Light-Stimulated Cotyledon Expansion in the blu3 and hy4 Mutants of Arabidopsis thaliana

Abstract

Cotyledon expansion in response to blue light was compared for wild-type Arabidopsis thaliana (1.) Heynh. and the mutants blu3 and hy4, which show reduced inhibition of hypocotyl growth in blue light. White, blue, and red light stimulated cotyledon expansion in both intact and excised cotyledons of wild-type seedlings (ecotypes No-O, WS, Co-O, La-er). Cotyledons on intact blu3 and hy4 seedlings did not grow as well as those on the wild type in response to blue light, but pretreatment of blu3 seedlings with low fluence rates of red light increased their responsiveness to blue light. Excision of cotyledons alleviated the mutant phenotype so that both mutant and wild-type cotyledons grew equally well in blue light. The loss of the mutant cotyledon phenotype upon excision indicates that the blu3 and hy4 lesions affect cotyledon expansion indirectly via a whole-plant response to light. Furthermore, the ability of excised, mutant cotyledons to grow normally in blue light shows that this growth response to blue light is mediated by a photosystem other than the ones impaired by the blu3 and hy4 lesions.Blue light-induced photomorphogenic responses in angiosperms include phototropism, inhibition of hypocotyl and epicotyl elongation, hook unfolding, and cotyledon and leaf expansion. The latter is a complicated process that can be driven through several photoreceptors acting at different sites. For example, in Phaseolus vulgaris (L.), full expansion of the primary leaves can be driven by both blue and red light acting photosynthetically and photomorphogenically (Van Volkenburgh and Cleland, 1990; Van Volkenburgh et al., 1990; Blum et al., 1992). Perception of the red light signal is at two sites, the hook and the primary leaves (De Greef et al., 1978).Cellular pathways mediating responses to specific stimuli can be elucidated by analyzing mutants deficient in these responses. Severa1 Arabidopsis thaliana mutants, blul, blu2, blu3 (Liscum and Hangarter, 1991), and hy4 (Koomneef et al., 1980), have been identified by a screen for altered hypocotyl growth inhibition. These mutants do not show wildtype inhibition of hypocotyl growth in response to blue light, but they display normal response to white and far-red light in the case of the blu mutants (Liscum and Hangarter, 1991) and to far-red light in hy4 (Koomneef et al., 1980). Addi- tionally blul, 61242, and blu3 do not exhibit full cotyledon expansion in blue light when compared to white light (Liscum and Hangarter, 1991). Of the mutants found by Liscum and Hangarter, blu3 showed the greatest difference between white-and blue-light-induced cotyledon expansion.To understand the cellular mechanism of blue-light-driven cell growth in cotyledons, their growth response in the mutants blu3 and hy4 has been investigated. Realizing that several sites of perception may be involved (Black and Shuttleworth, 1974; De Greef et al., 1978; Oelze-Karow and Mohr, 1988) and that excision sometimes changes responses (Cosgrove, 1981), we also examined whether the mutant phenotype woul...

Light-stimulated root elongation in Arabidopsis thaliana

“…For stimulation of cotyledon expansion by red light, both phytochrome A and B are required while, under far-red light, only phytochrome A is responsible (Reed et al, 1994). The cotyledon expansion elicited by phytochrome B is cell autonomous , while that induced by blue light is only observed in intact plants (Blum et al, 1994). We observed that cotyledon expansion induced by continuous white light was not blocked by DCMU (data not shown).…”

Cellular and Subcellular Localization of Phototropin 1

“…In addition to its role in activating stomatal opening and chloroplast movement, blue light activates at least two other responses in leaves: leaf expansion Blum et al, 1994;Van Volkenburgh, 1999) and, in certain species, solar tracking (Voerkel, 1934;Yin, 1938). Therefore, it was of interest to determine the distribution of phot1 in leaves.…”

Blue light-induced kinetics of H + and Ca 2+ fluxes in etiolated wild-type and phototropin-mutant Arabidopsis seedlings

“…Next are bending responses, operating within the time scale of hours. The slowest are reactions of cotyledon expansion, which take days (2). However, it is quite possible that all these reactions may be induced simultaneously (moreover, they could be linked), and the observed difference originates merely from different rates and delays in metabolic or long-distance-transport processes that mediate BL responses at the whole-plant level.…”

“…Among numerous physiological processes controlled by BL are phototropism (bending toward or away from the light source), cotyledon expansion, and inhibition of hypocotyl elongation (1)(2)(3). These reactions are preceded or accompanied by significant changes in electrochemical properties of cells and tissues, including changes in membrane potential and ion transport across membranes (4)(5)(6).…”