Characterization and expression of FLORICAULA/LEAFY homologues in Buddleja davidii

Abstract

Over a decade of intense research efforts, primarily in the model plants Arabidopsis thaliana, Antirrhinum majus and Petunia x hybrida, have augmented our understanding of physiological and anatomical processes in flower development with insights into their molecular underpinnings. Elucidation of sequences and functions of numerous genes and gene products involved in floral induction and development has added to our overall understanding of the molecular genetic control of meristematic phase change and inflorescence development in flowering plants. Insights into these components of plant development have the potential to greatly impact our ability to modify and cultivate plants for the nutritional, economical, social and emotional benefit of humans.

FLORICAULA (FLO) in Antirrhinum majus and LEAFY (LFY) in Arabidopsis thaliana are floral meristem identity genes that signal the transition from indeterminate inflorescence meristems to determinate floral meristems. LFY is expressed in both vegetative and reproductive tissues, and low expression during the vegetative phase prevents premature flowering. LFY encodes a DNA-binding transcription factor shown to localize to the nucleus and interact directly with floral organ development genes. Upregulation of FLO/LFY serves as a reliable indicator of the transition to a floral meristem from an inflorescence meristem with the associated cessation of further shoot elongation. With this in mind, it is clear that spatial and temporal expression of LFY plays a central role in the degree of inflorescence branching.

Buddleja, a cosmopolitan taxon of roughly 100 species, provides a unique model for studying inflorescence development at the molecular level. Great diversity in inflorescence architecture exists among Buddleja species, and numerous hybrids exist between and among these taxa. Breeding goals have included enhancement of floral architecture through increased panicle branching and total flowers per inflorescence. The B. davidii inflorescence is an indeterminate panicle of racemes, and several clones exhibiting enhanced inflorescence branching are known.

Homologues of FLO/LFY have been isolated from B. davidii in an effort to facilitate our understanding of the molecular contribution to inflorescence branching. Five full-length cDNAs were identified as FLO/LFY homologues. Although FLO/LFY homologues exist as a single copy in most diploid higher plants, we anticipated finding cDNAs representing at least two gene copies in the tetraploid B. davidii. Nucleotide sequence identity among the five clones was at least 96%. Three clones shared 100% identity at both the nucleotide and deduced amino acid sequence level with the exception of gaped regions. These three appear to represent alternative splice forms of a single allele (BdFL1α, BdFL1β and BdFL1γ) and two others (BdFL2 and BdFL3) represent separate alleles. Nucleotide sequence homology of BdFL clones was 86% to 88% with FLO and 62% to 68% with LFY.

Five unique cDNA isoforms of FLO/LFY homologues were isolated from Buddleja davidii. Analysis of the nucleotide and presumed amino acid sequences suggest that the five cDNAs are products of at least two different coding sequences. In addition, three of the five may be due to alternative splicing based on comparisons to similar isoforms in Arabidopsis. One additional clone is unique due to the absence of a proline-rich region near the N-terminal that is common among most FLO/LFY homologues reported to date. Expression analysis of the Buddleja FLO/LFY homologues showed similar expression patterns in seven different samples among four of the five clones. A fifth clone was undetectable in any of the samples.