title: Transcriptomics of shading-induced and NAA-induced abscission in apple (Malus domestica) reveals a shared pathway involving reduced photosynthesis, alterations in carbohydrate transport and signaling and hormone crosstalk.
abstract: BACKGROUND
  Naphthaleneacetic acid (NAA), a synthetic auxin analogue, is widely used as an effective thinner in apple orchards. When applied shortly after fruit set, some fruit abscise leading to improved fruit size and quality. However, the thinning results of NAA are inconsistent and difficult to predict, sometimes leading to excess fruit drop or insufficient thinning which are costly to growers. This unpredictability reflects our incomplete understanding of the mode of action of NAA in promoting fruit abscission.
  
  
  RESULTS
  Here we compared NAA-induced fruit drop with that caused by shading via gene expression profiling performed on the fruit abscission zone (FAZ), sampled 1, 3, and 5 d after treatment. More than 700 genes with significant changes in transcript abundance were identified from NAA-treated FAZ. Combining results from both treatments, we found that genes associated with photosynthesis, cell cycle and membrane/cellular trafficking were downregulated. On the other hand, there was up-regulation of genes related to ABA, ethylene biosynthesis and signaling, cell wall degradation and programmed cell death. While the differentially expressed gene sets for NAA and shading treatments shared only 25% identity, NAA and shading showed substantial similarity with respect to the classes of genes identified. Specifically, photosynthesis, carbon utilization, ABA and ethylene pathways were affected in both NAA- and shading-induced young fruit abscission. Moreover, we found that NAA, similar to shading, directly interfered with leaf photosynthesis by repressing photosystem II (PSII) efficiency within 10 minutes of treatment, suggesting that NAA and shading induced some of the same early responses due to reduced photosynthesis, which concurred with changes in hormone signaling pathways and triggered fruit abscission.
  
  
  CONCLUSIONS
  This study provides an extensive transcriptome study and a good platform for further investigation of possible regulatory genes involved in the induction of young fruit abscission in apple, which will enable us to better understand the mechanism of fruit thinning and facilitate the selection of potential chemicals for the thinning programs in apple.
authors: Alson H. Smith, Jr. Agricultural Research and Extension Center, Virginia Polytechnic Institute and State University, 595 Laurel Grove Road, Winchester, VA 22602, USA.
authors: Hong
authors: H
authors: Zhu
authors: Chris D
authors: CD
authors: Dardick
authors: Eric P
authors: EP
authors: Beers
authors: Ann M
authors: AM
authors: Callanhan
authors: Rui
authors: R
authors: Xia
authors: Rongcai
authors: R
authors: Yuan
mesh_terms: D050260:Carbohydrate Metabolism; D016000:Cluster Analysis; D003624:Darkness; D005030:Ethylenes / Q000096:biosynthesis; D005638:Fruit / Q000378:metabolism / Q000502:physiology; D020869:Gene Expression Profiling; D018506:Gene Expression Regulation, Plant; D027845:Malus / Q000235:genetics / Q000502:physiology* / Q000528:radiation effects; D009280:Naphthaleneacetic Acids / Q000378:metabolism*; D020411:Oligonucleotide Array Sequence Analysis; D010788:Photosynthesis; D010937:Plant Growth Regulators / Q000378:metabolism; D018515:Plant Leaves / Q000378:metabolism; D059467:Transcriptome
chemical_list: D005030:Ethylenes; D009280:Naphthaleneacetic Acids; D010937:Plant Growth Regulators; C036216:ethylene