Phytochrome contributes to blue-light-mediated stem elongation and flower initiation in mature Arabidopsis thaliana plants

To examine whether phytochromes contribute to blue-light-mediated stem elongation, plant phenotypic responses were investigated in wild type Arabidopsis thaliana (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 mol m−2 s−1 of FR (735 nm). A photosynthetic photon flux density of ≈100 μmol m−2 s−1 was provided for all the light treatments. 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. However, hypocotyl length, a growth trait characterizing the de-etiolation stage, was reduced under B, BR and BRF vs. R regardless of phytochrome absence. These findings suggest that for mature Arabidopsis plants, phytochrome plays a role in blue-light-mediated stem elongation and the associated shade-avoidance responses.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author