Victor Alexandre Vitorello

Recent advances in aluminum toxicity and resistance in higher plants

Aluminum toxicity is a major soil constraint to food and biomass production throughout the world. Considerable advances in the understanding of the mechanism of resistance involving exudation of organic acids have been made in recent years. However, despite intense research efforts, there are many aspects of Al toxicity that remain unclear. This article reviews the features of the chemistry of Al relevant to its toxicity followed by an examination of the mechanisms of toxicity and resistance. Emphasis, however, is given to the mechanisms of Al toxicity, since resistance has been covered recently by several reviews. Some topics which are specifically discussed in this review are: a) The possible role of cellular effects of low pH in Al toxicity, which has been largely ignored and needs to be addressed; b) The relevance of non-genotypic (cell-to-cell) variations in sensitivity to Al; c) Evidence indicating that although Al may well exert its toxic effects in the cell wall, it is highly unlikely that Al does so in a non-specific manner by mere exchangeable binding; and d) The hypothesis that the primary target of Al toxicity resides in the cell wall-plasma membrane-cytoskeleton (CW-PM-CSK) continuum has the potential to integrate and conciliate much of the apparently conflicting results in this field.

Exogenous ornithine is an effective precursor and the δ-ornithine amino transferase pathway contributes to proline accumulation under high N recycling in salt-stressed cashew leaves

The role of the δ-ornithine amino transferase (OAT) pathway in proline synthesis is still controversial and was assessed in leaves of cashew plants subjected to salinity. The activities of enzymes and the concentrations of metabolites involved in proline synthesis were examined in parallel with the capacity of exogenous ornithine and glutamate to induce proline accumulation. Proline accumulation was best correlated with OAT activity, which increased 4-fold and was paralleled by NADH oxidation coupled to the activities of OAT and Δ(1)-pyrroline-5-carboxylate reductase (P5CR), demonstrating the potential of proline synthesis via OAT/P5C. Overall, the activities of GS, GOGAT and aminating GDH remained practically unchanged under salinity. The activity of P5CR did not respond to NaCl whereas Δ(1)-pyrroline-5-carboxylate dehydrogenase was sharply repressed by salinity. We suggest that if the export of P5C from the mitochondria to the cytosol is possible, its subsequent conversion to proline by P5CR may be important. In a time-course experiment, proline accumulation was associated with disturbances in amino acid metabolism as indicated by large increases in the concentrations of ammonia, free amino acids, glutamine, arginine and ornithine. Conversely, glutamate concentrations increased moderately and only within the first 24h. Exogenous feeding of ornithine as a precursor was very effective in inducing proline accumulation in intact plants and leaf discs, in which proline concentrations were several times higher than glutamate-fed or salt-treated plants. Our data suggest that proline accumulation might be a consequence of salt-induced increase in N recycling, resulting in increased levels of ornithine and other metabolites involved with proline synthesis and OAT activity. Under these metabolic circumstances the OAT pathway might contribute significantly to proline accumulation in salt-stressed cashew leaves.

Seed phytate content and phosphorus uptake and distribution in dry bean genotypes

Phytic acid is the major storage form of phosphorus in seeds of legumes and cereals. Since phytate can form complexes with proteins and minerals, reducing the digestive availability of these nutrients, it is usually regarded as an antinutrient, although recent work indicates that it has important beneficial roles as an antioxidant and anticarcinogen. Therefore, there is an interest in the assessment and manipulation of phytate contents in important food grains such as beans. The objective of this work was to compare eleven dry bean genotypes with regard to grain contents of P, phytate, inorganic P and protein and to examine if differences between genotypes could be explained by differences in grain protein content or in uptake and partitioning of P. Plants were grown in a greenhouse in pots containing soil and commercial substrate amended with 7.4 and 37 mg P.dm-3. The experimental setup was a random, complete block design with five replications. Genetic variability in grain phytate contents was observed. At the higher dose of P fertilization, genotype Una presented the highest phytate content (1.48 %) and the highest fraction of P as phytate (72 %), whereas these features were lowest in Paraiso (0.70 % and 47 %, respectively). Inorganic P made up 8 % of total P in Paraiso. A correlation between phytate and protein contents among genotypes was significant (r = 0.73) only under the higher dose of P fertilization. With the exception of genotype 4AP, differences in phytate content could not be explained by differences in uptake and partitioning of P in the plant. In the case of Paraiso, it is inferred that the lower phytate contents were due to differences in the metabolism of P and dry matter accumulation in the grain.

ANTIOXIDANT ISOENZYME RESPONSES TO NICKEL-INDUCED STRESS IN TOBACCO CELL SUSPENSION CULTURE

Exposure to nickel (Ni) at high concentrations can lead to production of reactive oxygen species (ROS) resulting in oxidative damage at the cellular level. We investigated the antioxidative responses of Nicotiana tabacum cv BY-2 cell suspension to Ni stress (0.075 and 0.75 mM NiCl2) over a 72 h period with special attention to potential alterations in isoenzymes of superoxide dismutase (SOD), catalase (CAT) and glutathione reductase (GR). Two main SOD isoenzymes were observed, a Mn-SOD (band I) and a Fe-SOD (band II), as well as one CAT isoenzyme and four GR isoenzymes. Activity staining analysis revealed that CAT activity plays a major role in the early response to Ni-induced oxidative stress, particularly when the Ni concentration used was low, whilst a specific GR isoenzyme appears to respond to the Ni-induced oxidative stress when a much higher Ni concentration was used to induce the stress for the same period of treatment. These results illustrate the importance and advantages of determining individual isoenzyme activities.

Fotossíntese, remobilização de reservas e crescimento de grãos em dois híbridos de milho sob deficiência hídrica na fase de enchimento dos grãos

Photosynthesis, nitrogen and sugar remobilization during the grain filling period were examined in two maize (Zea mays L.) hybrids (Pioneer 3072 and IAC 701 x B) submitted to water stress. The plants were grown under natural conditions in 12 boxes (4 m x 0,5 m x 0.6 m each) containing soil. Water stress was imposed by with holding irrigation during the 5.th and 15.th days after anthesis. Both hybrids in control treatment (without water stress) showed no differences in relation to the avaliated parameters. Under water stress the sugar and N remobilization from vegetative organs to the grains, were more intense than of the control plants. The sugar and N remobilization were larger in IAC 701 x B. The photosynthesis rates were reduced in both hybrids under water stress, but the reduction was larger in IAC 701 x B.

Variation in phytate accumulation in common bean (Phaseolus vulgaris L.) fruit explants

O fosforo e armazenado na forma de fitato nas sementes, o qual forma complexos estaveis e insoluveis com minerais e proteinas, conferindo efeito antinutriente. A sintese de fitato foi estudada em cultivo de explantes de fruto de feijao in vitro sob diferentes concentracoes de sacarose, fosforo (P), mio-inositol, acido abscisico (ABA), glutamina e metionina. Fixada a concentracao destes compostos, testou-se os diferentes tempos de cultivo (0, 3, 6 e 9 dias). A variacao no acumulo de fitato ocorreu na presenca de sacarose, mio-inositol, P e ABA nas diferentes concentracoes e tempos testados. O acumulo mais efetivo de fitato ocorreu na presenca de mio-inositol e P. O acumulo de P variou menos do que fitato em todos os tratamentos. Em conclusao, P, sacarose, ABA e mio-inositol causaram aumento no fitato acumulado nas sementes, mostrando que foi possivel alterar a sintese de fitato em cultivo de explantes de fruto de feijao.