TYPE II MADS-BOX GENES ASSOCIATED WITH POPLAR APICAL BUD DEVELOPMENT AND DORMANCY

Abstract

MADS-box transcription factors regulate the development of vegetative and reproductive organs in plants. Little is known about the role of MADS-box genes in tree development. Using phylogenetic analysis, 57 putative type II MADS-box genes representing 14 functional classes were identified in the Populus trichocarpa genome. cDNA sequencing of the poplar type II MADS-box genes indicates that 28.1% of the transcripts differed in the intron-exon structures predicted in the genome database and 19.3% of the transcripts appear to be alternatively spliced. The majority of the poplar type II MADS-box genes were expressed in a wide variety of tissues including shoot apices, leaves, bark, xylem, root, and floral tissues and in shoot apices during bud development and dormancy. These results indicate that poplar MADS-box genes have diverse regulatory roles in a broad range of tissues and developmental processes.

Six poplar FLC-like genes, PtFLC1-PtFLC6, were identified in the poplar genome and expression of all six genes was detected in poplar shoot apices. The expression of one gene, PtFLC2, declined in apical buds during SD photoperiod and low temperature induced dormancy development suggesting a role in bud dormancy and may represent an analogous regulatory mechanism to the down-regulation of FLC during vernalization in Arabidopsis. In addition, several PtFLC2 splice isoforms (PtFLC2as1-9) were identified that were associated with the later stages of bud dormancy. Overexpression of the PtFLC2as1 isoform delayed photoperiod induced apical bud development and bud dormancy, growth cessation, and leaf senescence while overexpression of the PtFLC2as2 isoform appeared to accelerate bud development and dormancy and reduce the amount of chilling required to overcome dormancy.

These findings suggest that PtFLC2, unlike Arabidopsis FLC, could be an integration point for both photoperiod and cold signals that regulate bud development and dormancy. These results also suggest that in addition to transcriptional regulation, that cold-mediated production of PtFLC2 splicing isoforms may have an important regulatory role in bud dormancy. The regulated production of splicing isoforms could regulate bud dormancy either by dominate negative interactions, by forming different protein complexes or regulating different pathways that regulate growth, dormancy, and dormancy release.