Iron toxicity in field-cultivated rice: contrasting tolerance mechanisms in distinct cultivars

Iron toxicity is a major nutritional disorder in irrigated and rainfed waterlogged rice. To elucidate mechanisms involved in tolerance to iron toxicity, plants from one cultivar susceptible to iron toxicity (BR-IRGA409) and two tolerant cultivars (EPAGRI 108 and EPAGRI 109) were grown in the field, at an iron-toxic site and at a control site in Southern Brazil. We evaluated chlorophyll concentrations, carbonyl concentrations, iron concentrations in leaves and roots, antioxidative enzyme activities (SOD, APX, CAT, GR and DHR), concentrations of reduced and oxidized forms of ascorbate and glutathione, and gene expression profile of four SOD genes in rice leaves. Only plants from the susceptible cultivar showed symptoms of iron toxicity when grown at the iron-toxic site, accumulating high levels of iron in leaves. EPAGRI 108 plants had the lowest iron concentration in leaves and reached the highest iron concentration in the root symplast, suggesting that the capacity to safely store iron in root cells and to limit iron translocation to shoots could be a tolerance mechanism in this cultivar. Plants from the susceptible cultivar showed higher APX activity as well as higher DHA and GSSG concentrations. Plants from the EPAGRI 109 cultivar accumulated high iron levels in leaves, and showed the highest SOD, GR and DHR activities when grown in the iron-toxic site. The same cultivar also showed the highest expression of three out of four SOD genes tested. Therefore, the two tolerant cultivars seem to rely on different mechanisms to deal with iron toxicity in field conditions: limiting iron translocation to the shoot or inducing enzymes-dependent leaf tolerance.