Phytochrome contributes to blue-light-mediated stem elongation and associated shade-avoidance response in mature Arabidopsis plants

Our recent studies on ornamental plants and microgreens indicate that blue-light-mediated stem elongation is related to phytochrome activity, which was based on the calculated phytochrome photoequilibrium. To examine whether phytochromes really contribute to the blue light’s effect, plant phenotypic responses were investigated in wild type Arabidopsis (Col-0), and its quintuple phytochrome ( phyA phyB phyC phyD phyE ) mutant plants under the following light treatments: (1) R, a pure red light from 660-nm LED; (2) B, a pure blue light from 455-nm LED; (3) BR, a impure blue light from LED combination of 94% B and 6% R; and (4) BRF, another impure blue light from LED combination of BR and 6 µmol m −2 s −1 of FR (735 nm). For all the light treatments, a photosynthetic photon flux density of ≈100 μmol m −2 s −1 were provided by 24-h lighting daily inside a walk-in growth chamber, which had an air temperature of ≈ 23 °C. The calculated phytochrome photoequilibrium was 0.89, 0.50, 0.69, and 0.60 for R, B, BR, and BRF, respectively, indicating a higher phytochrome activity under R and BR than B and BRF. After 18 days of light treatment, B or BRF increased main stem length in wild-type plants compared with R, but BR had an inhibition effect similar to R. Also, B and BRF relative to R or BR induced earlier flowering and reduced leaf size in wild type plants, showing typical shade-avoidance responses. In phytochrome-deficient mutant plants, the above shade-avoidance responses were inhibited under B or BRF, and induced under BR. However, as an exception, hypocotyl length, a growth trait during the de-etiolation stage, was reduced under B, BR and BRF vs. R regardless of phytochrome absence. It suggests that for mature Arabidopsis plants, phytochrome plays an active role in blue-light-mediated stem elongation and associated shade-avoidance response.