Zinc deficiency differentially affects redox homeostasis of rice genotypes contrasting in ascorbate level

Zinc (Zn) deficiency is an important mineral disorder affecting rice production, and is associated with the formation of oxidative stress in plant tissue. In this study we investigated processes of oxidative stress formation as affected by ascorbate (AsA) in two pairs of contrasting rice genotypes: (i) two indica lines differing in field tolerance to Zn deficiency and AsA metabolism, i.e. RIL46 (tolerant) and IR74 (sensitive); (ii) the japonica wild-type Nipponbare (tolerant) and the AsA deficient TOS17 mutant line ND6172 (sensitive) having a 20-30% lower AsA level due to the knockout of an AsA biosynthetic gene (OsGME1). Plants were grown hydroponically under +Zn and -Zn conditions for 21 days and samples were investigated after 7, 14, and 21 days of treatment. Tissue Zn concentrations below 20mg kg(-1) in the -Zn treatment induced the formation of visible symptoms of Zn deficiency from day 14 in all genotypes, but especially in the sensitive IR74. Significant increases in lipid peroxidation were observed in the leaves of the sensitive genotypes IR74 and ND6172, and in the roots of IR74, but not in the tolerant genotypes. At day 21, the tolerant genotypes RIL46 and Nipponbare had significantly higher AsA levels in both shoots and roots compared to the sensitive lines. Consistently, higher levels of hydrogen peroxide formation in leaves and roots of the sensitive genotypes were detected using staining methods. Differences in foliar hydrogen peroxide formation between IR74 and RIL46 became apparent on day 7 and between ND6172 and Nipponbare on day 14. Similarly, genotypic differences in hydrogen peroxide formation in the roots were seen on day 21. In conclusion, our data demonstrate that Zn deficiency leads to a redox imbalance in roots and shoots prior to the occurrence of visible symptoms, and that the antioxidant AsA plays an important role in maintaining the redox homeostasis under Zn deficiency.