Light-stimulated root elongation in Arabidopsis thaliana

We investigated the effects of continuous irradiation either with white light or far-red light on elongation of roots of Arabidopsis thaliana. The white light used lacked most spectral components in the far-red region. The white-light irradiation of a whole seedling stimulated root growth; DCMU, an inhibitor of photosynthesis, completely inhibited this stimulation. Excised roots lost their responsiveness to light. Sugars produced during photosynthesis stimulated the elongation of roots in the dark. Root growth was also promoted by continuous exposure of a whole seedling to far-red light although chlorophyll did not accumulate at the same rate as under white light. The effect of far-red light was not inhibited by DCMU. In a phyA mutant, there was no stimulation of root growth under far-red light. These results show that root growth is primarily promoted by photosynthetic activity, and that phytochrome A stimulates root growth independently of photosynthesis. In a phyB mutant, there was no stimulation of root growth by either white, red or far-red light, suggesting that phytochrome B, either as Pr or Pfr, is necessary for the roots to respond to light