One of the characteristic aspects of odour sensing in humans is the activation of olfactory receptors in a slightly different manner to different enantiomers. Here, we focused on whether plants showed enantiomer-specific responses similar to that in humans. We exposed Arabidopsis seedlings to methanol (control) and (+)- or (−)-borneol, and found that only (+)-borneol reduced the root length. Furthermore, the root-tip width was more increased upon (+)-borneol exposure than upon (−)-borneol exposure. In addition, root-hair formation was observed near the root tip in response to (+)-borneol. Auxin signalling was strongly reduced in the root tip following exposure to (+)-borneol, but was detected following exposure to (−)-borneol and methanol. Similarly, in the root tip, the activity of Cyclin B1:1 was detected on exposure to (−)-borneol and methanol, but not on exposure to (+)-borneol, indicating that (+)-borneol inhibits the meristematic activity in the root. These results partially explain (+)-borneol-specific reduction in the root length of Arabidopsis. Our results indicate the presence of a sensing system specific for (+)-borneol in Arabidopsis.

Exposure of aboveground parts of Arabidopsis seedlings to airborne borneol

One seed was placed on the surface of 0.8% agar (9 mL) containing 1X Murashige and Skoog salt mixture and 1× Gamborg’s vitamin solution (Sigma-Aldrich, St. Louis, MO, USA) filled in a Bioramo centrifuge tube II VIO-15BN (15 mL: 120 mm long) (AS ONE Corporation, Osaka, Japan). The pH of the mixture was adjusted to 5.9 with KOH. The depth of the agar was approximately 75 mm in the tube. The remaining volume (volatile exposure area) in the tube was 7.6 cm3. Three days after germination, the seedlings were treated with one of the borneol enantiomers for 4 days.

We diluted (-) and (+)-borneol in methanol to obtain 5 and 25 mM solutions and added 20 µL of the solution into a moist piece of cotton wool (>1 cm³) hung in the uppermost part of the exposed area. The amount of the impregnated compound was 15.4 and 77.0 µg, respectively. The concentration was determined from a pilot test of various borneol concentrations. At lower concentrations, we did not identify any characteristic morphological changes in the root tips. At higher concentrations, leaf chlorosis occurred. Twenty micro-litres of methanol was used as the control.