Abstract
BackgroundThe production of heather (Calluna vulgaris) in Germany is highly dependent on cultivars with mutated flower morphology, the so-called diplocalyx bud bloomers. So far, this unique flower type of C. vulgaris has not been reported in any other plant species. The flowers are characterised by an extremely extended flower attractiveness, since the flower buds remain closed throughout the complete flowering season. The flowers of C. vulgaris bud bloomers are male sterile, because the stamens are absent. Furthermore, petals are converted into sepals. Therefore the diplocalyx bud bloomer flowers consist of two whorls of sepals directly followed by the gynoecium.ResultsA broad comparison was undertaken to identify genes differentially expressed in the bud flowering phenotype and in the wild type of C. vulgaris. Transcriptome sequence reads were generated using 454 sequencing of two flower type specific cDNA libraries. In total, 360,000 sequence reads were obtained, assembled to 12,200 contigs, functionally mapped, and annotated. Transcript abundances were compared and 365 differentially expressed genes detected. Among these differentially expressed genes, Calluna vulgaris PISTILLATA (CvPI) which is the orthologue of the Arabidopsis B gene PISTILLATA (PI) was considered as the most promising candidate gene. Quantitative Reverse Transcription Polymerase Chain Reaction (qRT PCR) was performed to analyse the gene expression levels of two C. vulgaris B genes CvPI and Calluna vulgaris APETALA 3 (CvAP3) in both flower types. CvAP3 which is the orthologue of the Arabidopsis B gene APETALA 3 (AP3) turned out to be ectopically expressed in sepals of wild type and bud bloomer flowers. CvPI expression was proven to be reduced in the bud blooming flowers.ConclusionsDifferential expression patterns of the B-class genes CvAP3 and CvPI were identified to cause the characteristic morphology of C. vulgaris flowers leading to the following hypotheses: ectopic expression of CvAP3 is a convincing explanation for the formation of a completely petaloid perianth in both flower types. In C. vulgaris, CvPI is essential for determination of petal and stamen identity. The characteristic transition of petals into sepals potentially depends on the observed deficiency of CvPI and CvAP3 expression in bud blooming flowers.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-014-0407-z) contains supplementary material, which is available to authorized users.
Highlights
The production of heather (Calluna vulgaris) in Germany is highly dependent on cultivars with mutated flower morphology, the so-called diplocalyx bud bloomers
454 sequencing and assembly For transcriptome comparison, the bud blooming cultivar ‘Maria’ and its wild type flowering descendent F1, resulting from a cross between ‘Maria’ and ‘Boskoop’, have been selected in order to keep the genetic difference not depending on the flower type as low as possible
The B genes Calluna vulgaris PISTILLATA (CvPI) und Calluna vulgaris APETALA 3 (CvAP3) have been found to play crucial roles in the development to the diplocalyx bud bloomer mutants of C. vulgaris, which are of major economic significance in this important landscaping plant
Summary
The production of heather (Calluna vulgaris) in Germany is highly dependent on cultivars with mutated flower morphology, the so-called diplocalyx bud bloomers. This unique flower type of C. vulgaris has not been reported in any other plant species. Due to the shielding from cross-pollination by closed perianth organs and the impossibility of self-pollination due to the loss of stamens and the presence of a second whorl of robust sepals instead of softer petals, the flower buds of bud bloomers display a prolonged flower attractiveness compared to other flower types of C. vulgaris. An attractive flower morphology is one of the major selection targets in ornamental breeding
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