Unique ethylene-regulated touch responses of Arabidopsis thaliana roots to physical hardness

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/square mm 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.