Regulation of HEMA1 expression by phytochrome and a plastid signal during de‐etiolation in Arabidopsis thaliana

Abstract

SummaryThe synthesis of 5-aminolevulinic acid (ALA) is the rate-limiting step for the formation of all plant tetrapyrroles, including chlorophyll and heme, and regulation of ALA synthesis is therefore critical to plant development. Glutamyl-tRNA reductase (GluTR) is the ®rst committed enzyme of this pathway and is encoded by a small family of nuclear HEMA genes. Here, we have used transgenic Arabidopsis (Arabidopsis thaliana L. Col) lines expressing chimeric HEMA1 promoter:gusA fusion genes, combined with RNA gel blot analyses, to characterise the light-mediated regulation of the Arabidopsis HEMA1 gene during de-etiolation. HEMA1 was expressed strongly, but not exclusively, in photosynthetic tissues and was shown to be light regulated at the transcriptional level by the phytochrome family of photoreceptors acting in both the far-red high irradiance and low¯uence response modes. The HEMA2 gene, which is expressed only in roots of seedlings, was not light regulated. Analysis of truncated HEMA1 promoter constructs demonstrated that a ±199/+252 promoter fragment was suf®cient to confer full light-responsiveness to gusA expression. This fragment contained GT-1/I-box and CCA-1 binding sites that are implicated as the light-responsive cis elements. Both the full-length and truncated HEMA1 promoters required the presence of intact chloroplasts for full expression, consistent with previous indications that light and plastid factor signals converge to co-ordinately regulate expression of photosynthesis-related nuclear genes. These results provide the most comprehensive analysis to date of the light-regulation of a tetrapyrrole biosynthetic gene and support a direct link between regulation of HEMA1 transcription and chlorophyll accumulation during seedling de-etiolation.

Loss of Nuclear Gene Expression during the Phytochrome A-Mediated Far-Red Block of Greening Response

“…Such a signal has been previously identified through the action of photobleaching herbicides (Taylor, 1989;Susek and Chory, 1992) and is required for the expression of numerous photosynthetically related nuclear genes (Kusnetsov et al, 1996), including Lhcb in Arabidopsis (Susek et al, 1993). This signal is also closely involved with phytochrome signaling pathways (Ló pez-Juez et al, 1998) and can be thought of as gating phytochrome responses (McCormac et al, 2001) in a similar way to that proposed for circadian control of light-induced gene expression (Millar and Kay, 1996).…”

“…This gene, which encodes glutamyl-tRNA reductase, the first committed enzyme of tetrapyrrole synthesis, is strongly expressed in photosynthetic tissues and is regulated by light, including an FR-HIR, and a plastid signal (Ilag et al, 1994;Kumar et al, 1999;McCormac et al, 2001). However, the lightregulated expression pattern of HEMA1 is not identical to other light-regulated genes, as HEMA1 does not exhibit a very low-fluence response (McCormac et al, 2001) and utilizes light signaling pathways that partly diverge from those for light regulation of Lhcb (A.C. McCormac and M.J. Terry, unpublished data).…”

GLK Transcription Factors Coordinate Expression of the Photosynthetic Apparatus in Arabidopsis

“…All RNA was isolated by guanidinium thiocyanate-phenol-chloroform extraction (Chomczynski and Sacchi, 2006), except for the data shown in Figure 9, for which a method more suitable for young Arabidopsis seedlings was employed (McCormac et al, 2001). For microarray analysis, RNA was further purified with an RNeasy plant RNA extraction kit (Qiagen).…”

The Photomorphogenic Signal: An Essential Component of Photoautotrophic Life

“…Of this family, the expression of HEMA1 correlates with the expression of Lhcb1 genes, which encode light-harvesting proteins of the photosystem II; in some way the expression of HEMA1 reflects the demand of chlorophyll synthesis. On the other hand, HEMA2 is not light regulated (Matsumoto et al, 2004;McCormac et al, 2001;McCormac & Terry, 2002a;McCormac & Terry, 2002b). Glutamyl-tRNA reductase activity is regulated by negative feedback loops; the accumulation of Haeme, Mg-Protoporphyrin IX or Divinyl protochlorofilide a antagonise Glutamyl-tRNA reductase activity (Srivastava et al, 2005).…”

“…Glutamyl-tRNA reductase activity is regulated by negative feedback loops; the accumulation of Haeme, Mg-Protoporphyrin IX or Divinyl protochlorofilide a antagonise Glutamyl-tRNA reductase activity (Srivastava et al, 2005). At the transcriptional level, HEMA1 expression is induced by red and far-red light, implicating at least phyA and phyB, and blue light perceived by cry1 (McCormac et al, 2001;McCormac & Terry, 2002a). pif1 and pif3 mutants contain higher levels of HEMA1 mRNA, higher levels of protochlorophyllide and partially developed chloroplasts in the dark, a phenotype observed in cop mutants.…”