Unraveling Molecular Mechanisms Regulating Dormancy and Bloom Time in Apple (Malus × domestica Borkh)

Bud dormancy is an essential characteristic of deciduous woody perennials, including apple, to cope with the low temperatures during winter. The release from dormancy and subsequent budburst in apple can only occur after fulfillment of chilling and heat requirements. In the Mid-Atlantic region, dormancy release and bud break of apple often coincide with late-spring freezes that cause severe damages to flowers, and small fruitlets. Therefore, the present study aimed to better understand mechanisms underlying bud dormancy in apple, with an ultimate goal of exploring chemical and/or genetic approaches for bloom-time modulation to avoid spring frost. Using two apple cultivars, 'Cripps Pink' and 'Honeycrisp,' representing early- and late-blooming cultivars, respectively, the present study specifically investigated the accumulation kinetics of plant hormones, carbohydrates, and reactive oxygen species (ROS) throughout the dormancy-regrowth cycle. Our results indicated that both cultivars required 1000 chilling hours for endodormancy release, but 'Honeycrisp' required 1000 growing degree hours (GDHs) more than 'Cripps Pink' for ecodormancy release and budburst. Among plant hormones, abscisic acid (ABA) showed remarkably elevated levels in the dormant buds of both cultivars during endodormancy, but its levels were significantly higher in 'Honeycrisp'. The decline of the ABA level at bud burst was combined with increased levels of cytokinin (CK). The ABA accumulation pattern during dormancy paralleled with an upregulation and downregulation of ABA biosynthetic and catabolic genes, respectively. On the other hand, the levels of hydrogen peroxide (H2O2) and superoxide (O2.-) were significantly higher in 'Cripps Pink' than 'Honeycrisp', particularly by the time of endodormancy and ecodormancy release, respectively. Our findings also showed a gradual decline in starch levels with the dormancy progression and increased levels of total soluble sugars (TSS) that were generally higher in the early-blooming cultivars. Transcriptomic profiling and module-trait relationship identified two modules that contrast between two cultivars mainly during eco-dormancy. Gene ontology (GO) analysis indicated that these DEGs were mostly involved in pathways related to hormones and signaling and co-expressed with H2O2 whereas, during ecodormancy pathways related to glutathione metabolism, auxin biosynthesis, carbohydrate metabolism and reproductive development were co-expressed with O2.-. Together, our results suggest that the contrasting bloom dates between 'Cripps Pink' and 'Honeycrisp' can be explained, at least partially, by the differential accumulation levels of ABA, ROS, antioxidants, and their associated genes in the buds of these cultivars throughout the dormancy cycle.