Flowering responses to light-breaks in photomorphogenic mutants of Arabidopsis thaliana, a long-day plant

Abstract

M. 1991. Flowering responses to light-breaks in photomorphogenic mutants of Arabidopsis thaliana, a long-day plant. Flowering response and plant form of photomorphogenic mutants (hyl, hy2, hy3, hy4 and hy5) oi Arabidopsis thaliana (L.), a long-day plant, were examined in loog and short days. There were only slight differences among genotypes including Landsberg wild type with respect to the flowering time tjnder long days. The effect of 1 h light-(night)-breaks of far-red, red, bloe and white light given in the middle of the dark period of plants grown under short days, was studied. Effects of far-red light applied at the end or the beginning of the main photoperiod on flowering and plant form were also examined. The light-breaks with all the above mentioned light qualities promoted floral initiation of all the genotypes including the wild type in terms of both the flowering time and the ntimber of rosette leaves. In general, far-red light was most effective. It is possible to classify the hy-mutants into 3 groups by their responses to light-breaks under short day conditions: (a) Mutants hy2 and hy3, which have a reduced number of rosette leaves, and flower early. Red ligbt is as effective as far-red light. The wavelength of light-breaks is relatively unimportant for flowering response, (b) Mutants hy4, hy5 and Landsberg wild type, which have a greater ntimber of rosette leaves, and flower relatively late. The effectiveness of light-breaks is in the following order, far-red, blue, and red light, which is in reverse order to the transformation of phytoehrome to the P[, form, (c) Mutant hyl, which behaves anomalously with respect to relatiotis between flowering time and number ol rosette leaves; late flowering with reduced number of rosette leaves. Red. blue and far-red light are effective, but white light is ineffective for reducitig the ntimber of rosette leaves. When far-red light was given in the middle of the night or at the end of the main photoperiod, it markedly reduced the number of rosette leaves compared to those grown under short days for all the genotypes, while when applied at the beginning of the main photoperiod far-red light did not affect the number of rosette leaves. Different effects on the plant form dependent on the time of treatment with far-red light-breaks are also discussed.

Flowering Responses to Altered Expression of Phytochrome in Mutants and Transgenic Lines of Arabidopsis thaliana (L.) Heynh

“…With a slower leaf production rate (c) an indirect effect of photosynthesis on flowering must also be considered especially for the altered-phytochrome, phyB mutants, which may have no more than 50% of WT leaf Chl (Wester et al, 1994) and a much slower leaf production rate (Goto et al, 1991).…”

“…With photoreceptor-deficient mutants of Arabidopsis, a lack of phytochrome A delays flowering in low-intensity incandescent light-extended days (Johnson et al, 1994), but flowering is somewhat earlier in the mutant pkyB-2 (Goto et al, 1991), which has a deficiency in phytochrome B (Nagatani et al, 1991;Reed et al, 1993). This latter response with loss of a photoreceptor is not easy to explain unless the Pfr form of phytochrome B mediates a floral inhibition, whereas phytochrome A Pfr could be responsible for floral promotion.…”

“…However, the vegetative growth habit for mutants can be quite atypical with altered plant morphology, a slow rate of leaf production relative to the WT (cf. Goto et al, 1991), and large (2-fold) reductions in leaf Chl content (Wester et al, 1994) so that effects on flowering could be quite indirect. Overexpression of phytochrome A and B genes in transgenic Arabidopsis allows a complementary approach to the use of mutants for the examination of the role of phytochrome(s) in the control of flowering of a LDP, notwithstanding the difficulties associated with evaluating the role of overexpressed phytochrome in transgenic plants (Cherry and Vierstra, 1994).…”

Floral transition mutants in Arabidopsis

“…The effect of phytochrome B (phyB) is rather different and appears antagonistic to the effect of phyA. In Arabidopsis, but also in other LD plants such as pea (Reid et al 1996) and even in the SD plant Sorghum (Childs et al 1992), phyB deficiency results in early flowering, indicating an inhibitory role of this phytochrome (Goto et al 1991). Although earliness of phyB-deficient mutants seems more pronounced under unfavourable flowering conditions (such as SDs in Arabidopsis), these mutants still confer a daylength reponse, especially in a late mutant background (Koornneef et al 1995), and therefore this inhibitory effeet of phyB might be general and unrelated to daylength.…”

“…However, the observation that in SD conditions, which delay flowering much more that the conditions used by Bagnall et al (1995), plants with the same hy4-101 allele flower earlier than Col wildtype plants (Zagotta et al 1996) seems at odds with the promotive effect of CRY. The involvement of phytochrome in flowering of Arabidopsis was derived from studies using different R/FR ratios during development (Halliday, Koornneef & Whitelam 1994;Johnson et al 1994), end of day red (EOD R) and far-red (EOD FR) treatments (Bagnall et al 1995), daylength extension with low fluences of FR and R (Bagnall et al 1995), phytochromedeficient mutants (Goto et al 1991;Johnson et al 1994;Reed et at. 1994) and transgenic plants overexpressing phytoehrome proteins (Bagnall et al 1995).…”