BioProject

In addition to apical growth, plants undergo radial growth to increase girth, a particularly prominent feature in trees. More...

In addition to apical growth, plants undergo radial growth to increase girth, a particularly prominent feature in trees. During both apical and radial growth the phytohormones auxin and cytokinins form symmetries that govern further growth patterns. But whereas the gene regulatory networks interpreting the hormonal fields during apical growth are well established, such networks are not known for the radial growth. We show here that the initiation of radial growth occurs around early protophloem sieve element (PSE) cell files of the root procambial tissue in Arabidopsis. In this domain cytokinin signalling promotes expression of a pair of novel mobile transcription factors, PHLOEM EARLY DOF (PEAR1, PEAR2) and their four homologs (OBP2, DOF6, TMO6 and HCA2), collectively called PEAR proteins. The PEAR proteins form a short-range concentration gradient peaking at PSE and activating gene expression that promotes radial growth. The expression and function of PEAR proteins are antagonized by well-established polarity transcription factors, HD-ZIP III, whose expression is concentrated in the more internal domain of radially non-dividing procambial cells by the function of auxin and mobile miR165/166. The PEAR proteins locally promote transcription of their inhibitory HD-ZIP III genes, thereby establishing a negative feedback loop that forms a sharp boundary demarking the zone of cell divisions. Taken together, we have established a network, in which the PEAR - HD-ZIP III module integrates the spatial information of the hormonal domains and miRNA gradients during root procambial development, to provide a pre-pattern with actively dividing and more quiescent zones, thus priming radial growth. Overall design: 4-day-old plants grown on MS were treated with the indicated compounds for the indicated time, whereafter roots were harvested and total RNA was isolated. PEAR1 = AT2G37590; PEAR2 = AT5G02460 Less...