Transposase-Derived Proteins FHY3/FAR1 Interact with PHYTOCHROME-INTERACTING FACTOR1 to Regulate Chlorophyll Biosynthesis by Modulating HEMB1 during Deetiolation in Arabidopsis

Abstract

Successful chlorophyll biosynthesis during initial light exposure is critical for plant survival and growth, as excess accumulation of chlorophyll precursors in darkness can cause photooxidative damage to cells. Therefore, efficient mechanisms have evolved to precisely regulate chlorophyll biosynthesis in plants. Here, we identify FAR-RED ELONGATED HYPOCOTYL3 (FHY3) and FAR-RED IMPAIRED RESPONSE1 (FAR1), two transposase-derived transcription factors, as positive regulators of chlorophyll biosynthesis in Arabidopsis thaliana. We show that null mutations in FHY3 and FAR1 cause reduced protochlorophyllide (a precursor of chlorophyll) levels in darkness and less photobleaching in the light. We find that FHY3 directly binds to the promoter and activates expression of HEMB1, which encodes 5-aminolevulinic acid dehydratase in the chlorophyll biosynthetic pathway. We reveal that PHYTOCHROME-INTERACTING FACTOR1 physically interacts with the DNA binding domain of FHY3, thereby partly repressing FHY3/FAR1-activated HEMB1 expression. Strikingly, FHY3 expression is upregulated by white light. In addition, our genetic data indicate that overexpression, severe reduction, or lack of HEMB1 impairs plant growth and development. Together, our findings reveal a crucial role of FHY3/FAR1 in regulating chlorophyll biosynthesis, thus uncovering a new layer of regulation by which light promotes plant dark-light transition in early seedling development.

Antagonistic Basic Helix-Loop-Helix/bZIP Transcription Factors Form Transcriptional Modules That Integrate Light and Reactive Oxygen Species Signaling inArabidopsis

“…We show that the first 5 min of exposure to relatively high levels of light (60 µmol m 22 s 21 ) are critical for the survival of etiolated seedlings and that PIF1, PIF3, and possibly also PIF5 are required for survival during the onset of illumination (Figure 1). In addition, the survival rate of plants exposed to light correlates inversely with the levels of Pchlide, in agreement with our previous report (Tang et al, 2012). Pchlide is a potent photosensitizer in the chloroplast that leads to photooxidative damage in plants (op den Camp et al, 2003).…”

“…Other transcription repressors or activators might also be involved in this process by interacting with PIFs and HY5 proteins. We recently found that FAR-RED ELONGATED HYPOCOTYL3 (FHY3) and FAR-RED-IMPAIRED RESPONSE1 (FAR1) participate in regulating Pchlide synthesis and seedling greening by interacting and cooperating with PIF1 (Tang et al, 2012). HY5 feedback regulates phyA signaling homeostasis by interacting with FHY3 and FAR1 (Li et al, 2010).…”

“…To examine whether these PIF proteins have redundant functions in regulating seedling greening, we examined the phenotypes of the pif1 pif3 double and pifq quadruple mutant (loss of PIF1, 3, 4, and 5) (Leivar et al, 2008). The levels of Pchlide of dark-grown seedlings were determined by monitoring the fluorescence emission of the samples with a fluorescence spectrophotometer (Tang et al, 2012). We found that pifq possessed the highest levels of Pchlide and that pif1 pif3 also accumulated more Pchlide than either of the single mutant parents and the Columbia (Col) wild type (see Supplemental Figure 1A online).…”

“…We then examined the cellular ROS levels of pif1, pif3, pifq, and the wild type by detecting the fluorescence of 2',7'-dichlorodihydrofluorescein diacetate (H 2 DCFDA), a ROS-sensitive dye, in the cotyledons (Tang et al, 2012). When 4-d-old dark-grown seedlings were illuminated with light for 24 h, H 2 DCFDA fluorescence ( Figure 1B, shown in green) was strong in pif1 and further increased in pif3 and pifq compared with the wild type, whereas chlorophyll autofluorescence was only observed in the wild-type cotyledons (Zhong et al, 2009;Figure 1B).…”

“…ROS fluorescence determination was performed as previously described (Tang et al, 2012). Imaging of singlet oxygen production was performed as described (Flors et al, 2006).…”