Early carbon mobilization and radicle protrusion in maize germination.

Considerable amounts of information is available on the complex carbohydrates that are mobilized and utilized by the seed to support early seedling development. These events occur after radicle has protruded from the seed. However, scarce information is available on the role of the endogenous soluble carbohydrates from the embryo in the first hours of germination. The present work analysed how the soluble carbohydrate reserves in isolated maize embryos are mobilized during 6-24 h of water imbibition, an interval that exclusively embraces the first two phases of the germination process. It was found that sucrose constitutes a very significant reserve in the scutellum and that it is efficiently consumed during the time in which the adjacent embryo axis is engaged in an active metabolism. Sucrose transporter was immunolocalized in the scutellum and in vascular elements. In parallel, a cell-wall invertase activity, which hydrolyses sucrose, developed in the embryo axis, which favoured higher glucose uptake. Sucrose and hexose transporters were active in the embryo tissues, together with the plasma membrane H(+)-ATPase, which was localized in all embryo regions involved in both nutrient transport and active cell elongation to support radicle extension. It is proposed that, during the initial maize germination phases, a net flow of sucrose takes place from the scutellum towards the embryo axis and regions that undergo elongation. During radicle extension, sucrose and hexose transporters, as well as H(+)-ATPase, become the fundamental proteins that orchestrate the transport of nutrients required for successful germination.

Enzymatic changes in pectic polysaccharides related to the beneficial effect of soaking on bean cooking time.

BACKGROUND: Cooking time decreases when beans are soaked first. However, the molecular basis of this decrease remains unclear. To determine the mechanisms involved, changes in both pectic polysaccharides and cell wall enzymes were monitored during soaking. Two cultivars and one breeding line were studied. RESULTS: Soaking increased the activity of the cell wall enzymes rhamnogalacturonase, galactanase and polygalacturonase. Their activity in the cell wall was detected as changes in chemical composition of pectic polysaccharides. Rhamnose content decreased but galactose and uronic acid contents increased in the polysaccharides of soaked beans. A decrease in the average molecular weight of the pectin fraction was induced during soaking. The decrease in rhamnose and the polygalacturonase activity were associated (r = 0.933, P = 0.01, and r = 0.725, P = 0.01, respectively) with shorter cooking time after soaking. CONCLUSION: Pectic cell wall enzymes are responsible for the changes in rhamnogalacturonan I and polygalacturonan induced during soaking and constitute the biochemical factors that give bean cell walls new polysaccharide arrangements. Rhamnogalacturonan I is dispersed throughout the entire cell wall and interacts with cellulose and hemicellulose fibres, resulting in a higher rate of pectic polysaccharide thermosolubility and, therefore, a shorter cooking time.

Redox-sensitive target detection in gibberellic acid-induced barley aleurone layer.

Reactive oxygen species (ROS) are involved in redox regulation by their capacity to reversibly oxidize cysteine residues. This regulation is used by cells to modulate and integrate different responses to extracellular stimuli. In the barley (Hordeum vulgare L.) aleurone layer, gibberellic acid (GA(3)) is perceived at the plasma membrane and induces the synthesis and secretion of alpha-amylase. All aleurone membrane systems participate in the elaboration of this response. During these events, ROS are generated as a by-product from intense lipid metabolism. Therefore, we hypothesized that redox regulation may be operating in the GA(3)-induced response. To test this hypothesis, we measured if GA(3) treatment induced changes in the redox state of aleurone membrane-associated proteins. Membrane proteins with sulfhydryl and disulfide groups were isolated from reduced and in situ NEM-alkylated microsomal fractions, respectively. Each fraction was enriched by thiol-affinity chromatography and separated by two-dimensional electrophoresis. The in vivo redox state of each membrane protein present in GA(3)-treated and -untreated tissue was determined. Results showed that GA(3) induced the reduced state in 17 constitutive proteins and the oxidized state in another 5. These data indicate that redox changes occur in membrane proteins after GA(3) signaling in the aleurone layer.

Control of superoxide production in mitochondria from maize mesocotyls.

To understand the biochemical events that control the generation of superoxide, the effect of inhibiting the respiratory complexes III and IV (C-III and C-IV) and alternative oxidase (AOX) on the rate of superoxide production was analyzed in mitochondria from maize seedlings. To increase superoxide production, it was required to inhibit C-III or C-IV by at least 30% or 50%, respectively. Below this inhibition threshold, AOX exerted the highest degree of control on superoxide production, whereas above it, the highest degree of control was exerted by C-IV. The contribution of C-III to control superoxide production became significant when AOX activity was modulated.