microRNA165 and 166 modulate response of the Arabidopsis root apical meristem to salt stress

In plants, developmental plasticity allows for the modulation of organ growth in response to environmental cues. Being in contact with soil, roots are the first organ that responds to various types of soil abiotic stress such as high salt concentration. In the root, developmental plasticity relies on changes in the activity of the apical meristem, the region at the tip of the root where a set of self-renewing undifferentiated stem cells sustain growth. Here, we show that salt stress promotes differentiation of root meristem cells via reducing the dosage of the microRNAs miR165 and 166. By means of genetic, molecular and computational analysis, we show that the levels of miR165 and 166 respond to high salt concentration, and that miR165 and 166-dependent PHABULOSA ( PHB ) modulation is central to the response of root growth to this stress. Specifically, we show that salt-dependent reduction of miR165 and 166 causes a rapid increase in PHB expression and, hence, production of the root meristem pro-differentiation hormone cytokinin. Our data provide direct evidence for how the miRNA-dependent modulation of transcription factor dosage mediates plastic development in plants. Genetic, molecular, and computational analyses reveal that salt stress alters root apical meristem size through miR165/166-mediated regulation of cytokinin levels via the PHB-IPT7 pathway.