ITRAQ-based proteomic analysis of apple buds provides new insights into regulatory mechanisms of flowering in response to shoot bending

Flowering is an important life cycle stage in apple trees, and young 'Nagafu No. 2' produce limited inferior flower buds. Shoot bending, an effective agronomic measure, is widely used to promote flowering but little is known regarding the underlying molecular mechanisms. Using isobaric tags for relative and absolute quantification LC-MS/MS, 8,585 proteins expressed in buds during flower bud differentiation were identified. The 1,060 up and 1,079 down-regulated proteins after shoot bending were divided into 7 pattern-based groups: I (22/5), II (30/57), III (34/21), IV (77/61), V (381/452), VI (218/170), and VII (298/313) types. Number of proteins were associated with carbohydrate, fatty acid, and lipid metabolism, as well as hormone, transcription factor, circadian rhythm, and photoperiod and flowering pathways. The differentially expressed proteins involved in fatty acid and lipid pathways were up-regulated by shoot bending, while other proteins that were involved in starch synthesis were down-regulated. In addition, circadian rhythm-and flowering-related genes were up-regulated by shoot bending. This research provides a valuable inventory of proteins involved in floral bud development and contributes to the understanding of the regulatory mechanisms of flower bud differentiation caused by shoot bending in apple trees.