Salinity and osmotic stress trigger different antioxidant responses related to cytosolic ascorbate peroxidase knockdown in rice roots

Abstract

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 H2O2 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.