Plant roots have evolved a variety of anatomical patterns. For example, it is known that the number of root cortical layers varies amongst plant species. But what are the genetic mechanisms that underlie such morphological differences? Raffaele Dello Ioio and colleagues investigate this by analysing root development in Arabidopsis thaliana and Cardamine hirsuta, a genetically tractable close relative of Arabidopsis (dev153858). They first show that, unlike Arabidopsis (which has a single cortical layer), Cardamine has two cortical layers that arise during embryogenesis from a tissue with mixed cortical/endodermal (CEM) identity. They further reveal that Arabidopsis mutants in which the miRNA biogenesis machinery is perturbed also exhibit an additional cortical layer, likely owing to ectopic expression of the HD-ZIPIII transcription factor PHABULOSA (PHB), which is known to be regulated by the microRNA miR165/6. Following this, the authors show that HD-ZIPIII factors are required in the Cardamine CEM tissue for double cortex formation and that the activity domain of miR165/6 differs between the two species; the absence of miR165/6 in Cardamine CEM tissue allows a broader expression domain of PHB, resulting in the development of an extra cortical layer. Together, these findings highlight that variations in miRNA distribution can lead to differences in plant morphology.
