Functional plasticity of miR165/166 in plant development revealed by small tandem target mimic.

MicroRNA 165 and 166 (miR165/166) is composed of nine members and targets five members (PHB, PHV, REV, ATHB8 and ATHB15) of the HD-ZIP III transcription factor family. Mutants generated by traditional methods could hardly reveal the overall functions of miR165/166 in plant development. In this study, the expressions of all miR165/166 members were simultaneously blocked by over-expressing STTM165/166-31 in Arabidopsis and tomato for functional dissection of miR165/166 family. Following a down-regulation of over 90% endogenous miR165/166, the target HD-ZIP III genes were correspondingly up-regulated in the STTM transgenic Arabidopsis and tomato plants. Notably, the STTM165/166-31 over-expressed Arabidopsis and tomato displayed pleiotropic effects on development which were not frequently observed in previously identified genetic mutants of either individual miR165/166 gene or any of the five target genes. Furthermore, the transgenic Arabidopsis showed increased IAA content and decreased IAA sensitivity accompanied by enhanced expressions of genes responsible for auxin biosynthesis and signaling, suggesting possible roles of auxin in mediation of miR165/166-regulated processes. Importantly, the transgenic Arabidopsis exhibited the improved behavior under salt stress. Overall, such diverse variations in plant development and physiological process revealed by STTM165/166 demonstrate a key role of miR165/166-mediated network in regulating plant development and responses to abiotic stresses.