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.

Figure 1.

Effect of paclobutrazol (PAC) on photomorphogenesis of dark-grown Arabidopsis wild-type (WT) seedlings. A, Phenotypes of representative 4- and 6-d-old dark-grown seedlings for each treatment (see text). B, Hook angle of 4-d-old seedlings grown in darkness. Measurements are average of 10 to 15 seedlings ± se per treatment. C, Cotyledon angle of 6-d-old seedlings grown in darkness. Measurements are average of 10 to 15 seedlings ± se per treatment. D, Northern-blot analysis of CAB2 and RbcS gene expression in Arabidopsis WT seedlings grown in darkness or in continuous white light for 10 d. Four micrograms of total RNA from light-grown seedlings, and 8 μg of total RNA from dark-grown seedlings was loaded per lane.

Figure 2.

Effect of Arabidopsis GA biosynthesis and signaling mutants on photomorphogenesis in darkness. A, Phenotypes of representative 5- and 8-d-old dark-grown Arabidopsis GA biosynthesis and signaling mutants. B, Northern-blot analysis of CAB2 and RbcS gene expression in Arabidopsis GA biosynthesis and signaling mutant seedlings grown in darkness or in continuous white light for 10 d. Four micrograms of total RNA from light-grown seedlings and 7 μg of total RNA from dark-grown seedlings were loaded per lane.

Figure 3.

Interactions between GAs and BRs in the control of Arabidopsis photomorphogenesis in darkness. A, EBR treatment rescues the CAB2 and RbcS WT expression level in 10-d-old WT Arabidopsis seedlings treated with PAC. CAB2 and RcbS mRNA levels were analyzed by northern blot. Four micrograms of total RNA was loaded per lane. B, GA₃ does not rescue the molecular phenotype of CAB2 and RbcS in det2 mutants. WT and det2-1 seedlings were grown for 10 d in darkness. CAB2 and RcbS mRNA levels were analyzed by northern blot. Five micrograms of total RNA was loaded per lane.

Figure 4.

Effects of GA deficiency on photomorphogenesis of pea plants in darkness. A and B, Effect of PAC on photomorphogenesis of pea seedlings in the dark. Seeds of WT cv Alaska were watered with 10 μm PAC and cultured at 22°C in the dark for 6 (A) or 24 (B) d after seeding. GA₃ (1 μg) was applied to the seeds before planting, and to the seedlings after germination. C and D, Photomorphogenesis of na seedlings cultured in the dark for 6 (C) or 24 (D) d after seeding and effect of GA₃ (GA) and brassinolide (BL) application. E, GA₃ dose response on the reversal of photomorphogenesis of na pea seedlings cultured for 15 d in the dark. A single GA₃ dose was applied to dry seeds before germination. Numbers mean micrograms of GA₃ applied.

Figure 5.

Effect of PAC and na mutation on RbcS transcript levels in pea. Twenty micrograms of total RNA was loaded per lane. s, Stem; a, apex.