Genetic and Genomic Analyses of Vegetative Budbreak in Response to Chilling Units in European Pear (Pyrus Communis L.)

Abstract

Dormancy is critical for the normal yearly cycle of fruit trees in temperate zones due to their requirements of exposure to certain numbers of chilling hours. Once the chilling requirement is fulfilled, vegetative budbreak can occur when climatic conditions are favorable. Exposure to insufficient chilling units can lead to delayed vegetative budbreak. Bud dormancy has been studied in perennial fruit trees within the context of the effects of climate change. The recent rise in temperatures worldwide has led to a reduction in chilling units accumulation. Pear cultivars are highly influenced by the number of chilling units accumulated during the winter. However, fruit of most low-chilling cultivars is considered to be of low quality. Study of the genetic mechanism underlying chilling requirements would greatly accelerate adaptation of new pear cultivars to warm climates. As vegetative budbreak date shows high heritability, the potential for breeding a low-chilling requirement pear cultivar is high. However, chilling requirements are subject to a complex genetic mechanism which is probably determined by, or partially derived from, multiple genes. Genetic factors affecting dormancy have been identified for the first time in peach, wherein MADS-box genes associated with dormancy regulation have been reported. Six DORMANCY-ASSOCIATED MADS-BOX (DAM) genes, and a genomic region, designated as the evergrowing (evg) locus, have been identified. To date, three DAM genes, including PpDAM1, PpDAM2, and PpDAM3, have been identified in Asian pear (Pyrus spp.). In previous genetic studies in apple, which has a high level of synteny with pear, quantitative trait loci (QTLs) for chilling requirements have been identified. A QTL common to all families has been located on linkage group 9, suggesting stability of this QTL over different families, climate regions, and years. However, in European pear, a major QTL has been detected on linkage group 8, and an additional QTL on linkage group 9 has also been confirmed. Differentially expressed genes in these regions include PcDAM1 and PcDAM2, putative orthologs of PpDAM1 and PpDAM2. Due to a significant genotype × environment (G × E) effect, QTLs associated with G × E vegetative budbreak date have been detected. It has long been known that content levels of metabolites are highly correlated with dormancy phase transitions. Metabolites, such as phospholipids, sugars, and fatty acids, including alpha-linolenic acid, play major roles in dormancy regulation in pear. Several pear genes, such as 12-oxophytodienoate reductase 2-like (alpha-linolenic acid pathway), have been found to be linked to dormancy regulation. A proposed model for pear selection of traits under a changing climate will be discussed.