Molecular and cellular insights into auxin-regulated primary root growth: a comparative study of Arabidopsis and rice.

Auxin regulation of primary root growth in Arabidopsis and rice was compared by analyzing root growth in response to changes in auxin levels. A bell-shaped root-growth curve was identified in both Arabidopsis and rice in response to change in auxin levels. In Arabidopsis, cell division was the main regulator of root growth in response to auxin; in rice, auxin promoted root growth by regulating cell division and cell length. The expression levels of PLETHORA (PLT) genes in response to change in auxin level followed a bell-shaped curve and closely correlated with cell division in Arabidopsis but not in rice, implying that PLT gene expression plays key role to control root growth in Arabidopsis. The level of auxin in Arabidopsis was optimal for primary root elongation, while in rice it was higher than optimal. These differences may explain the species-dependent development of root systems.

Effect of an auxin biosynthesis inhibitor, p-phenoxyphenyl boronic acid, on auxin biosynthesis and development in rice.

p-Phenoxyphenyl boronic acid (PPBo) is a specific inhibitor of auxin biosynthesis in Arabidopsis. We examined the inhibitory activity of PPBo in rice. The activity of OsYUCCA, a key enzyme for auxin biosynthesis, was inhibited by PPBo in vitro. The endogenous indole-3-acetic acid (IAA) level and the expression levels of auxin-response genes were significantly reduced in PPBo-treated rice seedlings, which showed typical auxin-deficiency phenotypes. Seminal root growth was promoted by 1 µM PPBo, which was reversed by co-treatment of IAA and PPBo. By contrast, the inhibition of root growth by 10 µM PPBo was not recovered by IAA. The root meristem morphology and cell division were restored by IAA at 60 µM, but that concentration may be too high to support root growth. In conclusion, PPBo is an inhibitor of auxin biosynthesis that targets YUCCA in rice.