Abstract
Many apple cultivars are subject to biennial fluctuations in flowering and fruiting. It is believed that this phenomenon is caused by a repressive effect of developing fruit on the initiation of flowers in the apex of proximal bourse shoots. However, the genetic pathways of floral initiation are incompletely described in apple, and the biological nature of floral repression by fruit is currently unknown. In this study, we characterized the transcriptional landscape of bourse shoot apices in the biennial cultivar, ’Honeycrisp’, during the period of floral initiation, in trees bearing a high fruit load and in trees without fruit. Trees with high fruit load produced almost exclusively vegetative growth in the subsequent year, whereas the trees without fruit produced flowers on the majority of the potential flowering nodes. Using RNA-based sequence data, we documented gene expression at high resolution, identifying >11,000 transcripts that had not been previously annotated, and characterized expression profiles associated with vegetative growth and flowering. We also conducted a census of genes related to known flowering genes, organized the phylogenetic and syntenic relationships of these genes, and compared expression among homeologs. Several genes closely related to AP1, FT, FUL, LFY, and SPLs were more strongly expressed in apices from non-bearing, floral-determined trees, consistent with their presumed floral-promotive roles. In contrast, a homolog of TFL1 exhibited strong and persistent up-regulation only in apices from bearing, vegetative-determined trees, suggesting a role in floral repression. Additionally, we identified four GIBBERELLIC ACID (GA) 2 OXIDASE genes that were expressed to relatively high levels in apices from bearing trees. These results define the flowering-related transcriptional landscape in apple, and strongly support previous studies implicating both gibberellins and TFL1 as key components in repression of flowering by fruit.
Highlights
In many tree fruits and nuts, flowering follows a biennial cycle, with maximal and minimal flowering alternating yearly [1,2,3]
For phylogenetic analyses of the 16 intensively studied flowering gene families, we considered only the 25 highest-scoring apple transcript translations and only the 25 highestscoring Arabidopsis gene translations identified with each apple sequence query
We identified a set of 180 Arabidopsis genes with flowering-related annotations, and searched the combined GDDH13 / ’Honeycrisp’ transcriptome for expressed sequences with significant homology (Evalue < 1e-12)
Summary
In many tree fruits and nuts, flowering follows a biennial cycle, with maximal and minimal flowering alternating yearly [1,2,3]. This extends considerably the previously annotated gene content of the GDDH13 genome, which was based on nine RNA-seq libraries representing diverse structures, including the shoot apex, along with cDNAs and expressed sequence tags (ESTs) cataloged in NCBI databases.
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