NaCl-induced expression of ASCORBATE PEROXIDASE 8 in roots of rice (Oryza sativa L.) seedlings is not associated with osmotic component

Ascorbate peroxidase (APx; EC 1.11.1.11) plays an important role in scavenging the toxic effects of H2O2 in higher plants. Eight types of APx have been described for Oryza sativa: two cytosolic (OsAPx1 and OsAPx2), two putative peroxisomal (OsAPx3 and OsAPx4), and four chloroplastic isoforms (OsAPx5, OsAPx6, OsAPx7 and OsAPx8). We have recently demonstrated that Na+ but not Cl-is required for the NaCl-induced expression of OsAPx8 in rice roots. Evidence is also provided to show that Na+-induced expression of OsAPx8 is mediated through an accumulation of ABA. In addition to its known component of ion toxicity, there is an osmotic effect resulting from salt concentration in the soil. Here we show that ABA level but not OsAPx8 expression was enhanced at a concentration of mannitol iso-osmotic with 150 mM NaCl suggests that NaCl-enhanced OsAPx8 expression is not associated with osmotic component.