HY5 regulates light-responsive transcription of microRNA163 to promote primary root elongation in Arabidopsis seedlings

Micro RNAs(mi RNAs) play key roles in the posttranscriptional regulation of gene expression in plants. Many mi RNAs are responsive to environmental signals. Light is the first environmental signal perceived by plants after emergence from the soil.However, less is known about the roles and regulatory mechanism of mi RNAs in response to light signal. Here, using small RNA sequencing, wedetermined that mi R163 is significantly rapidly induced by light signaling in Arabidopsis thaliana seedlings. The light-inducible response of mi R163 functions genetically downstream of LONG HYPOCOTYL 5(HY5), a central positive regulator of photomorphogenesis. HY5 directly binds to the two G/Chybrid elements in the mi R163 promoter with unequal affinity; one of these elements, which is located next to the transcription start site, plays a major role in light-induced expression of mi R163. Overexpression of mi R163 rescued the defective primary root elongation of hy5 seedlings without affecting lateral root growth, whereas overexpressing of mi R163 target PXMT1 inhibited primary root elongation. These findings provide insight into understanding the post-transcriptional regulation of root photomorphogenesis mediated by the HY5-mi R163-PXMT1 network.