title: Involvements of PCD and changes in gene expression profile during self-pruning of spring shoots in sweet orange (Citrus sinensis).
abstract: BACKGROUND
  Citrus shoot tips abscise at an anatomically distinct abscission zone (AZ) that separates the top part of the shoots into basal and apical portions (citrus self-pruning). Cell separation occurs only at the AZ, which suggests its cells have distinctive molecular regulation. Although several studies have looked into the morphological aspects of self-pruning process, the underlying molecular mechanisms remain unknown.
  
  
  RESULTS
  In this study, the hallmarks of programmed cell death (PCD) were identified by TUNEL experiments, transmission electron microscopy (TEM) and histochemical staining for reactive oxygen species (ROS) during self-pruning of the spring shoots in sweet orange. Our results indicated that PCD occurred systematically and progressively and may play an important role in the control of self-pruning of citrus. Microarray analysis was used to examine transcriptome changes at three stages of self-pruning, and 1,378 differentially expressed genes were identified. Some genes were related to PCD, while others were associated with cell wall biosynthesis or metabolism. These results strongly suggest that abscission layers activate both catabolic and anabolic wall modification pathways during the self-pruning process. In addition, a strong correlation was observed between self-pruning and the expression of hormone-related genes. Self-pruning plays an important role in citrus floral bud initiation. Therefore, several key flowering homologs of Arabidopsis and tomato shoot apical meristem (SAM) activity genes were investigated in sweet orange by real-time PCR and in situ hybridization, and the results indicated that these genes were preferentially expressed in SAM as well as axillary meristem.
  
  
  CONCLUSIONS
  Based on these findings, a model for sweet orange spring shoot self-pruning is proposed, which will enable us to better understand the mechanism of self-pruning and abscission.
authors.forename: Jin-Zhi
authors.initials: JZ
authors.lastname: Zhang
authors.forename: Kun
authors.initials: K
authors.lastname: Zhao
authors.forename: Xiao-Yan
authors.initials: XY
authors.lastname: Ai
authors.affiliation: Key Laboratory of Horticultural Plant Biology (Ministry of Education), College of Horticulture and Forestry Science, Huazhong Agricultural University, Wuhan, 430070, China. chungen@mail.hzau.edu.cn.
authors.forename: Chun-Gen
authors.initials: CG
authors.lastname: Hu
mesh_terms: D001343:Autophagy; D002135:Calcium-Binding Proteins / Q000235:genetics / Q000378:metabolism; D032084:Citrus sinensis / Q000235:genetics / Q000378:metabolism*; D016000:Cluster Analysis; D018744:DNA, Plant / Q000378:metabolism; D051034:Inhibitor of Apoptosis Proteins / Q000235:genetics / Q000378:metabolism; D046529:Microscopy, Electron, Transmission; D020411:Oligonucleotide Array Sequence Analysis; D010940:Plant Proteins / Q000737:chemistry / Q000378:metabolism; D018520:Plant Shoots / Q000235:genetics / Q000378:metabolism; D017382:Reactive Oxygen Species / Q000378:metabolism; D059467:Transcriptome
chemical_list: D002135:Calcium-Binding Proteins; D018744:DNA, Plant; D051034:Inhibitor of Apoptosis Proteins; D010940:Plant Proteins; D017382:Reactive Oxygen Species