Gibberellins Repress Photomorphogenesis in Darkness

Abstract

Plants undergo two different developmental programs depending on whether they are growing in darkness (skotomorphogenesis) or in the presence of light (photomorphogenesis). It has been proposed that the latter is the default pathway followed by many plants after germination and before the seedling emerges from soil. The transition between the two pathways is tightly regulated. The conserved COP1-based complex is central in the light-dependent repression of photomorphogenesis in darkness. Besides this control, hormones such as brassinosteroids (BRs), cytokinins, auxins, or ethylene also have been shown to regulate, to different extents, this developmental switch. In the present work, we show that the hormone gibberellin (GA) widely participates in this regulation. Studies from Arabidopsis show that both chemical and genetic reductions of endogenous GA levels partially derepress photomorphogenesis in darkness. This is based both on morphological phenotypes, such as hypocotyl elongation and hook and cotyledon opening, and on molecular phenotypes, such as misregulation of the light-controlled genes CAB2 and RbcS. Genetic studies indicate that the GA signaling elements GAI and RGA participate in these responses. Our results also suggest that GA regulation of this response partially depends on BRs. This regulation seems to be conserved across species because lowering endogenous GA levels in pea (Pisum sativum) induces full de-etiolation in darkness, which is not reverted by BR application. Our results, therefore, attribute an important role for GAs in the establishment of etiolated growth and in repression of photomorphogenesis.One of the most dramatic changes in plant growth and development occurs during the transition from life in the dark just after germination, to life in a light environment when the seedling emerges from soil. Development in darkness is referred to as skotomorphogenesis, whereas development in the light is referred to as photomorphogenesis. Skotomorphogenesis is characterized by an etiolated appearance of seedlings with a fast-growing hypocotyl or epicotyl, presence of an apical hook, and small and closed cotyledons or primary leaves. Moreover, these seedlings present etioplasts instead of chloroplasts, and the expression of genes that are normally lightregulated is repressed or kept at low, basal levels. When light triggers the photomorphogenic development, growth of hypocotyl or epicotyl is slowed down, cotyledons or primary leaves open and expand, etioplasts develop into chloroplasts, and the expression of light-controlled genes is up-regulated (Neff et al., 2000).After germination, it is extremely important for plants to be able to maintain the skotomorphogenic development before reaching the light to preserve and protect both the shoot apical meristem and cotyledons or primary leaves. In many plants, photomorphogenesis is the default developmental pathway after germination (Wei et al., 1994). This is based on the fact that many lower plants lack an etiolated growth phase, and the new program...

Diurnal Regulation of the Brassinosteroid-Biosynthetic CPD Gene in Arabidopsis

“…For instance, light was shown to promote the germination of Arabidopsis and lettuce seeds by increasing their GA content via the induction GA 3b-hydroxylases (Toyomasu et al, 1998;Yamaguchi et al, 1998). Alabadí et al (2004) reported that both mutational and chemical disruption of GA synthesis can interfere with normal skotomorphogenesis and cause aberrant expression of light-regulated nuclear genes. Furthermore, ethylene and GA were found to act coordinately in the phytochrome-mediated shade avoidance response, but their effect could be abolished by paclobutrazol, an inhibitor of GA synthesis (Pierik et al, 2004).…”

A molecular framework for light and gibberellin control of cell elongation

“…Gibberellins (GAs) exert an opposite effect to light on photomorphogenesis 2 . GAs promote etiolated growth, whereas GA-deficiency induces a partially de-etiolated phenotype in the dark, which is reverted by a lack of DELLA function 2,9 . DELLAs function as key repressors of GA-responsive growth, by inhibiting GA-regulated gene expression 5 .…”

“…Cell elongation during seedling development is antagonistically regulated by light and gibberellins (GAs) 1,2 . Light induces photomorphogenesis, leading to inhibition of hypocotyl growth, whereas GAs promote etiolated growth, characterized by increased hypocotyl elongation.…”

DELLAs Contribute to Plant Photomorphogenesis

“…Reed et al, 1996;Ait-ali et al, 1999;Alabadí et al, 2004), none have systematically investigated the role of DELLAs in light-mediated growth regulation. Recent advances have led to the development of Arabidopsis mutant lines that are multiply DELLA deficient and to the use of these lines in experiments that reveal the role of DELLAs in many aspects of plant growth and development, including those that are influenced by environmental conditions (Lee et al, 2002;Cheng et al, 2004;Yu et al, 2004;Cao et al, 2005;Achard et al, 2006).…”