Osmotic stress-responsive promoter upstream transcripts (PROMPTs) act as carriers of MYB transcription factors to induce the expression of target genes in Populus simonii.

Complex RNA transcription and processing produces a diverse range catalog of long noncoding RNAs (lncRNAs), important biological regulators that have been implicated in osmotic stress responses in plants. Promoter upstream transcript (PROMPT) lncRNAs share some regulatory elements with the promoters of their neighbouring protein-coding genes. However, their function remains unknown. Here, using strand-specific RNA sequencing, we identified 209 differentially regulated osmotic-responsive PROMPTs in poplar (Populus simonii). PROMPTs are transcribed bidirectionally and are more stable than other lncRNAs. Co-expression analysis of PROMPTs and protein-coding genes divided the regulatory network into five independent subnetworks including 27 network modules. Significantly enriched PROMPTs in the network were selected to validate their regulatory roles. We used delaminated layered double hydroxide lactate nanosheets (LDH-lactate-NS) to transport synthetic nucleic acids into live tissues to mimic overexpression and interference of a specific PROMPT. The altered expression of PROMPT_1281 induced the expression of its cis and trans targets, and this interaction was governed by its secondary structure rather than just its primary sequence. Based on this example, we proposed a model that a concentration gradient of PROMPT_1281 is established, which increases the probability of its interaction with targets near its transcription site that shares common motifs. Our results firstly demonstrated that PROMPT_1281 act as carriers of MYB transcription factors to induce the expression of target genes under osmotic stress. In sum, our study identified and validated a set of poplar PROMPTs that likely have regulatory functions in osmotic responses.