Lateral root elongation inside the basal cortex of the seminal root is caused by the changes in auxin distribution to the root system of rice (Oryza sativa L.) seedlings

Polar auxin transport was inhibited in rice seedlings when they were treated with N1-naphthylphthalamic acid (NPA). The treatment reduced total root length and the number of lateral roots, and negatively affected gravitropism. The auxin content at the base of the seminal root significantly increased in the NPA treatment seedlings compared to the control. Lateral roots elongated along the seminal root axis after NPA treatment, but growth remained within the cortex. Lignin content in the basal region also increased at the same time and accumulated in the epidermis. These results suggest that NPA treatment prevents lateral roots from penetrating the hypodermis due to the hardening of hypodermis cell walls through the enhanced lignification, and the disturbed gravitropism caused by NPA treatment affected auxin flow.