title: Unique ethylene-regulated touch responses of Arabidopsis thaliana roots to physical hardness.
abstract: Although touch responses of plant roots are an important adaptive behavior, the molecular mechanism remains unclear. We have developed a bioassay for measuring root-bending responses to physical hardness in Arabidopsis thaliana seedlings. Our test requires a two-layer solid medium. Primary roots growing downward through an upper layer of 0.3% phytagel either penetrate the lower layer or bend along an interface between the upper and lower layers with different concentrations (0.2-0.5%, corresponding to 1.57-6.79 gw mm(-2) in hardness). In proportion to increasing hardness of the lower layer, we found that the percentage of bending roots increased and ethylene production decreased, suggesting an inverse relationship between the root-bending response and ethylene production. Studies with ethylene biosynthesis modulators and mutants also suggested that bending and non-bending responses of roots to medium hardness depend, respectively, on decreased and increased ethylene biosynthesis. In addition, the degrees of root-tip softening and differential root-cell growth, both possible factors determining root-bending response, were enhanced and attenuated by decreased and increased amounts of ethylene, respectively--also in bending roots and non-bending roots. Our findings indicate that ethylene regulates root touch responses, probably through a combination of root-tip softening (or hardening) and differential root-cell growth.
authors.affiliation: Faculty of Applied Bioscience, Tokyo University of Agriculture, 1-1-1 Sakuragaoka, Setagaya-ku, Tokyo, 156-8502, Japan.
authors.forename: Chigusa
authors.initials: C
authors.lastname: Yamamoto
authors.forename: Yoichi
authors.initials: Y
authors.lastname: Sakata
authors.forename: Teruaki
authors.initials: T
authors.lastname: Taji
authors.forename: Tadashi
authors.initials: T
authors.lastname: Baba
authors.forename: Shigeo
authors.initials: S
authors.lastname: Tanaka
mesh_terms: D000598:Amino Acids, Cyclic / Q000378:metabolism; D017360:Arabidopsis / Q000235:genetics / Q000254:growth & development* / Q000378:metabolism; D005030:Ethylenes / Q000378:metabolism*; D006244:Hardness; D009154:Mutation; D010812:Physical Stimulation; D018517:Plant Roots / Q000254:growth & development* / Q000378:metabolism; D036226:Seedlings / Q000254:growth & development* / Q000378:metabolism; D018523:Tropism
chemical_list: D000598:Amino Acids, Cyclic; D005030:Ethylenes; C023863:1-aminocyclopropane-1-carboxylic acid; C036216:ethylene