Rice miR172 induces flowering by suppressing OsIDS1 and SNB, two AP2 genes that negatively regulate expression of Ehd1 and florigens

Background: Rice is a facultative short-day plant that flowers under long days (LD) after a lengthy vegetative phase. Although several inhibitors that delay flowering have been identified, the process by which rice eventually flowers under non-permissive LD conditions is not well understood. Results: Overexpression of miR172 reduced flowering time significantly, suggesting its role as an inducer. Levels of miR172 increased as plants aged, further supporting our findings. Transcripts of SNB and OsIDS1, two members of the AP2 family that have the miR172 target site, were reduced in older plants as the level of miR172 rose. Overexpression of those AP2 genes delayed flowering; overexpression of miR172-resistant forms of SNB or OsIDS1 further delayed this process. This demonstrated that the AP2 genes function downstream of miR172. Two florigen genes -- Hd3a and RFT1 -- and their immediate upstream regulator Ehd1 were suppressed in the AP2 overexpression plants. This suggested that the AP2 genes are upstream repressors of Ehd1. In phytochrome mutants, miR172d levels were increased whereas those of SNB and OsIDS1 were decreased. Thus, it appears that phytochromes inhibit miR172d, an AP2 suppresser. Conclusions: We revealed that miR172d developmentally induced flowering via repressing OsIDS1 and SNB, which suppressed Ehd1. We also showed that phytochromes negatively regulated miR172.