[PubMed] [Google Scholar]Rich PR. growth of soybean seedlings can be largely explained by decreases in maximal rates of electron transport via COX. Flux via AOX Tideglusib is usually increased so that the ubiquinone pool is usually maintained in a moderately reduced state. The rate of herb respiration is usually linked to the rate of metabolism and growth due to requirements for ATP, reductant, and carbon skeletons during cell maintenance, division, and growth (Hunt and Loomis, 1979; Lambers et al., 1983). For example, respiration rates are often lower in species with intrinsically slower growth rates GSS (Poorter et al., 1991). Moreover, respiration is usually rapid in tissues with high energy demands, such as thermogenic floral spadices (Meeuse, 1975), and in rapidly growing tissues, such as the elongation zone of roots (Lambers et al., 1996). Herb respiration can also increase rapidly in response to both biotic and abiotic stress (for a recent review, see Lambers et al., 1996). Conversely, decreases in respiratory rate often occur as plant tissues age (Azcon-Bieto et al., 1983; McDonnell and Farrar, 1993; Atkin and Cummins, 1994; Winkler et al., 1994). Various factors may be responsible for these changes, including substrate availability, enzyme activation, specific protein degradation or de novo protein synthesis, and alterations in mitochondrial numbers. The extent to which such changes in respiration rate alter the rate of oxidative phosphorylation also depends on the partitioning of electron flux between the Cyt and the alternative pathways of electron transport. The Cyt Tideglusib Tideglusib pathway (terminating at COX) couples the reduction of O2 to water with the translocation of protons across the inner mitochondrial membrane, thereby building a proton-motive pressure that drives ATP synthesis. The alternative pathway branches directly from Q and reduces O2 to water without further proton translocation. Tideglusib This pathway appears to consist of a single-subunit cyanide-resistant quinol oxidase, AOX. Electron flow via AOX in plants can allow carbon flux through the TCA cycle when ADP is usually limiting, thereby providing carbon skeletons for other cellular processes (Lambers and Steingr?ver, 1978). This pathway may also protect against harmful reactive O2 generation when the Q pool is usually highly reduced (Purvis and Shewfelt, 1993; Wagner and Krab, 1995), allow respiration to proceed in the presence of nitric oxide (Millar and Day, 1996), and help avoid the production of fermentation products when pyruvate accumulates (Vanlerberghe et al., 1995). Partitioning between COX and AOX can be dramatically affected by factors that influence the AOX activation state (Hoefnagel et al., 1995; Ribas-Carbo et al., 1995a, 1997). AOX exists as a dimer in plants, and sulfhydryl linkages between paired subunits must be reduced for maximal AOX activity (Umbach and Siedow, 1993). A variety of 2-oxo acids, notably pyruvate, have been shown to specifically and reversibly stimulate AOX activity at micromolar concentrations (Millar et al., 1993, 1996). These activators apparently increase the L. cv Stevens) seedlings propagated in trays of vermiculite in a growth cabinet at 28/25C with a 16-h light/8-h dark cycle. At d 4 the cotyledons and hypocotyls were greening and the root system (approximately 150 mg fresh mass/seedling) consisted of a single taproot without branches. At d 7 cotyledons were green and beginning to open, and the primary leaf was expanding. The primary root (approximately 300 mg fresh mass/seedling) had designed branches at the base in a classic taproot structure. At d 17 cotyledons were fully open and slightly yellowing, primary leaves were fully expanded, and the first trifoliate leaf was expanding. The root system (approximately Tideglusib 600 mg fresh mass/seedling) was a network of first- and second-order branches. Published methods were used to isolate mitochondria from roots of 4-, 7-, and 17-d-old seedlings (Day et al., 1985). Mitochondrial Assays O2 consumption was measured at 25C using an electrode (Rank Brothers, Cambridge, UK). A standard reaction medium (0.3 m Suc, 10 mm TES (to provide the rate of COX activity. In isolated mitochondria endogenous ascorbate-dependent O2 consumption was negligible. Protein content was determined by the method of Lowry et al. (1951). NAD-ME activities were assayed as NADH production at 340 nm, according to the method of Day et al. (1984), in a reaction medium consisting of 2 mm NAD+, 2 mm MnCl2, 4 mm DTT, 0.02% (v/v) Triton TX-100, 1 m antimycin A, 50 m and the supernatant filtered and neutralized.