The Class III Peroxidase Encoding Gene AtPrx62 Positively and Spatiotemporally Regulates the Low pH-Induced Cell Death in Arabidopsis thaliana Roots.

Exogenous low pH stress causes cell death in root cells, limiting root development, and agricultural production. Different lines of evidence suggested a relationship with cell wall (CW) remodeling players. We investigated whether class III peroxidase (CIII Prx) total activity, CIII Prx candidate gene expression, and reactive oxygen species (ROS) could modify CW structure during low pH-induced cell death in Arabidopsis thaliana roots. Wild-type roots displayed a good spatio-temporal correlation between the low pH-induced cell death and total CIII Prx activity in the early elongation (EZs), transition (TZs), and meristematic (MZs) zones. In situ mRNA hybridization showed that AtPrx62 transcripts accumulated only in roots treated at pH 4.6 in the same zones where cell death was induced. Furthermore, roots of the atprx62-1 knockout mutant showed decreased cell mortality under low pH compared to wild-type roots. Among the ROS, there was a drastic decrease in O2·- levels in the MZs of wild-type and atprx62-1 roots upon low pH stress. Together, our data demonstrate that AtPrx62 expression is induced by low pH and that the produced protein could positively regulate cell death. Whether the decrease in O2·- level is related to cell death induced upon low pH treatment remains to be elucidated.

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.

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

The in vitro synthesis of phytate was studied in common bean fruit explants. Different concentrations of sucrose; phosphorus (P); myo-inositol; abscisic acid (ABA); glutamine and methionine, were tested. Fixed concentrations of these compounds were tested at different periods (0, 3, 6 and 9 days). Variation in phytate coincided with different concentrations of sucrose, myo-inositol, P and ABA for the duration tested. These compounds caused an accumulation of phytate and were more effective in the presence of myo-inositol and P. The accumulation of P varied less than phytate for the different treatments tested in vitro. In conclusion, P, sucrose, ABA, and myo-inositol caused an increase in the phytate of bean seed, showing that it could be possible to alter its content by culturing bean fruit explants in vitro.

O fósforo é armazenado na forma de fitato nas sementes, o qual forma complexos estáveis e insolúveis com minerais e proteínas, conferindo efeito antinutriente. A síntese de fitato foi estudada em cultivo de explantes de fruto de feijão in vitro sob diferentes concentrações de sacarose, fósforo (P), mio-inositol, ácido abscísico (ABA), glutamina e metionina. Fixada a concentração destes compostos, testou-se os diferentes tempos de cultivo (0, 3, 6 e 9 dias). A variação no acúmulo de fitato ocorreu na presença de sacarose, mio-inositol, P e ABA nas diferentes concentrações e tempos testados. O acúmulo mais efetivo de fitato ocorreu na presença de mio-inositol e P. O acúmulo de P variou menos do que fitato em todos os tratamentos. Em conclusão, P, sacarose, ABA e mio-inositol causaram aumento no fitato acumulado nas sementes, mostrando que foi possível alterar a síntese de fitato em cultivo de explantes de fruto de feijão.

Dynamics of inositol phosphate pools (tris-, tetrakis- and pentakisphosphate) in relation to the rate of phytate synthesis during seed development in common bean (Phaseolus vulgaris).

Four cultivars of Phaseolus vulgaris were grown in a greenhouse and each flower was Labeled with date of anthesis. Seeds were collected at six different stages of development and inositol phosphates (InsPs) were analyzed by ion-pair reversed-phase HPLC. Phytate accumulation was similar in all cultivars, and the specific rate of phytate synthesis (Rs) peaked at about 22 days after flowering (DAF). Variations in the concentrations of the InsP3 and InsP4 pools matched changes in Rs in cultivars Una and Aruã. These results suggest mass-action effects. Thus, the rates of conversion of InsP3 to InsP5 appeared to be at least partly dependent on substrate concentration. Proportional increases in size of all InsP pools up to 21 DAF are also consistent with Little regulation in this part of the pathway. However, this did not appear to be the case in cv. Diamante Negro or with the conversion of InsP5 to InsP6 in all cultivars, where concentrations of the InsP precursor pools peaked earlier or even dropped as Rs peaked, suggesting activation of enzyme activity. Therefore, the evidence is consistent with a control point regulating this metabolic route upstream of InsP3 and possibly in the conversion of InsP5 to InsP6.