Date Published: July 6, 2017
Publisher: Public Library of Science
Author(s): Yonghong Zhang, Chen Li, Jingxuan Zhang, Jiajing Wang, Jingwei Yang, Yanxia Lv, Nian Yang, Jianping Liu, Xuanbin Wang, Gergo Palfalvi, Guodong Wang, Lanlan Zheng, Keqiang Wu.
ELONGATED HYPOCOTYL 5 (HY5), a member of the bZIP gene family, is a positive regulator of the light signaling pathway in Arabidopsis thaliana. Whereas the hy5 mutant exhibits an elongated hypocotyl when grown in the light, the hy5 homolog (hyh) mutant does not. Although the functions of HY5 and HYH in light-mediated seedling development have been revealed, the tissue-specific expression patterns of HY5 and HYH and their interconnected regulation are largely unknown. Here, we report that HY5 regulates HYH expression in roots and contributes to root growth under different light conditions. We generated HY5 and HYH transcriptional and translational fusion reporter lines to investigate their expression patterns. HY5 was constitutively expressed in all root tissues, while HYH was predominantly expressed in root xylem cells. Root growth after a dark-to-light transition was perturbed in the hy5 and hy5hyh mutant lines, but not in the hyh mutant line, indicating that HY5 plays a major role in light-regulated root growth. Light-induced HY5/HYH expression occurred autonomously in roots. HYH expression in roots was decreased in the hy5 mutant, suggesting that HY5 regulates HYH expression. Collectively, these results indicate that an organ-specific HY5-mediated pathway controls root photomorphogenic development independently of light signaling in the shoot.
Light is essential for plant growth and development, providing energy for photosynthesis and regulating seedling photomorphogenesis, seed germination, shade avoidance, and photoperiod responses [1,2]. Among these developmental processes, photomorphogenesis is one of the most extensively studied . Molecular genetic studies have revealed various regulators downstream of photoreceptors [4,5], including the RING E3 ubiquitin ligase CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), a major integrator of light responses . COP1 ubiquitinates two downstream basic domain/leucine zipper (bZIP) transcription factors, ELONGATED HYPOCOTYL 5 (HY5) and HY5 HOMOLOG (HYH), and mediates their degradation [7–9]. HY5 was the first transcription factor shown to be involved in promoting photomorphogenesis, and has been extensively studied [10–12]. Although mutations in HY5 result in elongated hypocotyls even under constant light, the hyh mutant exhibits no obvious phenotype [7,13,14]. Both of these genes are required for UV RESISTANCE LOCUS8 (UVR8)-regulated UV-B signaling . PROLIFERATING CELL FACTOR 2 (TCP2) was reported to transcriptionally activate HY5 and HYH expression via the cryptochrome 1 (CRY1)-mediated photosensory signaling pathway .
The roles of HY5 in plant photomorphogenesis have been extensively studied, with a major focus on the shoot system [10,14,37]. Furthermore, large-scale omics data support the notion that HY5 is a key signal integration point during dark-to-light transitions [10,36,38]. It was recently demonstrated that phyB-activated HY5 mediates cellular responses to light in the root, and is important for primary root growth and root gravitropism [26,39]. HY5 has been reported to be a shoot-to-root mobile signal that mediates the light-responsive coupling of shoot growth and carbon assimilation with root growth and nitrogen uptake in Arabidopsis . These findings suggest that HY5 expression in the root affects both its growth and the expression of other genes in this tissue through two distinct pathways [26,27]. Nevertheless, our knowledge of the molecular mechanisms underlying root photomorphogenesis is fragmented. The accelerated root growth of hy5 was reported to be associated with auxin signaling ; however, although HY5 is known to integrate the light and hormone signaling pathways , the details of this crosstalk are unclear.