<scp>HOS</scp>1 regulates Argonaute1 by promoting transcription of the micro<scp>RNA</scp> gene <i><scp>MIR</scp>168b</i> in Arabidopsis

Summary Proper accumulation and function of mi RNA s is essential for plant growth and development. While core components of the mi RNA biogenesis pathway and mi RNA ‐induced silencing complex have been well characterized, cellular regulators of mi RNA s remain to be fully explored. Here we report that HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENES 1 ( HOS 1) is a regulator of an important mi RNA , mi168a/b, that targets the ARGONAUTE 1 ( AGO 1 ) gene in Arabidopsis. HOS 1 functions as an ubiquitin E3 ligase to regulate plant cold‐stress responses, associates with the nuclear pores to regulate mRNA export, and regulates the circadian clock and flowering time by binding to chromatin of the flowering regulator gene FLOWERING LOCUS C ( FLC ). In a genetic screen for enhancers of sic– 1, we isolated a loss‐of‐function Arabidopsis mutant of HOS 1 that is defective in mi RNA biogenesis. Like other hos1 mutant alleles, the hos1–7 mutant flowered early and was smaller in stature than the wild‐type. Dysfunction in HOS 1 reduced the abundance of miR168a/b but not of other mi RNA s. In hos1 mutants, pri‐ MIR 168b and pre‐ MIR 168b levels were decreased, and RNA polymerase II occupancy was reduced at the promoter of MIR 168b but not that of MIR 168a . Chromatin immunoprecipitation assays revealed that HOS 1 protein is enriched at the chromatin of the MIR 168b promoter. The reduced miR168a/b level in hos1 mutants results in an increase in the mRNA and protein levels of its target gene, AGO 1 . Our results reveal that HOS 1 regulates miR168a/b and AGO 1 levels in Arabidopsis by maintaining proper transcription of MIR 168b .