The effects of auxin on lateral root initiation and root gravitropism in a lateral rootless mutant Lrt1 of rice (Oryza sativa L.)

Auxins control growth and development in plants, including lateral root initiation and root gravity response. However, how endogenous auxin regulates these processes is poorly understood. In this study, the effects of auxins on lateral root initiation and root gravity response in rice were investigated using a lateral rootless mutant Lrt1, which fails to form lateral roots and shows a reduced root gravity response. Exogenous application of IBA to the Lrt1 mutant restored both lateral root initiation and root gravitropism. However, application of IAA, a major form of natural auxin, restored only root gravitropic response but not lateral root initiation. These results suggest that IBA is more effective than IAA in lateral root formation and that IBA also plays an important role in root gravitropic response in rice. The application of NAA restored lateral root initiation, but did not completely restore root gravitropism. Root elongation assays of Lrt1 displayed resistance to 2,4-D, NAA, IBA, and IAA. This result suggests that the reduced sensitivity to exogenous auxins may be due to the altered auxin activity in the root, thereby affecting root morphology in Lrt1.