Effect of Light on Root Growth and Gravitropic Response of Phytochrome Mutants of Arabidopsis

Light, one of the environmental stimuli, is fundamental to the growth and development of plants. Red and far-red light are sensed using the phytochrome family of plant photoreceptors. To investigate the effect of light on root growth and gravitropism, we used the Arabidopsis phytochrome mutants grown in several light conditions. The root growth of phyA reared in all light conditions except white light and was stimulated compared to the WT. The stimulation of root growth was obvious in phyA grown in red light. On the other hand, the root growth of phyB grown in all light conditions decreased, and the lowest rate of decrease was observed in phyAB grown in white and red light. The gravitropic response of phyA was stimulated compared to the WT when it was grown in all light conditions except far-red light. PhyAB grown in all light conditions showed the inhibition of gravitropic response. The transcript level of ACS, one of the enzymes regulating ethylene biosynthesis, increased in phyA grown in white and red light, but not in phyA grown in far-red light. In conclusion, these results suggested that the Pfr form of phyB regulates the root growth and gravitropism.