Phototropin is partly involved in blue-light-mediated stem elongation, flower initiation, and leaf expansion: A comparison of phenotypic responses between wild Arabidopsis and its phototropin mutants

Our previous study on bedding plants indicated that blue light in association with low phytochrome activity, relative to red light, can promote elongation growth as one of the shade-avoidance responses. We hypothesized that phototropin is involved in the blue light promotion effects. To test the hypothesis, plant growth and morphology were examined in wild Arabidopsis (Col-0), and its three phototropin-deficient mutants (phot1, phot2, and phot1phot2) under four light quality treatments: (1) R, pure red light (660 nm); (2) B, pure blue light (455 nm); (3) BR, unpure blue light created by mixing B with low-level (6 %) R; and (4) BRF, unpure blue light created by adding low-level far-red light (735 nm) to BR with red/far-red ≈ 1. Continuous (24-h) light-emitting diode lighting with 100 μmol m−2·s−1 photosynthetic photon flux density, and an air temperature of ≈ 23 ℃ were used with the above treatments. The calculated phytochrome photoequilibrium, an indicator of phytochrome activity, was 0.89, 0.69, 0.50, and 0.60 for R, BR, B, and BRF, respectively. After 20 days of light treatments, for the wild-type plants, B or BRF, compared to R or BR, promoted stem elongation, plant flowering, and leaf expansion, showing typical shade-avoidance responses. However, for the phototropin mutants, the promotion effects of B or BRF, relative to R or BR, on the above plant traits were reduced. In the absence of the phototropin(s), plants had reduced stem elongation and delayed flower initiation under B or BRF rather than R or BR, or reduced leaf expansion to a larger degree under B or BRF than R or BR. Our results suggest that phototropin is partly involved in the promotion of stem elongation, plant flowering, and leaf expansion mediated by blue light in association with low phytochrome activity.