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Barley ( Hordeum vulgare L.), genetically modified with the Al3+ resistance gene of wheat ( TaALMT1), was compared with a non-transformed sibling line when grown on an acidic and highly phosphate-fixing ferrosol supplied with a range of phosphorus concentrations. In short-term pot trials (26 days), transgenic barley expressing TaALMT1 (GP-ALMT1) was more efficient than a non-transformed sibling line (GP) at taking up phosphorus on acid soil, but the genotypes did not differ when the soil was limed. Differences in phosphorus uptake efficiency on acid soil could be attributed not only to the differential effects of aluminium toxicity on root growth between the genotypes, but also to differences in phosphorus uptake per unit root length. Although GP-ALMT1 out-performed GP on acid soil, it was still not as efficient at taking up phosphorus as plants grown on limed soil. GP-ALMT1 plants grown in acid soil possessed substantially smaller rhizosheaths than those grown in limed soil, suggesting that root hairs were shorter. This is a probable reason for the lower phosphorus uptake efficiency. When grown to maturity in large pots, GP-ALMT1 plants produced more than twice the grain as GP plants grown on acid soil and 80% of the grain produced by limed controls. Expression of TaALMT1 in barley was not associated with a penalty in either total shoot or grain production in the absence of Al3+, with both genotypes showing equivalent yields in limed soil. These findings demonstrate that an important crop species can be genetically engineered to successfully increase grain production on an acid soil. [ABSTRACT FROM AUTHOR]

To assess the effectiveness of manipulating citrate metabolism with the aim of increasing citrate efflux from roots, we generated transgenic tobacco (Nicotiana tabacum L.) lines that either overexpressed mitochondrial citrate synthase (EC 4.1.3.7) activity or had reduced activity of cytosolic isocitrate dehydrogenase (EC 1.1.1.42). Despite increases in citrate synthase activities in transgenic lines of up to 5-fold, neither internal citrate concentrations nor citrate efflux were increased compared to controls suggesting that, in tobacco, citrate synthase activity does not directly determine citrate accumulation and efflux. Consistent with a lack of effect on citrate efflux, the increase in citrate synthase activity did not enhance the aluminium resistance of the transgenic lines. Preliminary data collected on two transgenic lines with cytosolic isocitrate dehydrogenase activities reduced to one-tenth and one third of the control for shoot and root tissues respectively, showed that while these changes in activities were associated with a 1.5-fold increase in internal citrate concentrations of both types of tissue, citrate efflux from roots was not increased. Further work is needed to establish whether the increase in internal citrate concentration is associated with enhanced aluminium resistance of these lines. We conclude that in tobacco internal citrate concentrations and citrate efflux are largely insensitive to large changes in either mitochondrial citrate synthase or cytosolic isocitrate dehydrogenase activities and suggest that other factors, such as transport out of the roots, control citrate efflux. [ABSTRACT FROM AUTHOR]