), a simultaneous reduction in both activities was observed. Severe stress (ψmodlower than: -2.0 MPa) stimulated in vitro PEPC. Bacteroid β-OH-BDFI activity was stimulated by slight (-1.3 MPa) and moderate (-2.0 MPa) drought. MDH activity rose in slightly stressed nodules (ψmod -1.3 MPa). Greater water deficits sharply decreased. MDH activity to! values significant]y lower than those found in control nodules. Nodule malate content followed the same pattern as MDH. The plant fraction of the nodule showed constitutive ADH activity and contained ethanol, ADH was stimulated at slight (-1.3 MPa) and moderate drought levels (-2.0 MPa). Ethanol content showed similar behavior to ADH activity. Inhibition of ARA, reduction of Lb con tent and stimulation of the fermentative metabolism induced by water stress suggest some reduction in O2 availability within the nodule. [ABSTRACT FROM AUTHOR]

The genes encoding malate synthase (glcB) and isocitrate lyase (aceA) and a 240-bp open reading frame (SMc00767) located downstream of aceA were isolated and functionally characterized in Sinorhizobium meliloti. Independent and double interposon mutants of each gene were constructed, and the corresponding phenotypes were analyzed. aceA mutants failed to grow on acetate, and mutants deficient in SMc00767 were also affected in acetate utilization. In contrast, mutants deficient in glcB grew on acetate similar to wild-type strain Rm5000. Complementation experiments showed that aceA and SMc00767 gene constructs were able to restore the growth on acetate in the corresponding single mutants. aceA-glcB, aceA-SMc00767, and glcB-SMc00767 double knockouts were also unable to grow on acetate, but this ability was recovered when the wild-type aceA-glcB or aceA-SMc00767 loci were introduced into the double mutants. These data confirm the functional role of aceA and SMc00767 and show that glcB, in the absence of SMc00767, is required for acetate metabolism. Isocitrate lyase and malate synthase activities were measured in strain Rm5000, the mutant derivatives, and complemented strains. aceA and glcB were able to complement the enzymatic activity lacking in the corresponding single mutants. The enzymatic activities also showed that SMc00767 represses the activity of isocitrate lyase in cells grown on acetate. Gene fusions confirmed the repressor role of SMc00767, which regulates aceA expression at the transcriptional level. Comparison of the transcriptional profiles of the SMc00767 mutant and wild-type strain Rm5000 showed that SMc00767 represses the expression of a moderate number of open reading frames, including aceA; thus, we propose that SMc00767 is a novel repressor involved in acetate metabolism in S. meliloti. Genetic and functional analyses indicated that aceA and SMc00767 constitute a functional two-gene operon, which is conserved in other α-proteobacteria. Alfalfa plants infected with the aceA and glcB mutants were not impaired in nodulation or nitrogen fixation, and so the glyoxylate cycle is not required in the Rhizobium-legume symbiosis. [ABSTRACT FROM AUTHOR]