Arabidopsis BBX14 is involved in high light acclimation and seedling development.

The development of photosynthetically competent seedlings requires both light and retrograde biogenic signaling pathways. The transcription factor GLK1 functions at the interface between these pathways and receives input from the biogenic signal integrator GUN1. BBX14 was previously identified, together with GLK1, in a core module that mediates the response to high light (HL) levels and biogenic signals, which was studied by using inhibitors of chloroplast development. Our chromatin immunoprecipitation-Seq experiments revealed that BBX14 is a direct target of GLK1, and RNA-Seq analysis suggests that BBX14 may function as a regulator of the circadian clock. In addition, BBX14 plays a role in chlorophyll biosynthesis during early onset of light. Knockout of BBX14 results in a long hypocotyl phenotype dependent on a retrograde signal. Furthermore, the expression of BBX14 and BBX15 during biogenic signaling requires GUN1. Investigation of the role of BBX14 and BBX15 in GUN-type biogenic (gun) signaling showed that the overexpression of BBX14 or BBX15 caused de-repression of CA1 mRNA levels, when seedlings were grown on norflurazon. Notably, transcripts of the LHCB1.2 marker are not de-repressed. Furthermore, BBX14 is required to acclimate plants to HL stress. We propose that BBX14 is an integrator of biogenic signals and that BBX14 is a nuclear target of retrograde signals downstream of the GUN1/GLK1 module. However, we do not classify BBX14 or BBX15 overexpressors as gun mutants based on a critical evaluation of our results and those reported in the literature. Finally, we discuss a classification system necessary for the declaration of new gun mutants.

Natural genetic variation in GLK1-mediated photosynthetic acclimation in response to light.

BACKGROUND: GOLDEN-like (GLK) transcription factors are central regulators of chloroplast biogenesis in Arabidopsis and other species. Findings from Arabidopsis show that these factors also contribute to photosynthetic acclimation, e.g. to variation in light intensity, and are controlled by retrograde signals emanating from the chloroplast. However, the natural variation of GLK1-centered gene-regulatory networks in Arabidopsis is largely unexplored. RESULTS: By evaluating the activities of GLK1 target genes and GLK1 itself in vegetative leaves of natural Arabidopsis accessions grown under standard conditions, we uncovered variation in the activity of GLK1 centered regulatory networks. This is linked with the ecogeographic origin of the accessions, and can be associated with a complex genetic variation across loci acting in different functional pathways, including photosynthesis, ROS and brassinosteroid pathways. Our results identify candidate upstream regulators that contribute to a basal level of GLK1 activity in rosette leaves, which can then impact the capacity to acclimate to different environmental conditions. Indeed, accessions with higher GLK1 activity, arising from habitats with a high monthly variation in solar radiation levels, may show lower levels of photoinhibition at higher light intensities. CONCLUSIONS: Our results provide evidence for natural variation in GLK1 regulatory activities in vegetative leaves. This variation is associated with ecogeographic origin and can contribute to acclimation to high light conditions.

TCP15 interacts with GOLDEN2-LIKE 1 to control cotyledon opening in Arabidopsis.

After germination, exposure to light promotes the opening and expansion of the cotyledons and the development of the photosynthetic apparatus in a process called de-etiolation. This process is crucial for seedling establishment and photoautotrophic growth. TEOSINTE BRANCHED 1, CYCLOIDEA, and PROLIFERATING CELL FACTORS (TCP) transcription factors are important developmental regulators of plant responses to internal and external signals that are grouped into two main classes. In this study, we identified GOLDEN2-LIKE 1 (GLK1), a key transcriptional regulator of photomorphogenesis, as a protein partner of class I TCPs during light-induced cotyledon opening and expansion in Arabidopsis. The class I TCP TCP15 and GLK1 are mutually required for cotyledon opening and the induction of SAUR and EXPANSIN genes, involved in cell expansion. TCP15 also participates in the expression of photosynthesis-associated genes regulated by GLK1, like LHCB1.4 and LHCB2.2. Furthermore, GLK1 and TCP15 bind to the same promoter regions of different target genes containing either GLK or TCP binding motifs and binding of TCP15 is affected in a GLK1-deficient background, suggesting that a complex between TCP15 and GLK1 participates in the induction of these genes. We postulate that GLK1 helps to recruit TCP15 for the modulation of cell expansion genes in cotyledons and that the functional interaction between these transcription factors may serve to coordinate the expression of cell expansion genes with that of genes involved in the development of the photosynthetic apparatus.

Transcription factor GLK1 promotes anthocyanin biosynthesis via an MBW complex-dependent pathway in Arabidopsis thaliana.

Anthocyanins are important natural plant pigments and play diverse roles in plant growth and adaptation. Anthocyanins function as screens to protect photosynthetic tissues from photoinhibition. However, the regulatory mechanisms underlying the biosynthesis and spatial accumulation pattern of anthocyanins remain some unresolved issues. Here, we demonstrate that the GARP-type transcription factor GOLDEN2-LIKE 1 (GLK1) functions as a positive factor in anthocyanin accumulation. GLK1 enhances the transcriptional activation activities of MYB75, MYB90, and MYB113 via direct protein-protein interactions to increase the expression of anthocyanin-specific biosynthetic genes. Anthocyanins accumulate in an acropetal manner in Arabidopsis. We also found that the expression pattern of GLK1 overall mimicked the accumulation pattern of anthocyanin from the base of the main stem to the shoot apex. Based on these findings, we established a working model for the role of GLK1 in anthocyanin accumulation and propose that GLK1 mediates the spatial distribution pattern of anthocyanins by affecting the transcriptional activation activities of MYB75, MYB90, and MYB113.

Nitrogen-inducible GLK1 modulates phosphate starvation response via the PHR1-dependent pathway.

Phosphorus (P) is a limiting nutrient for plant growth and productivity. Thus, a deep understanding of the molecular mechanisms of plants' response to phosphate starvation is significant when breeding crops with higher phosphorus-use efficiency. Here, we found that GARP-type transcription factor GLK1 acted as a positive regulator for phosphate-starvation response (PSR) via the PHR1-dependent pathway in Arabidopsis thaliana. GLK1 increased the transcription activity of PHR1 through the direct physical interaction and regulated the multiple responses to inorganic orthophosphate (Pi) starvation. Nitrogen (N) is a key factor in the regulation of PSR. We also found that the N status controlled the function of the GLK1-PHR1 signaling module under Pi-deficient (LP) conditions by regulating the accumulation of GLK1 and PHR1. Ultimately, we showed that the presence of GLK1 effectively promoted the protein accumulation of PHR1 at low N concentrations, and this action was helpful to maintain the activation of PSR. According to these findings, we establish the working model for GLK1 in PSR and propose that GLK1 mediates the interaction between N and P by influencing the effect of N on PHR1 in Arabidopsis thaliana.

BBX16 mediates the repression of seedling photomorphogenesis downstream of the GUN1/GLK1 module during retrograde signalling.

Plastid-to-nucleus retrograde signalling (RS) initiated by dysfunctional chloroplasts impact photomorphogenic development. We have previously shown that the transcription factor GLK1 acts downstream of the RS regulator GUN1 in photodamaging conditions to regulate not only the well established expression of photosynthesis-associated nuclear genes (PhANGs) but also to regulate seedling morphogenesis. Specifically, the GUN1/GLK1 module inhibits the light-induced phytochrome-interacting factor (PIF)-repressed transcriptional network to suppress cotyledon development when chloroplast integrity is compromised, modulating the area exposed to potentially damaging high light. However, how the GUN1/GLK1 module inhibits photomorphogenesis upon chloroplast damage remained undefined. Here, we report the identification of BBX16 as a novel direct target of GLK1. BBX16 is induced and promotes photomorphogenesis in moderate light and is repressed via GUN1/GLK1 after chloroplast damage. Additionally, we showed that BBX16 represents a regulatory branching point downstream of GUN1/GLK1 in the regulation of PhANG expression and seedling development upon RS activation. The gun1 phenotype in lincomycin and the gun1-like phenotype of GLK1OX are markedly suppressed in gun1bbx16 and GLK1OXbbx16. This study identified BBX16 as the first member of the BBX family involved in RS, and defines a molecular bifurcation mechanism operated by GLK1/BBX16 to optimise seedling de-etiolation, and to ensure photoprotection in unfavourable light conditions.

Long-term abscisic acid promotes golden2-like1 degradation through constitutive photomorphogenic 1 in a light intensity-dependent manner to suppress chloroplast development.

Abiotic stress, a serious threat to plants, occurs for extended periods in nature. Abscisic acid (ABA) plays a critical role in abiotic stress responses in plants. Therefore, stress responses mediated by ABA have been studied extensively, especially in short-term responses. However, long-term stress responses mediated by ABA remain largely unknown. To elucidate the mechanism by which plants respond to prolonged abiotic stress, we used long-term ABA treatment that activates the signalling against abiotic stress such as dehydration and investigated mechanisms underlying the responses. Long-term ABA treatment activates constitutive photomorphogenic 1 (COP1). Active COP1 mediates the ubiquitination of golden2-like1 (GLK1) for degradation, contributing to lowering expression of photosynthesis-associated genes such as glutamyl-tRNA reductase (HEMA1) and protochlorophyllide oxidoreductase A (PORA), resulting in the suppression of chloroplast development. Moreover, COP1 activation and GLK1 degradation upon long-term ABA treatment depend on light intensity. Additionally, plants with COP1 mutation or exposed to higher light intensity were more sensitive to salt stress. Collectively, our results demonstrate that long-term treatment of ABA leads to activation of COP1 in a light intensity-dependent manner for GLK1 degradation to suppress chloroplast development, which we propose to constitute a mechanism of balancing normal growth and stress responses upon the long-term abiotic stress.

Silencing of AtRAP, a target gene of a bacteria-induced small RNA, triggers antibacterial defense responses through activation of LSU2 and down-regulation of GLK1.

Plants fine-tune their sophisticated immunity systems in response to pathogen infections. We previously showed that AtlsiRNA-1, a bacteria-induced plant endogenous small interfering RNA, silences the AtRAP gene, which encodes a putative RNA binding protein. In this study, we demonstrate that AtRAP functions as a negative regulator in plant immunity by characterizing molecular and biological responses of the knockout mutant and overexpression lines of AtRAP upon bacterial infection. AtRAP is localized in chloroplasts and physically interacts with Low Sulfur Upregulated 2 (LSU2), which positively regulates plant defense. Our results suggest that AtRAP negatively regulates defense responses by suppressing LSU2 through physical interaction. We also detected downregulation of the transcription factor GOLDEN2-LIKE 1 (GLK1) in atrap-1 using microarray analysis. The glk1 glk2 double mutant showed enhanced resistance to Pseudomonas syringae pv. tomato, which is consistent with a previous study showing enhanced resistance of a glk1 glk2 double mutant to Hyaloperonospora arabidopsidis. Taken together, our data suggest that silencing of AtRAP by AtlsiRNA-1 upon bacterial infection triggers defense responses through regulation of LSU2 and GLK1.

A Specific Protective Mechanism Against Chloroplast Photo-Reactive Oxygen Species in Phosphate-Starved Rice Plants.

Phosphorus (Pi) starvation prevents a good match between light energy absorption and photosynthetic carbon metabolism, generating photo-reactive oxygen species (photo-ROS) in chloroplasts. Plants have evolved to withstand photo-oxidative stress, but the key regulatory mechanism underlying it remains unclear. In rice (Oryza sativa), DEEP GREEN PANICLE1 (DGP1) is robustly up-regulated in response to Pi deficiency. DGP1 decreases the DNA-binding capacities of the transcriptional activators GLK1/2 on the photosynthetic genes involved in chlorophyll biosynthesis, light harvesting, and electron transport. This Pi-starvation-induced mechanism dampens both electron transport rates through photosystem I and II (ETRI and ETRII) and thus mitigates the electron-excessive stress in mesophyll cells. Meanwhile, DGP1 hijacks glycolytic enzymes GAPC1/2/3, redirecting glucose metabolism toward the pentose phosphate pathway with superfluous NADPH production. Phenotypically, light irradiation induces O2 - production in Pi-starved WT leaves but is observably accelerated in dgp1 mutant and impaired in GAPCsRNAi and glk1glk2 lines. Interestingly, overexpressed DGP1 in rice caused hyposensitivity to ROS-inducers (catechin and methyl viologen), but the dgp1 mutant shows a similar inhibitory phenotype with the WT seedlings. Overall, the DGP1 gene serves as a specific antagonizer against photo-ROS in Pi-starved rice plants, which coordinates light-absorbing and anti-oxidative systems by orchestrating transcriptional and metabolic regulations, respectively.