The physiological role of peroxisomal ascorbate peroxidases (pAPX) is unknown; therefore, we utilized pAPX4 knockdown rice and catalase (CAT) inhibition to assess its role in CAT compensation under high photorespiration. pAPX4 knockdown induced co-suppression in the expression of pAPX3. The rice mutants exhibited metabolic changes such as lower CAT and glycolate oxidase (GO) activities and reduced glyoxylate content; however, APX activity was not altered. CAT inhibition triggered different changes in the expression of CAT, APX and glutathione peroxidase (GPX) isoforms between non-transformed (NT) and silenced plants. These responses were associated with alterations in APX, GPX and GO activities, suggesting redox homeostasis differences. The glutathione oxidation-reduction states were modulated differently in mutants, and the ascorbate redox state was greatly affected in both genotypes. The pAPX suffered less oxidative stress and photosystem II (PSII) damage and displayed higher photosynthesis than the NT plants. The improved acclimation exhibited by the pAPX plants was indicated by lower H2 O2 accumulation, which was associated with lower GO activity and glyoxylate content. The suppression of both pAPXs and/or its downstream metabolic and molecular effects may trigger favourable antioxidant and compensatory mechanisms to cope with CAT deficiency. This physiological acclimation may involve signalling by peroxisomal H2 O2 , which minimized the photorespiration.
Although plant physiological responses to drought have been widely studied, the interaction between photoprotection, photorespiration and antioxidant metabolism in water-stressed plants is scarcely addressed. This study aimed to evaluate the physiological adjustments preserving photosynthesis and growth in two plant species with different tolerance to drought: Jatropha curcas and Ricinus communis. We measured stress indicators, gas exchange, photochemistry of PSII and PSI, antioxidant enzymes, cyclic electron flow and photorespiration. Physiological stress indicators associated with reduction in growth confirmed R. communis as sensitive and J. curcas as tolerant to drought. Drought induced loss of photosynthesis in R. communis, whereas J. curcas maintained higher leaf gas exchange and photochemistry under drought. In addition, J. curcas showed higher dissipation of excess energy and presented higher cyclic electron flow when exposed to drought. Although none of these mechanisms have been triggered in R. communis, this species showed increases in photorespiration. R. communis displayed loss of Rubisco content while the Rubisco relative abundance did not change in J. curcas under drought. Accordingly, the in vivo maximum Rubisco carboxylation rate (V ) and the maximum photosynthetic electron transport rate driving RuBP regeneration (J ) were less affected in J. curcas. Both species displayed an efficient antioxidant mechanism by increasing activities of ascorbate peroxidase (APX) and superoxide dismutase (SOD). Overall, we suggest that the modulation of different photoprotective mechanisms is crucial to mitigate the effects caused by excess energy, maintaining photosynthetic apparatus efficiency and promoting the establishment of young plants of these two species under drought.
Salinity and osmotic stress trigger distinct signals in roots, which might induce differences in antioxidant responses. To clarify these relationships, transgenic rice plants silenced in both cytosolic ascorbate peroxidases (apx1/2) and non-transformed (NT) were exposed to iso-osmotic concentrations of NaCl and mannitol. Under both stress conditions, apx1/2 roots did not suffer oxidative stress, revealing that cytosolic APXs were not crucial to oxidative protection. Silenced and non-transformed roots triggered different responses to high salinity and osmotic stress and these stressful factors induced also distinct antioxidant changes. High salinity up-regulated expression of important OsAPX isoforms and these changes were related to increased APX activity, especially in NT roots. Intriguingly, salt stress triggered up-regulation of OsCAT isoforms but CAT activity did not change in both genotypes. In contrast, mannitol trigged very low increment in expression of OsAPX isoforms but induced substantial up-regulation in APX activity in NT roots. Mannitol also remarkably up-regulated OsCATB expression in parallel to CAT activity, in both apx1/2 and NT roots. POD and GPX (glutathione peroxidases) activities were strongly increased by high salinity but did not change in response to mannitol, in both genotypes. The two stress types as well as apx1/2 and NT roots displayed different response in terms of modulation in the H 2 O 2 levels but lipid peroxidation did not change. Membrane integrity was drastically affected by both stressful factors and similarly in both genotypes, whereas root fresh matter was affected only by salt stress. Altogether, the obtained data reveal that high salinity and osmotic stress trigger different antioxidant responses and these strategies were genotype-dependent. The different antioxidant molecular-biochemical mechanisms employed by cytosolic APX knockdown and non-transformed roots allowed reaching similar physiological performance. 2016 Elsevier B.V. All rights reserved.
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