A rice mutant sensitive to Al toxicity is defective in the specification of root outer cell layers

Outer cell layers of rice roots, which comprise epidermis, exodermis and sclerenchyma, have been proposed to protect the roots from various stresses in soil. Here, we report a mutant which is defective in the specification of outer cell layers, and examined the role of these layers in Al and other metal resistance. Morphological and histochemical observations revealed that the mutant isolated based on Al sensitivity frequently showed a disordered pattern of periclinal cell division in the epidermal layers at a region close to the root apical meristem. The lateral root caps in the mutant became difficult to peel off from the epidermis, and epidermal cells became smaller and irregular with far fewer root hairs. Furthermore, some exodermal cells were transformed into additional sclerenchyma cells. However, there was no difference in the inner cell layers between the wild-type rice and the mutant. The mutant showed similar root growth to the wild-type rice in the absence of Al, but greater inhibition of root elongation by Al was found in the mutant. Morin staining showed that Al penetrated into the inner cortical cells in the mutant. Furthermore, the mutant was also sensitive to other metals including Cd and La. Taken together, our results indicate that root outer cell layers protect the roots against the toxicity of Al and other metals by preventing metal penetration into the inner cells. Genetic analysis showed that the mutant phenotypes were controlled by a single recessive gene, which was located on the short arm of rice chromosome 2.