RETENTION OF PHYTOCHROME-MEDIATED SHADE AVOIDANCE RESPONSES IN PHYTOCHROME-DEFICIENT MUTANTS OF ARABIDOPSIS, CUCUMBER AND TOMATO

Phytochrome-mediated shade avoidance responses have been studied in light-grown wild type and phytochrome-deficient mutant seedlings of Arabidopsis thaliana, tomato and cucumber. Light-grown seedlings of the long-hypocotyl hy-1, hy-2 and hy-6 mutants of A. thaliana retain a qualitatively normal shade avoidance response to growth under a low red: far-red (R: FR) ratio, namely an increase in elongation growth and a acceleration of flowering. This indicates that, despite the fact that etiolated tissues of these mutants are severely deficient in spectrophotometrically-detectable phytochrome, light-grown seedlings of the mutants possess some functional phytochrome. The hy-3 mutant, which is deficient in seed phytochrome, retains a similar, but less dramatic, shade avoidance response. The aurea (au) mutant of tomato, in which etiolated tissues are deficient in spectrophotometrically- and immunochemically-detectable phytochrome, also retains a qualitatively normal shade avoidance response to reduced R: FR ratio. This is consistent with the hypothesis that au seedlings are deficient only in a light-labile phytochrome species that normally accumulates in etiolated tissues and is involved in the de-etiolation process. Light-grown seedlings of the long-hypocotyl (lh) mutant of cucumber, a mutation that appears to lead to a deficiency in a light-stable phytochrome species, retain a small, but significant, phytochrome-mediated increase in elongation growth in response to reduced R:FR ratio.