Mechanism underlying regulated expression of RFL, a conserved transcription factor, in the developing rice inflorescence

Abstract

LFY and its orthologues are necessary for flower specification in diverse dicotyledonous plants. The spatial and temporal RNA expression pattern of a rice LFY-like gene: RFL differs significantly from that in several other species studied thus far. The onset of RFL expression coincides with inflorescence meristem (panicle meristem) initiation, and continues during panicle branching. Notably, incipient flower primordia have lower expression levels, and during floral organogenesis the expression is restricted to second-whorl lodicules. To explore mechanisms underlying this distinct expression pattern, we have tested the transcription regulatory functions of sequences upstream to RFL coding sequences either alone, or together with intragenic segments. Sequences 3.0 kb upstream of the RFL reading frame do not confer correctly regulated reporter gene expression in transgenic rice. In contrast, RFL intron1 or 2 can individually confer the expected profile in the developing panicle and floret. However, the additional repression of expression in vegetative tissues, is a pattern achieved by intron2 together with far-upstream sequences. Strikingly, RFL intron2 sequences can even utilize the Arabidopsis thaliana LFY promoter to confer regulated transcription in young rice panicles. By sequence comparison of RFL upstream sequences, intron1, intron2 and the Arabidopsis LFY promoter, we identify putative cis-regulatory elements unique to RFL. These data exemplify the use of regulatory circuits specific to rice RFL that may underlie its divergent function.