Arabidopsis Mutants Lacking Blue Light-Dependent Inhibition of Hypocotyl Elongation

We have isolated a new class of photomorphogenic mutants in Arabidopsis. Hypocotyl elongation is not inhibited in the mutant seedlings by continuous blue light but is inhibited by far red light, indicating that these mutations are phenotypically different from the previously isolated long hypocotyl (hy) mutants. Complementation analysis indicated that recessive nuclear mutations at three genetic loci, designated blu1, blu2, and blu3, can result in the blu mutant phenotype and that these mutants are genetically distinct from other long hypocotyl mutants. The BLU genes appear to be important only during seedling development because the blu mutations have little effect on mature plants, whereas hypocotyl elongation and cotyledon expansion are altered in seedlings. The genetic separation of the blue and far red sensitivities of light-induced hypocotyl inhibition in the blu and hy mutants demonstrates that two photosensory systems function in this response.