Identification of the minimal promoter element of LBD27 driving pollen-specific expression in Arabidopsis.

Activating and repressive promoter elements and the 5' UTR coordinately regulate spatiotemporal expression of the male gametophyte-specific transcription factor LBD27 during pollen development in Arabidopsis. The Lateral Organ Boundaries Domain (LBD) family consists of plant-specific transcription factors that regulate diverse aspects of growth and development. In Arabidopsis thaliana, SIDECAR POLLEN/LATERAL ORGAN BOUNDARIES DOMAIN 27 (SCP/LBD27) is a male-gametophyte-specific LBD transcription factor that controls asymmetric division and cell fate specification during early pollen development. Although precise spatiotemporal expression of LBD27 is essential for its developmental function, the molecular mechanisms and cis-regulatory elements underlying this regulation remain largely unknown. In this study, we investigated the regulatory elements that govern LBD27 expression during pollen development by dissecting its promoter and 5' untranslated region (UTR). Analysis of a series of LBD27 promoter deletion constructs fused to GUS:eGFP or histone H2B:eGFP reporters showed that the 5' UTR is necessary for expression in both the vegetative cell nucleus and sperm cell nuclei during pollen development. In addition, a distal promoter region spanning -1501--801 bp contains regulatory elements that repress LBD27 expression at later stages of pollen development. Together, these results suggest that the spatiotemporal expression of LBD27 is regulated through combinatorial interactions between distinct promoter regions and the 5' UTR, thereby ensuring precise control during male gametophyte development in Arabidopsis.