Cytidine analogs in plant epigenetic research and beyond

Cytosine (DNA) methylation plays important roles in silencing transposable elements, plant development, genomic imprinting, stress responses, and maintenance of genome stability. To better understand the functions of this epigenetic modification, several tools have been developed to manipulate DNA methylation levels. These tools include mutants of DNA methylation writers and readers, targeted manipulation of locus-specific methylation, and the use of chemical inhibitors. Here, we summarize the effects of commonly used cytidine analog chemical inhibitors, represented by zebularine, 5-azacytidine, and their related compounds, on plants. These analogs are incorporated into chromosomal DNA, where they block the activity of the replicative CG DNA methyltransferase 1 (MET1). This leads to manifold alterations in the plant epigenome, modified developmental programs, or suppression of hybridization barriers. We also highlight the DNA-damaging effects of cytidine analogs, particularly the formation of stable DNA-protein crosslinks between DNA and MET1. This phenomenon sheds new light on specific phenotypes observed upon treatment with cytidine analogs. In conclusion, cytidine analogs are a vital tool for plant genome research and have the potential to open new promising avenues for applications in plant biotechnology and breeding. Inhibition of DNA methylation by cytidine analogs led to a surprising variety of epigenetic discoveries, and recently, these chemicals also became a cornerstone for understanding the mechanisms of genome stability.