DELLAs contribute to plant photomorphogenesis

Abstract

Plant morphogenesis is profoundly influenced by light (a phenomenon known as photomorphogenesis). For example, light inhibits seedling hypocotyl growth via activation of phytochromes and additional photoreceptors. Subsequently, information is transmitted through photoreceptor-linked signal transduction pathways and used (via previously unknown mechanisms) to control hypocotyl growth. Here we show that light inhibition of Arabidopsis (Arabidopsis thaliana) hypocotyl growth is in part dependent on the DELLAs (a family of nuclear growth-restraining proteins that mediate the effect of the phytohormone gibberellin [GA] on growth). We show that light inhibition of growth is reduced in DELLA-deficient mutant hypocotyls. We also show that light activation of phytochromes promotes the accumulation of DELLAs. A green fluorescent protein (GFP)-tagged DELLA (GFP-RGA) accumulates in elongating cells of light-grown, but not dark-grown, transgenic wild-type hypocotyls. Furthermore, transfer of seedlings from light to dark (or vice versa) results in rapid changes in hypocotyl GFP-RGA accumulation, changes that are paralleled by rapid alterations in the abundance in hypocotyls of transcripts encoding enzymes of GA metabolism. These observations suggest that light-dependent changes in hypocotyl GFP-RGA accumulation are a consequence of light-dependent changes in bioactive GA level. Finally, we show that GFP accumulation and quantitative modulation of hypocotyl growth is proportionate with light energy dose (the product of exposure duration and fluence rate). Hence, DELLAs inhibit hypocotyl growth during the light phase of the day-night cycle via a mechanism that is quantitatively responsive to natural light variability. We conclude that DELLAs are a major component of the adaptively significant mechanism via which light regulates plant growth during photomorphogenesis.

Figure 1.

Light inhibits hypocotyl growth via a DELLA-dependent mechanism. Comparison of hypocotyl lengths of 7-d-old wild-type, ga1-3, quadruple-DELLA mutant, and ga1-3 quadruple-DELLA mutant seedlings grown in white light (white bar) or in the dark (black bar). Results are presented as means; error bars represent se.

Figure 2.

DELLA-dependent light-mediated inhibition of hypocotyl growth is affected by photoperiod duration. Comparison of hypocotyl length ratios: wild type/quadruple-DELLA mutant (black bar) versus ga1-3/wild type (white bar) grown in different photoperiods as indicated. Results are presented as the ratio between the means of hypocotyl length; error bars represent se.

Figure 3.

Light promotes accumulation of GFP-RGA in seedling hypocotyls. A, GFP fluorescence in nuclei of elongating cells of hypocotyls of 7-d-old pRGA:GFP-RGA or ga1-3 pRGA:GFP-RGA seedlings grown in white light or in the dark and then transferred to dark or light, respectively; times as indicated. Arrows indicate nuclear GFP-RGA fluorescence. B, Immunodetection of GFP-RGA in hypocotyls of pRGA:GFP-RGA seedlings grown in the dark for 5 d and then transferred to white light for 8 or 16 h. β-Tubulin serves as a sample-loading control.

Figure 4.

Light regulates the transcript level of GA metabolism genes. Levels of GA metabolism GA5 (GA20ox1), GA4 (GA3ox1), GA2ox1, GA2ox2, and GA2ox3, and GA-signaling RGA, SLY1, SLY2, AtGID1a, AtGID1b gene transcripts (determined by reverse transcription-PCR) in 7-d-old seedling hypocotyls grown in white light (L) or in the dark (D), then transferred to dark or light, respectively, for the time as indicated (1–24 h). ELF4a transcripts provide loading control.

Figure 5.

Involvement of DELLAs in PHYA-dependent FR-mediated inhibition of hypocotyl growth. A, DELLAs contribute to the activated PHYA-mediated inhibition of hypocotyl growth. Mean (±se) hypocotyl length of 7-d-old wild type; gai; quadruple-DELLA mutant; phyA-1 and phyA-1 gai seedlings grown in FR light. B, Immunodetection of GFP-RGA in 7-d-old pRGA:GFP-RGA and phyA-1 pRGA:GFP-RGA seedlings grown in continuous FR light. β-Tubulin (β-TUB) serves as a sample-loading control.

Figure 6.

DELLAs contribute to the activated PHYB-mediated inhibition of hypocotyl growth. A, Mean (±se) hypocotyl length of 7-d-old wild type, quadruple-DELLA, ga1-3, and ga1-3 quadruple-DELLA seedlings grown in R light. B, Immunodetection of GFP-RGA in hypocotyls of 4-d-old pRGA:GFP-RGA and phyB-1 pRGA:GFP-RGA seedlings grown in the dark and then transferred to white light for 1 or 2 h as indicated. β-Tubulin (β-TUB) serves as a sample-loading control.

Figure 7.

DELLA inhibition of hypocotyl growth is proportionate to light dose. A, Comparison of wild type/quadruple-DELLA mutant hypocotyl length ratio for 7-d-old seedlings grown in dark (0 μmol s⁻¹ m⁻²) or in the presence of continuous white light at fluence rates of 6, 20, or 75 μmol s⁻¹ m⁻². B, Comparison of wild type/quadruple-DELLA mutant hypocotyl length ratio for 7-d-old seedlings grown in photoperiod (light duration per 24-h cycle)/fluence rate combinations as indicated. C, Immunodetection of GFP-RGA in hypocotyls of 5-d-old pRGA:GFP-RGA seedlings grown in photoperiod (light duration per 24-h cycle)/fluence rate combinations as indicated. β-Tubulin (β-TUB) serves as a sample-loading control.