AGO104 is an RdDM effector of paramutation at the maize b1 locus

Paramutation is an exception among eukaryotes, in which epigenetic information is conserved through mitosis and meiosis. It has been studied for over 70 years in maize, but the mechanisms involved are largely unknown. Previously described actors of paramutation encode components of the RNA-dependent DNA-methylation (RdDM) pathway all involved in the biogenesis of 24-nt small RNAs. However, no actor of paramutation have been identified in the effector complex of RdDM. Here, through a combination of reverse genetics, immunolocalization and immunoprecipitation (siRNA-IP) we found that ARGONAUTE104 (AGO104), AGO105 and AGO119 are members of the RdDM effector complex in maize and bind siRNAs produced from the tandem repeats required for paramutation at the b1 locus. We also showed that AGO104 is an effector of the b1 paramutation in maize. Author summaryReprogramming of epigenetic information has been described in both plants and mammals. Here, we show that maize ARGONAUTE (AGO) AGO104 and AGO105/AGO119, respectively the close homologs of A. thaliana AGO9 and AGO4, are required to enable paramutation at the b1 locus in maize. Paramutation is an epigenetic phenomenon that is stable over many generations (both mitotically and meiotically). A classic example is the booster1 (b1) gene in maize, where the weakly expressed Booster (B) allele stably decreases the expression of the Booster-Intense (B-I) allele, and changes it into a new B allele. This new B allele will in turn change B-I into new B in subsequent crosses. Previous research demonstrated that paramutation requires several proteins involved in the biosynthesis of small interfering RNAs (siRNAs) all related to the RNA-dependent DNA-methylation (RdDM) pathway. Yet, few members of the RdDM were functionally identified in maize. Here, we identify two new members of the maize RdDM pathway, and provide evidence that they are also involved in paramutation at the b1 locus.