title: A possible involvement of autophagy in amyloplast degradation in columella cells during hydrotropic response of Arabidopsis roots.
abstract: Seedling roots display not only gravitropism but also hydrotropism, and the two tropisms interfere with one another. In Arabidopsis (Arabidopsis thaliana) roots, amyloplasts in columella cells are rapidly degraded during the hydrotropic response. Degradation of amyloplasts involved in gravisensing enhances the hydrotropic response by reducing the gravitropic response. However, the mechanism by which amyloplasts are degraded in hydrotropically responding roots remains unknown. In this study, the mechanistic aspects of the degradation of amyloplasts in columella cells during hydrotropic response were investigated by analyzing organellar morphology, cell polarity and changes in gene expression. The results showed that hydrotropic stimulation or systemic water stress caused dramatic changes in organellar form and positioning in columella cells. Specifically, the columella cells of hydrotropically responding or water-stressed roots lost polarity in the distribution of the endoplasmic reticulum (ER), and showed accelerated vacuolization and nuclear movement. Analysis of ER-localized GFP showed that ER redistributed around the developed vacuoles. Cells often showed decomposing amyloplasts in autophagosome-like structures. Both hydrotropic stimulation and water stress upregulated the expression of AtATG18a, which is required for autophagosome formation. Furthermore, analysis with GFP-AtATG8a revealed that both hydrotropic stimulation and water stress induced the formation of autophagosomes in the columella cells. In addition, expression of plastid marker, pt-GFP, in the columella cells dramatically decreased in response to both hydrotropic stimulation and water stress, but its decrease was much less in the autophagy mutant atg5. These results suggest that hydrotropic stimulation confers water stress in the roots, which triggers an autophagic response responsible for the degradation of amyloplasts in columella cells of Arabidopsis roots.
authors.affiliation: Graduate School of Life Sciences, Tohoku University, Sendai 980-8577, Japan.
authors.forename: Mayumi
authors.initials: M
authors.lastname: Nakayama
authors.forename: Yasuko
authors.initials: Y
authors.lastname: Kaneko
authors.forename: Yutaka
authors.initials: Y
authors.lastname: Miyazawa
authors.forename: Nobuharu
authors.initials: N
authors.lastname: Fujii
authors.forename: Nahoko
authors.initials: N
authors.lastname: Higashitani
authors.forename: Shinya
authors.initials: S
authors.lastname: Wada
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Ishida
authors.forename: Kohki
authors.initials: K
authors.lastname: Yoshimoto
authors.forename: Ken
authors.initials: K
authors.lastname: Shirasu
authors.forename: Kenji
authors.initials: K
authors.lastname: Yamada
authors.forename: Mikio
authors.initials: M
authors.lastname: Nishimura
authors.forename: Hideyuki
authors.initials: H
authors.lastname: Takahashi
mesh_terms: D017360:Arabidopsis / Q000235:genetics / Q000502:physiology* / Q000648:ultrastructure; D029681:Arabidopsis Proteins / Q000235:genetics; D001343:Autophagy / Q000502:physiology*; D000071183:Autophagy-Related Proteins; D002467:Cell Nucleus / Q000502:physiology / Q000648:ultrastructure; D016764:Cell Polarity; D003681:Dehydration; D004721:Endoplasmic Reticulum / Q000502:physiology / Q000648:ultrastructure; D018506:Gene Expression Regulation, Plant; D018613:Microscopy, Confocal; D046529:Microscopy, Electron, Transmission; D009154:Mutation; D018517:Plant Roots / Q000235:genetics / Q000502:physiology / Q000648:ultrastructure; D030821:Plants, Genetically Modified; D018087:Plastids / Q000235:genetics / Q000502:physiology* / Q000648:ultrastructure; D011993:Recombinant Fusion Proteins; D036226:Seedlings / Q000235:genetics / Q000502:physiology* / Q000648:ultrastructure; D013312:Stress, Physiological / Q000502:physiology*; D013997:Time Factors; D014157:Transcription Factors / Q000235:genetics; D018523:Tropism / Q000502:physiology*; D014617:Vacuoles / Q000502:physiology / Q000648:ultrastructure
chemical_list: C502739:ATG18a protein, Arabidopsis; D029681:Arabidopsis Proteins; D000071183:Autophagy-Related Proteins; C114579:DREB2A protein, Arabidopsis; D011993:Recombinant Fusion Proteins; D014157:Transcription Factors