HD-ZIPIII Transcription Factors Pattern the Root Cortex Modulating Cytokinin Activity

One of the central questions in biology is the basis of interspecific morphological variability. The plant root cortex layer number is a paradigmatic example of this variability, as this can vary from none to several, depending on the plant species. Through secondary growth of the cortex, roots control air quantity in plants growing on wet soils (aerenchyma), and store nutrients when growing under adverse weather and nutritive conditions (storage parenchyma). Recently, via comparing the development of Arabidopsis thaliana (one cortex layer) and Cardamine hirsuta (two cortex layers), we identified a novel mechanism where the spatial distribution of a miRNA (miR165/6) in the root ground tissue controls the localization of the HOMEODOMAINLEUCINEZIPPERIII (HD-ZIPIII) transcription factor family that in turn controls the number of cortex layers. We also propose a plausible mechanism, involving cytokinin, through which HD-ZIPIIIs regulate the cortical patterning.