Opposite Root Growth Phenotypes of hy5 versus hy5 hyh Mutants Correlate with Increased Constitutive Auxin Signaling

Abstract

The Arabidopsis transcription factor HY5 controls light-induced gene expression downstream of photoreceptors and plays an important role in the switch of seedling shoots from dark-adapted to light-adapted development. In addition, HY5 has been implicated in plant hormone signaling, accounting for the accelerated root system growth phenotype of hy5 mutants. Mutants in the close HY5 homolog HYH resemble wild-type, despite the largely similar expression patterns and levels of HY5 and HYH, and the functional equivalence of the respective proteins. Moreover, the relative contribution of HYH to the overall activity of the gene pair is increased by an alternative HYH transcript, which encodes a stabilized protein. Consistent with the enhanced root system growth observed in hy5 loss-of-function mutants, constitutively overexpressed alternative HYH inhibits root system growth. Paradoxically, however, in double mutants carrying hy5 and hyh null alleles, the hy5 root growth phenotype is suppressed rather than enhanced. Even more surprisingly, compared to wild-type, root system growth is diminished in hy5 hyh double mutants. In addition, the double mutants display novel shoot phenotypes that are absent from either single mutant. These include cotyledon fusions and defective vasculature, which are typical for mutants in genes involved in the transcriptional response to the plant hormone auxin. Indeed, many auxin-responsive and auxin signaling genes are misexpressed in hy5 mutants, and at a higher number and magnitude in hy5 hyh mutants. Therefore, auxin-induced transcription is constitutively activated at different levels in the two mutant backgrounds. Our data support the hypothesis that the opposite root system phenotypes of hy5 single and hy5 hyh double mutants represent the morphological response to a quantitative gradient in the same molecular process, that is gradually increased constitutive auxin signaling. The data also suggest that HY5 and HYH are important negative regulators of auxin signaling amplitude in embryogenesis and seedling development.

COP1 mediates the coordination of root and shoot growth by light through modulation of PIN1- and PIN2-dependent auxin transport in Arabidopsis

“…However, although HY5 binding sites were found in the PIN1 locus , genome-wide expression analyses did not detect significant changes in PIN1 expression in the hy5 mutant (Cluis et al, 2004;Zhang et al, 2011). This could still be explained by a functional redundancy between HY5 and HYH in the regulation of PIN1 (Sibout et al, 2006) but further investigations will be needed to clarify this.…”

“…How this is achieved is still unknown but an attractive possibility is that one of the targets of COP1-mediated degradation might directly activate PIN1 transcription. The bZIP transcription factor LONG HYPOCOTYL 5 (HY5), which is the bestcharacterized target of COP1-mediated proteolysis (Osterlund et al, 2000), would be a good candidate as several functional links between HY5 and auxin have been established (Cluis et al, 2004;Sibout et al, 2006). However, although HY5 binding sites were found in the PIN1 locus , genome-wide expression analyses did not detect significant changes in PIN1 expression in the hy5 mutant (Cluis et al, 2004;Zhang et al, 2011).…”

The Differential Transcription Network between Embryo and Endosperm in the Early Developing Maize Seed

“…For example, alternative processing of the rice WAXY gene contributes to glutinous rice (Isshiki et al, 1998), and that of the disease resistance gene RPS4 is dynamically regulated during the resistance response (Zhang and Gassmann, 2007); FCA, which undergoes both polyadenylation and AS, is involved in the regulation of flowering time (Macknight et al, 2002); SR45.1 and SR45.2, the two alternatively spliced isoforms of SR45, play a major role in flower petal development and root growth, respectively (Zhang and Mount, 2009); alternative HYH transcript contributes to the increased activity of the HYH-Hy5 gene pair (Sibout et al, 2006); and a C2H2-domain protein with alternatively spliced transcripts is essential for endosperm development in Arabidopsis (Lu et al, 2012). Additionally, AS has also been functionally implicated in nutrient metabolism.…”

DNA-Binding Study Identifies C-Box and Hybrid C/G-Box or C/A-Box Motifs as High-Affinity Binding Sites for STF1 and LONG HYPOCOTYL5 Proteins

“…One of the best characterized bZIP factors thought to play a role in photomorphogenic seedling development and hormone signaling in Arabidopsis (Arabidopsis thaliana) is LONG HYPOCOTYL5 (HY5) (Oyama et al, 1997;Ang et al, 1998;Holm et al, 2002;Cluis et al, 2004). The function of HY5 in photomorphogenesis is well illustrated in hy5 mutant seedlings, which have defects in light inhibition of hypocotyl elongation, in light-induced chlorophyll, and in anthocyanin accumulation (Oyama et al, 1997;Sibout et al, 2006;Shin et al, 2007). The model for light-signaling pathways in photomorphogenic development includes the photoreceptor phytochromes, the ubiquitin ligase CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1), and the positive signaling component HY5 (Deng et al, 1992;Ang and Deng, 1994;Holm et al, 2002).…”

“…The hy5 mutation affects several aspects of root morphogenesis, resulting in an elevated number of lateral roots, less responsiveness to gravitropic stimulus and touching, and longer root hairs in hy5 seedlings than in wild type. The hy5 mutant traits are partly the result of an altered balance in the signaling of auxin (Sibout et al, 2006). Microarray analyses have shown that many auxinresponsive and auxin-signaling genes are misexpressed in hy5 mutants, an indication that the genes encoding auxin-signaling components are one group of the HY5 downstream genes (Cluis et al, 2004;Sibout et al, 2006).…”