Polymerase IV Plays a Crucial Role in Pollen Development in Capsella.

Zhenxing Wang,Nicolas Butel,Filipe Borges,Claudia Köhler,Robert A Martienssen,German Martinez,Jun Yi,Juan Santos-González

doi:10.1105/tpc.19.00938

Abstract

In Arabidopsis (Arabidopsis thaliana), DNA-dependent RNA polymerase IV (Pol IV) is required for the formation of transposable element (TE)-derived small RNA transcripts. These transcripts are processed by DICER-LIKE3 into 24-nucleotide small interfering RNAs (siRNAs) that guide RNA-directed DNA methylation. In the pollen grain, Pol IV is also required for the accumulation of 21/22-nucleotide epigenetically activated siRNAs, which likely silence TEs via post-transcriptional mechanisms. Despite this proposed role of Pol IV, its loss of function in Arabidopsis does not cause a discernible pollen defect. Here, we show that the knockout of NRPD1, encoding the largest subunit of Pol IV, in the Brassicaceae species Capsella (Capsella rubella), caused postmeiotic arrest of pollen development at the microspore stage. As in Arabidopsis, all TE-derived siRNAs were depleted in Capsella nrpd1 microspores. In the wild-type background, the same TEs produced 21/22-nucleotide and 24-nucleotide siRNAs; these processes required Pol IV activity. Arrest of Capsella nrpd1 microspores was accompanied by the deregulation of genes targeted by Pol IV-dependent siRNAs. TEs were much closer to genes in Capsella compared with Arabidopsis, perhaps explaining the essential role of Pol IV in pollen development in Capsella. Our discovery that Pol IV is functionally required in Capsella microspores emphasizes the relevance of investigating different plant models.

Highlights

The formation of male and female plant gametes is associated with the partial resetting of epigenetic marks, which likely serves to achieve meiotic competence (Borges and Martienssen, 2013; Baroux and Autran, 2015; Borg and Berger, 2015)
Microspore arrest in Capsella nrpd[1] mutant plants correlated with a substantially stronger deregulation of genes compared to Arabidopsis nrpd[1] microspores, including genes encoding known regulators of pollen development. These findings indicate that polymerase IV (Pol IV) in Capsella generates small interfering RNAs (siRNAs) ranging from 21-24-nt in size with important functional roles in pollen development
Like the Arabidopsis thaliana nrpd[1] mutant, transposable element (TE)-derived 24-nt siRNAs were abolished in Cr nrpd[1] leaves (Figure 1B) (Wierzbicki et al, 2012), which was associated with a strong reduction in CHH methylation levels over TEs (Figure 1C). These results reveal that Pol IV has a conserved function in siRNA biogenesis and is required for RNA-directed DNA methylation (RdDM) in Capsella and Arabidopsis

Summary

Introduction

The formation of male and female plant gametes is associated with the partial resetting of epigenetic marks, which likely serves to achieve meiotic competence (Borges and Martienssen, 2013; Baroux and Autran, 2015; Borg and Berger, 2015). Epigenetic modifications can be applied directly on the DNA in the form of DNA methylation, or on histones, the proteins that package DNA into nucleosomes. The specific type of modification and its position on the genomic locus define the transcriptional outcome. DNA methylation is generally (but not always) a repressive modification and is used to silence transposable elements (TEs) and genes during specific stages of plant. DNA methylation can occur in the CG, CHG, and CHH context. 25 (where H corresponds to A, T or C) and is established and maintained by different DNA. Methylation in the CG context is maintained by

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