No abstract

No abstract

Plant glutathione peroxidases (GPX) catalyze the reduction of H 2 O 2 or organic hydroperoxides to water, mitigating the toxicity of these compounds to cells. In rice plants, the GPX gene family is composed of five members that are distributed in a range of sub-cellular compartments including cytosol, mitochondria, chloroplasts, or endoplasmic reticulum. Of these, OsGPX1 and OsGPX4 are located in mitochondria and chloroplasts, respectively. To understand the role of these GPX in rice, the effect of knockdown of OsGPX1 and OsGPX4 in rice plants was evaluated. Our data show that OsGPX4 was essential for in vitro rice regeneration because no plants were obtained from calli carrying a hairpin construct against OsGPX4. Although the knockdown of OsGPX1 did not impair plant regeneration, the plants with silenced OsGPX1 (GPX1s plants) showed reduced shoot length and a reduced number of seeds compared to the non-transformed rice plants. These results indicate that OsGPX1 and OsGPX4 are essential for redox homeostasis which leads to normal growth and development of rice.

The biochemical mechanisms underlying the involvement of cytosolic ascorbate peroxidases (cAPXs) in photosynthesis are still unknown. In this study, rice plants doubly silenced in these genes (APX1/2) were exposed to moderate light (ML) and high light (HL) to assess the role of cAPXs in photosynthetic efficiency. APX1/2 mutants that were exposed to ML overexpressed seven and five proteins involved in photochemical activity and photorespiration, respectively. These plants also increased the pheophytin and chlorophyll levels, but the amount of five proteins that are important for Calvin cycle did not change. These responses in mutants were associated with Rubisco carboxylation rate, photosystem II (PSII) activity and potential photosynthesis, which were similar to non-transformed plants. The upregulation of photochemical proteins may be part of a compensatory mechanism for APX1/2 deficiency but apparently the finer-control for photosynthesis efficiency is dependent on Calvin cycle proteins. Conversely, under HL the mutants employed a different strategy, triggering downregulation of proteins related to photochemical activity, Calvin cycle and decreasing the levels of photosynthetic pigments. These changes were associated to strong impairment in PSII activity and Rubisco carboxylation. The upregulation of some photorespiratory proteins was maintained under that stressful condition and this response may have contributed to photoprotection in rice plants deficient in cAPXs. The data reveal that the two cAPXs are not essential for photosynthesis in rice or, alternatively, the deficient plants are able to trigger compensatory mechanisms to photosynthetic acclimation under ML and HL conditions. These mechanisms involve differential regulation in protein expression related to photochemistry, Calvin cycle and photorespiration.

The salt stress effect in root growth and antioxidative response were investigated in two cowpea cultivars which differ in salt tolerance in terms of plant growth and leaf oxidative response. Four-day-old seedlings (establishment stage) were exposed to 100 mM NaCl for two days. The roots of the two cultivars presented distinct response in terms of salt-induced changes in elongation and dry weight. Root dry weight was only decreased in Pérola (sensitive) cultivar while root elongation was mainly hampered in Pitiúba (tolerant). Root relative water content remained unchanged under salinity, but root Na + content achieved toxic levels as revealed by the K + /Na + ratio in both cultivars. Then, root growth inhibition might be due to ionic toxicity rather than by salt-induced water deficit. Although electrolyte leakage markedly increased mainly in the Pérola genotype, lipid peroxidation decreased similarly in both saltstressed cultivars. APX and SOD activities were reduced by salinity in both cultivars reaching similar values despite the decrease in Pitiúba had been higher compared to respective controls. CAT decreased significantly in Pitiúba but did not change in Pérola, while POX increased in both cultivars. The salt-induced decrease in the CAT activity of Pitiúba root is, at principle, incompatible to allow a more effective oxidative protection. Our results support the idea that the activities of SOD, APX, CAT and POX and lipid peroxidation in cowpea seedling roots were not associated with differential salt tolerance as previously characterized in terms of growth rate and oxidative response in plant leaves.Key words: antioxidative enzymes, oxidative stress, root growth inhibition, salt stress, Vigna unguiculata resuMo O objetivo deste trabalho foi avaliar o efeito do estresse salino sob o crescimento radicular e a atividade de enzimas antioxidantes em duas cultivares de feijão caupi contrastantes em termos de crescimento e resposta oxidativa de folhas. Plântulas com quatro dias de idade foram expostas a NaCl 100 mM durante dois dias sob condições controladas. A massa seca das raízes foi afetada somente na cultivar Pérola (sensível) enquanto que o comprimento radicular foi mais reduzido na Pitiúba (resistente). O estresse salino não afetou o conteúdo relativo de água, contudo o conteúdo de Na + nas raízes atingiu níveis tóxicos de acordo com a razão /Na + em ambas as cultivares, indicando que a inibição do crescimento radicular pode ser estar mais relacionada à toxicidade iônica mais do ao estresse osmótico induzido pelo sal. Embora o vazamento de eletrólitos tenha aumentado principalmente na cultivar sensível, a peroxidação de lipídeos decresceu de forma similar em ambas as cultivares. As atividades de APX e SOD foram diminuídas pela salinidade nas duas cultivares, atingindo valores similares, apesar do decréscimo em Pitiúba ter sido maior, em relação aos controles. A atividade de CAT diminuiu significantemente em Pitiúba, mas permaneceu inalterada em Pérola. O decréscimo na atividade de CAT em Pitiúba é, ...