The molecular control of bud dormancy establishment and release is still not well understood, although some genes have already been demonstrated to play important roles in this process. The dormancy-associated MADS-box (DAM) genes were first identified in the peach EVERGROWING locus and are considered the main regulators of bud dormancy control. In this work, the apple (Malus × domestica Borkh.), a perennial plant adapted to temperate climates that displays cycles of growth and bud dormancy, was screened for the presence of DAM genes. The candidate genes retrieved were characterized in comparison to DAM genes from other species. Four of them (MdDAM1-4) are structurally very similar to the reported DAM genes. When apple genomic segments containing these candidates were compared to the peach EVERGROWING locus, a highly conserved noncoding region was detected inside their largest intron. Similar sequences were also identified inside introns of apricot and pear DAM genes. Organ expression patterns revealed that MdDAM1-4 are mainly expressed in dormant buds and seeds, with low transcript accumulation in vegetative structures. In addition, the MdDAM genes showed seasonally oscillating patterns of steady-state messenger RNA (mRNA) levels and were downregulated by artificial chilling. Motif analyses in the promoter and in the intronic conserved region of the MdDAM genes disclosed some clues to the regulation of the expression patterns observed. Possible roles for the conserved intronic sequence in dormancy regulation are discussed.

Galactinol and raffinose act together to protect dormant buds against limited availability of winter water; the apple galactinol synthases MdGolS1 and MdGolS2 are responsible for their seasonal accumulation during dormancy.

Chilling requirement (CR) for bud dormancy completion determines the time of bud break in apple (Malus × domestica Borkh.). The molecular control of bud dormancy is highly heritable, suggesting a strong genetic control of the trait. An available Infinium II SNP platform for genotyping containing 8,788 single nucleotide polymorphic markers was employed, and linkage maps were constructed in a F1 cross from the low CR M13/91 and the moderate CR cv. Fred Hough. These maps were used to identify quantitative trait loci (QTL) for bud break date as a trait related to dormancy release. A major QTL for bud break was detected at the beginning of linkage group 9 (LG9). This QTL remained stable during seven seasons in two different growing sites. To increase mapping efficiency in detecting contributing genes underlying this QTL, 182 additional SNP markers located at the locus for bud break were used. Combining linkage mapping and structural characterization of the region, the high proportion of the phenotypic variance in the trait explained by the QTL is related to the coincident positioning of Arabidopsis orthologs for ICE1, FLC, and PRE1 protein-coding genes. The proximity of these genes from the most explanatory markers of this QTL for bud break suggests potential genetic additive effects, reinforcing the hypothesis of inter-dependent mechanisms controlling dormancy induction and release in apple trees.

A genetic linkage map was constructed using a pseudo-testcross strategy based on a cross between the seedless Vitis vinifera 'Crimson Seedless' and the complex hybrid 'Villard Blanc', resistant to downy mildew. A total of 315 DNA markers, including 262 AFLP, 48 simple sequence repeats (SSR), 2 SCARs (sequence characterized amplified region) and 3 minisatellite markers were used to generate a map for each parent. For both parents, 19 linkage groups were obtained, covering 1,111.0 cM and 926.0 cM for 'Villard Blanc' and 'Crimson Seedless', respectively. The position of SSR loci in the obtained maps is consistent with the genomic sequence. Quantitative Trait Loci (QTLs) for seedlessness and resistance to downy mildew were investigated. Two major effect QTLs for downy mildew resistance and seedlessness were mapped on the same region of linkage group 18. These QTLs explain 25.0-55.7% and 54.0-62.,4% of the total variance, respectively. The MIKC C-Type MADS box gene VvAG3, whose orthologue is involved in Arabidopsis carpel and ovule development, and located in the confidence interval of the seedlessness QTL detected on the LG 18, could be considered as a candidate gene to control seed development in grape. Colocalizations were found in the same region, between the position of the Rpv3 locus, which is very rich in TIR-NBS-LRR genes, and the main QTL identified for downy mildew resistance. Our results demonstrate that the same region of LG 18 contains important genetic determinants for seedlessness and downy mildew resistance in grapevine. Moreover, assessing the allelic variation at these agronomically important loci provides a basis for the development of marker-assisted selection for seedlessness and downy mildew simultaneously.

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