• Cardamine hirsuta
• Cortex
• HD-ZIP III
• microRNAs

Variability in plant root anatomy is a key factor for allowing plants to colonize different lands. Among the various tissues compounding the root, the cortical layer has a fundamental role for permitting plants to adapt in different environment. Indeed, secondary growth of the cortical layers controls air/water ratio in plant living on wet soils or stores carbohydrates as starch in plants living in adverse weather and nutritive conditions. The number of cortical layers can largely vary among different species, spanning from one to several. Utilizing a comparative development approach between Arabidopsis thaliana, one cortex, and its close relative Cardamine hirsuta, two cortexes, it has been shown that a differential cellular distribution of microRNA165 and microRNA166 (miR165 and 166) and its target PHABULOSA (PHB), a member of the HD-ZIPIII transcription factor family, is at the basis of cortical patterning diversity. In Arabidopsis PHB action in the cortex initials is eliminated thanks to the activity of miR165 and 166 in this domain. Differently, in Cardamine PHB is present in cortex initials cells where miR165 and 166 seems to be less active.
The goal of my PhD project is to gain in-depth knowledge about the differential distribution of miR165 and 166 and to identify the molecular mechanisms that drive the differential expression of these miRNAs between two species with different cortical layer number: Arabidopsis and Cardamine. Regarding these goals, in-depth analysis and evaluation of the expression of these miRNAs, the assessment of the modulation of their activity on cortex development, and the analysis of their cis-regulatory factors in Arabidopsis and Cardamine have provided new insights into the conserved developmental mechanisms that structure the patterning of cortical tissue across species. The obtained data suggest that different quantitative expression of miR166 drives cortical layer number variability. To corroborate this idea, I generated a tool to measure miR165/166 activity in vivo at cellular resolution to provide quantitative data on the radial gradient of miRNAs at the cellular level in both species.