Julia L. Baulies

Plants display a remarkable regeneration capacity, which allows them to replace damaged or lost cells, tissues and organs, and thus recover from a broad spectrum of injuries^1,2. Even lost stem cells can be regenerated from non-stem cells after competence acquisition, highlighting the enormous plasticity of plant cells. However, the molecular mechanisms underlying this process are still poorly understood. In the root, the highly conserved microRNA miR396 and its targets, the GROWTH-REGULATING FACTORs (GRFs), control the transition from stem cells to proliferative cells. miR396 promotes stem cell activity by repressing and excluding the GRFs from the stem cell area. In turn, the GRFs promote cell division in the proliferation zone³. Here we show that the miR396–GRF regulatory module guides stem cell reconstitution after root tip excision, playing a dual role: while miR396 promotes competence, the GRFs control regeneration speed. Moreover, plants with ectopic miR396 expression have defined stem cell niches before the excision but do not reconstitute them afterwards, remaining in an open state despite continuing to grow. We propose that this phenomenon is caused by dispersed stem cell activity, which supports growth after root tip excision without reconstituting the organized and spatially restricted stem cell niche typical of Arabidopsis roots.

Key message Proper root growth depends on the clearance of TCP transcripts from the root apical meristem by microRNA miR319. Abstract The evolutionarily conserved microRNA miR319 regulates genes encoding TCP transcription factors in angiosperms. The miR319-TCP module controls cell proliferation and differentiation in leaves and other aerial organs. The current model sustains that miR319 quantitatively tunes TCP activity during leaf growth and development, ultimately affecting its size. In this work we studied how this module participates in Arabidopsis root development. We found that misregulation of TCP activity through impairment of miR319 binding decreased root meristem size and root length. Cellular and molecular analyses revealed that high TCP activity affects cell number and cyclin expression but not mature cell length, indicating that, in roots, unchecking the expression of miR319-regulated TCPs significantly affects cell proliferation. Conversely, tcp multiple mutants showed no obvious effect on root growth, but strong defects in leaf morphogenesis. Therefore, in contrast to the quantitative regulation of the TCPs by miR319 in leaves, our data suggest that miR319 clears TCP transcripts from root cells. Hence, we provide new insights into the functions of the miR319-TCP regulatory system in Arabidopsis development, highlighting a different modus operandi for its action mechanism in roots and shoots.