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.