Cytokinin-Dependent Control of GH3 Group II Family Genes in the Arabidopsis Root

Abstract

<b>A</b><b>bstract</b><b>:</b> The <i>Arabidopsis</i> root is a dynamic system where the interaction between different plant hormones controls root meristem activity and, thus, organ growth. In the root, a characteristic graded distribution of the hormone auxin provides positional information, coordinating the proliferating and differentiating cell status. The hormone cytokinin shapes this gradient by positioning an auxin minimum in the last meristematic cells. This auxin minimum triggers a cell developmental switch necessary to start the differentiation program, thus, regulating the root meristem size. To position the auxin minimum, cytokinin promotes the expression of the <i>IAA-</i><i>amido</i> <i>synthase group II</i> gene <i>GH3.17</i>, which conjugates auxin with amino acids, in the most external layer of the root, the lateral root cap tissue. Since additional <i>GH3</i> genes are expressed in the root, we questioned whether cytokinin to position the auxin minimum also operates via different <i>GH3</i> genes. Here, we show that cytokinin regulates meristem size by activating the expression of <i>GH3.5</i> and <i>GH3.6</i> genes, in addition to <i>GH3.17</i>. Thus, cytokinin activity provides a robust control of auxin activity in the entire organ necessary to regulate root growth.