Root indeterminate growth and its outstanding ability to produce new tissues continuously make this organ a highly dynamic structure able to respond promptly to external environmental stimuli. Developmental processes therefore need to be finely tuned, and hormonal cross-talk plays a pivotal role in the regulation of root growth. In contrast to what happens in animals, plant development is a post-embryonic process. A pool of stem cells, placed in a niche at the apex of the meristem, is a source of self-renewing cells that provides cells for tissue formation. During the first days post-germination, the meristem reaches its final size as a result of a balance between cell division and cell differentiation. A complex network of interactions between hormonal pathways co-ordinates such developmental inputs. In recent years, by means of molecular and computational approaches, many efforts have been made aiming to define the molecular components of these networks. In this review, we focus our attention on the molecular mechanisms at the basis of hormone cross-talk during root meristem size determination.

Highlights d Upon germination, cell divisions generate a PLT drop that forms the transition zone d A PLTs-ARR12 mutual antagonism restricts early root meristem expansion d ARR1 repression of cell division via KRP2 is key for meristem size stabilization

Root growth is modulated by different factors, including phytohormones, transcription factors, and microRNAs (miRNAs). MicroRNA156 and its targets, the SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) genes, define an age-dependent pathway that controls several developmental processes, including lateral root emergence. However, it remains unclear whether miR156-regulated SPLs control root meristem activity and root-derived de novo shoot regeneration. Here, we show that MIR156 and SPL genes have opposing expression patterns during the progression of primary root (PR) growth in Arabidopsis, suggesting that age cues may modulate root development. Plants with high miR156 levels display reduced meristem size, resulting in shorter primary root (PRs). Conversely, plants with reduced miR156 levels show higher meristem activity. Importantly, loss of function of SPL10 decreases meristem activity, while SPL10 de-repression increases it. Meristem activity is regulated by SPL10 probably through the reduction of cytokinin responses, via the modulation of type-B ARABIDOPSIS RESPONSE REGULATOR1(ARR1) expression. We also show that SPL10 de-repression in the PRs abolishes de novo shoot regenerative capacity by attenuating cytokinin responses. Our results reveal a cooperative regulation of root meristem activity and root-derived de novo shoot regeneration by integrating age cues with cytokinin responses via miR156-targeted SPL10.

A clear example of interspecific variation is the number of root cortical layers in plants. The genetic mechanisms underlying this variability are poorly understood, partly because of the lack of a convenient model. Here, we demonstrate that , unlike, has two cortical layers that are patterned during late embryogenesis. We show that a miR165/6-dependent distribution of the HOMEODOMAIN LEUCINE ZIPPER III (HD-ZIPIII) transcription factor PHABULOSA (PHB) controls this pattern. Our findings reveal that interspecies variation in miRNA distribution can determine differences in anatomy in plants.

During organogenesis a key step towards the development of a functional organ is the separation of cells in specific domains with different activities. Mutual inhibition of gene expression has been shown to be sufficient to establish and maintain these domains during organogenesis of several multicellular organisms. Here we show that the mutual inhibition between the PLTs and the ARRs transcription factors is sufficient to separate cell division and cell differentiation during root organogenesis. In particular, we show that ARR1 suppresses PLTs activities and that PLTs suppress ARR1 and ARR12 by targeting their protein for degradation via the KMD2 F-box protein. These findings reveal new important aspects of the complex process of root zonation and development.