Enhancers associated with unstable RNAs are rare in plants

Unstable transcripts have emerged as markers of active enhancers in vertebrates and shown to be involved in many cellular processes and medical disorders. However, their prevalence and role in plants is largely unexplored. Here, we comprehensively captured all actively initiating (nascent) transcripts across diverse crops and other plants using capped small (cs)RNA sequencing. We discovered that unstable transcripts are rare in plants, unlike in vertebrates, and when present, often originate from promoters. In addition, many 'distal' elements in plants initiate tissue-specific stable transcripts and are likely bona fide promoters of as-yet-unannotated genes or non-coding RNAs, cautioning against using reference genome annotations to infer putative enhancer sites. To investigate enhancer function, we integrated data from self-transcribing active regulatory region (STARR) sequencing. We found that annotated promoters and other regions that initiate stable transcripts, but not those marked by unstable or bidirectional unstable transcripts, showed stronger enhancer activity in this assay. Our findings underscore the blurred line between promoters and enhancers and suggest that cis-regulatory elements can encompass diverse structures and mechanisms in eukaryotes, including humans.