GOLDEN 2-LIKE transcription factors for chloroplast development affect ozone tolerance through the regulation of stomatal movement

Identification and transcriptome analysis of a photosynthesis deficient mutant of Populus davidiana Dode

Photosynthesis is the main source of energy for plants to sustain growth and development. Abnormalities in photosynthesis may cause defects in plant development. The elaborate regulatory mechanism underlying photosynthesis remains unclear. In this study, we identified a natural mutant from the Greater Khingan Mountains and named it as "1-T". This mutant had variegated leaf with irregular distribution of yellow and green. Chlorophyll contents and photosynthetic capacity of 1-T were significantly reduced compared to other poplar genotypes. Furthermore, a transcriptome analysis revealed 3269 differentially expressed genes (DEGs) in 1-T. The products of the DEGs were enriched in photosystem I and photosystem II. Three motifs were significantly enriched in the promoters of these DEGs. Yeast one-hybrid, Electrophoretic mobility shift assays and tobacco transient transformation experiments indicated that PdGLKs may bind to the three motifs. Further analysis indicated that these photosystem related genes were also significantly down-regulated in PdGLK-RNAi poplars. Therefore, we preliminarily concluded that the down-regulation of PdGLKs in 1-T may affect the expression of photosystem-related genes, resulting in abnormal photosystem development and thus affecting the growth and development. Our results provide new insights into the molecular mechanism of photosynthesis regulating plant growth.

Maize GOLDEN2-LIKE proteins enhance drought tolerance in rice by promoting stomatal closure

Drought has become one of the most severe abiotic stresses experienced in agricultural production across the world. Plants respond to water deficit via stomatal movements in the leaves, which are mainly regulated by abscisic acid (ABA). A previous study from our lab showed that constitutive expression of maize (Zea mays L.) GOLDEN2-LIKE (GLK) transcription factors in rice (Oryza sativa L.) can improve stomatal conductance and plant photosynthetic capacity under field conditions. In the present study, we uncovered a function of ZmGLK regulation of stomatal movement in rice during drought stress. We found that elevated drought tolerance in rice plants overexpressing ZmGLK1 or GOLDEN2 (ZmG2) was conferred by rapid ABA-mediated stomatal closure. Comparative analysis of RNA-sequencing (RNA-seq) data from the rice leaves and DNA affinity purification sequencing (DAP-seq) results obtained in vitro revealed that ZmGLKs played roles in regulating ABA-related and stress-responsive pathways. Four upregulated genes closely functioning in abiotic stress tolerance with strong binding peaks in the DAP-seq data were identified as putative target genes of ZmGLK1 and ZmG2 in rice. These results demonstrated that maize GLKs play an important role in regulating stomatal movements to coordinate photosynthesis and stress tolerance. This trait is a valuable target for breeding drought-tolerant crop plants without compromising photosynthetic capacity.

A genome‑wide approach to the systematic and comprehensive analysis of LIM gene family in sorghum (Sorghum bicolor L.)

The LIM domain-containing proteins are dominantly found in plants and play a significant role in various biological processes such as gene transcription as well as actin cytoskeletal organization. Nevertheless, genome-wide identification as well as functional analysis of the LIM gene family have not yet been reported in the economically important plant sorghum (Sorghum bicolor L.). Therefore, we conducted an in silico identification and characterization of LIM genes in S. bicolor genome using integrated bioinformatics approaches. Based on phylogenetic tree analysis and conserved domain, we identified five LIM genes in S. bicolor (SbLIM) genome corresponding to Arabidopsis LIM (AtLIM) genes. The conserved domain, motif as well as gene structure analyses of the SbLIM gene family showed the similarity within the SbLIM and AtLIM members. The gene ontology (GO) enrichment study revealed that the candidate LIM genes are directly involved in cytoskeletal organization and various other important biological as well as molecular pathways. Some important families of regulating transcription factors such as ERF, MYB, WRKY, NAC, bZIP, C2H2, Dof, and G2-like were detected by analyzing their interaction network with identified SbLIM genes. The cis-acting regulatory elements related to predicted SbLIM genes were identified as responsive to light, hormones, stress, and other functions. The present study will provide valuable useful information about LIM genes in sorghum which would pave the way for the future study of functional pathways of candidate SbLIM genes as well as their regulatory factors in wet-lab experiments.

Molecular Evolution and Genetic Variation of G2-Like Transcription Factor Genes in Wheat (Triticum aestivum L.)

The GOLDEN2-LIKE (G2-like) gene family members provide significant contributions to the growth and development of plants. In this study, a total of 76 wheat G2-like gene family members (TaG1-TaG76) were detected in the wheat genome and were categorized into three groups (including six subgroups) based on the gene structure and protein motif analyses. These genes were unevenly distributed in 19 of 21 wheat chromosomes. A total of 63 segmental duplication pairs of TaG2-like genes were identified in the wheat genome. The expression levels of all the TaG2-like genes indicated that TaG2-like genes showed different expression patterns in various organs and tissues. Moreover, the transcriptions of TaG2-like genes were significantly affected under abiotic stress (cold, ABA, NaCl, and PEG). This study offered valuable insights into the functional characterization of TaG2-like genes in wheat.

Chloroplasts prevent precocious flowering through a GOLDEN2-LIKE-B-BOX DOMAIN PROTEIN module

The timing of flowering is tightly controlled by signals that integrate environmental and endogenous cues. Sugars produced by carbon fixation in the chloroplast are a crucial endogenous cue for floral initiation. Chloroplasts also convey information directly to the nucleus through retrograde signaling to control plant growth and development. Here, we show that mutants defective in chlorophyll biosynthesis and chloroplast development flowered early, especially under long-day conditions, although low sugar accumulation was seen in some mutants. Plants treated with the bleaching herbicide norflurazon also flowered early, suggesting that chloroplasts have a role in floral repression. Among retrograde signaling mutants, the golden2-like 1 (glk1) glk2 double mutants showed early flowering under long-day conditions. This early flowering was completely suppressed by constans (co) and flowering locus t (ft) mutations. Leaf vascular-specific knockdown of both GLK1 and GLK2 phenocopied the glk1 glk2 mutants. GLK1 and GLK2 repress flowering by directly activating the expression of B-BOX DOMAIN PROTEIN 14 (BBX14), BBX15, and BBX16 via CCAATC cis-elements in the BBX genes. BBX14/15/16 physically interact with CO in the nucleus, and expression of BBXs hampered CO-mediated FT transcription. Simultaneous knockdown of BBX14/15/16 by artificial miRNA (35S::amiR-BBX14/15/16) caused early flowering with increased FT transcript levels, whereas BBX overexpression caused late flowering. Flowering of glk1/2 and 35S::amiR-BBX14/15/16 plants was insensitive to norflurazon treatment. Taking these observations together, we propose that the GLK1/2-BBX14/15/16 module provides a novel mechanism explaining how the chloroplast represses flowering to balance plant growth and reproductive development.

Functions of nitric oxide-mediated post-translational modifications under abiotic stress

Environmental conditions greatly impact plant growth and development. In the current context of both global climate change and land degradation, abiotic stresses usually lead to growth restriction limiting crop production. Plants have evolved to sense and respond to maximize adaptation and survival; therefore, understanding the mechanisms involved in the different converging signaling networks becomes critical for improving plant tolerance. In the last few years, several studies have shown the plant responses against drought and salinity, high and low temperatures, mechanical wounding, heavy metals, hypoxia, UV radiation, or ozone stresses. These threats lead the plant to coordinate a crosstalk among different pathways, highlighting the role of phytohormones and reactive oxygen and nitrogen species (RONS). In particular, plants sense these reactive species through post-translational modification (PTM) of macromolecules such as nucleic acids, proteins, and fatty acids, hence triggering antioxidant responses with molecular implications in the plant welfare. Here, this review compiles the state of the art about how plant systems sense and transduce this crosstalk through PTMs of biological molecules, highlighting the S-nitrosylation of protein targets. These molecular mechanisms finally impact at a physiological level facing the abiotic stressful traits that could lead to establishing molecular patterns underlying stress responses and adaptation strategies.

Transcriptome profiling reveals characteristics of hairy root and the role of AhGLK1 in response to drought stress and post-drought recovery in peanut

BACKGROUND

HR (hairy root) has emerged as a valuable tissue for the rapid characterization of plant gene function and enzyme activity in vivo. AhGLK1 (Arachis hypogaea L. golden2-like 1) is known to play a role in post-drought recovery. However, it is unclear (a) whether HR has properties that are distinct from those of PR (primary root); and (b) which gene networks are regulated by AhGLK1 in response to drought stress and recovery in peanut.

RESULTS

We found that cells of the root tip cortex were larger in HR than in PR, while a total of 850 differentially expressed genes (DEGs) were identified in HR compared to PR. Eighty-eight of these DEGs, relating to chlorophyll and photosynthesis, were upregulated in HR. In addition, AhGLK1-OX (AhGLK1-overexpressing) HR showed a green phenotype, and had a higher relative water content than 35 S::eGFP (control) HR during drought stress. RNA-seq analysis showed that 74 DEGs involved both in the drought response and the post-drought recovery process were significantly enriched in the galactose metabolism pathway. GO terms enrichment analysis revealed that 59.19%, 29.79% and 17.02% of the DEGs mapped to the 'biological process' (BP), 'molecular function' (MF) and 'cellular component' (CC) domains, respectively. Furthermore, 20 DEGs involved in post-drought recovery were uniquely expressed in AhGLK1-OX HR and were significantly enriched in the porphyrin metabolism pathway. GO analysis showed that 42.42%, 30.30% and 27.28% of DEGs could be assigned to the BP, MF and CC domains, respectively. Transcription factors including bHLH and MYB family members may play a key role during drought stress and recovery.

CONCLUSION

Our data reveal that HR has some of the characteristics of leaves, indicating that HR is suitable for studying genes that are mainly expressed in leaves. The RNA-seq results are consistent with previous studies that show chlorophyll synthesis and photosynthesis to be critical for the role of AhGLK1 in improving post-drought recovery growth in peanut. These findings provide in-depth insights that will be of great utility for the exploration of candidate gene functions in relation to drought tolerance and/or post-drought recovery ability in peanut.

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.

Identification of GOLDEN2-like transcription factor genes in soybeans and their role in regulating plant development and metal ion stresses

The Golden 2-Like (G2-like or GLK) transcription factors are essential for plant growth, development, and many stress responses as well as heavy metal stress. However, G2-like regulatory genes have not been studied in soybean. This study identified the genes for 130 G2-Like candidates' in the genome of Glycine max (soybean). These GLK genes were located on all 20 chromosomes, and several of them were segmentally duplicated. Most GLK family proteins are highly conserved in Arabidopsis and soybean and were classified into five major groups based on phylogenetic analysis. These GmGLK gene promoters share cis-acting elements involved in plant responses to abscisic acid, methyl jasmonate, auxin signaling, low temperature, and biotic and abiotic stresses. RNA-seq expression data revealed that the GLK genes were classified into 12 major groups and differentially expressed in different tissues or organs. The co-expression network complex revealed that the GmGLK genes encode proteins involved in the interaction of genes related to chlorophyll biosynthesis, circadian rhythms, and flowering regulation. Real-time quantitative PCR analysis confirmed the expression profiles of eight GLK genes in response to cadmium (Cd) and copper (Cu) stress, with some GLK genes significantly induced by both Cd and Cu stress treatments, implying a functional role in defense responsiveness. Thus, we present a comprehensive perspective of the GLK genes in soybean and emphasize their important role in crop development and metal ion stresses.

Genome-Wide Identification and Characterization of GARP Transcription Factor Gene Family Members Reveal Their Diverse Functions in Tea Plant (Camellia sinensis)

Golden2, ARR-B, Psr1 (GARP) proteins are plant-specific transcription factors that play vital and diverse roles in plants. However, systematic research on the GARP gene family in plants, including tea plant (Camellia sinensis), is scarce. In this study, a total of 69 GARP genes were identified and characterized from the tea plant genome based on the B-motif sequence signature. The CsGARP genes were clustered into five subfamilies: PHR1/PHL1, KAN, NIGT1/HRS1/HHO, GLK and ARR-B subfamilies. The phylogenetic relationships, gene structures, chromosomal locations, conserved motifs and regulatory cis-acting elements of the CsGARP family members were comprehensively analyzed. The expansion of CsGARP genes occurred via whole-genome duplication/segmental duplication, proximal duplication, and dispersed duplication under purifying selective pressure. The expression patterns of the CsGARP genes were systematically explored from various perspectives: in different tissues during different seasons; in different leaf color stages of tea plant; under aluminum treatment and nitrogen treatment; and in response to abiotic stresses such as cold, drought and salt and to biotic stress caused by Acaphylla theae. The results demonstrate that CsGARP family genes are ubiquitously expressed and play crucial roles in the regulation of growth and development of tea plant and the responses to environmental stimuli. Collectively, these results not only provide valuable information for further functional investigations of CsGARPs in tea plant but also contribute to broadening our knowledge of the functional diversity of GARP family genes in plants.

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.

Antagonistic modules regulate photosynthesis-associated nuclear genes via GOLDEN2-LIKE transcription factors

GOLDEN2-LIKE (GLK) transcription factors drive the expression of photosynthesis-associated nuclear genes (PhANGs) indispensable for chloroplast biogenesis. Salicylic acid (SA)-induced SIGMA FACTOR-BINDING PROTEIN 1 (SIB1), a transcription coregulator and positive regulator of cell death, interacts with GLK1 and GLK2 to reinforce the expression of PhANGs, leading to photoinhibition of photosystem II and singlet oxygen (1O2) burst in chloroplasts. 1O2 then contributes to SA-induced cell death via EXECUTER 1 (EX1; 1O2 sensor protein)-mediated retrograde signaling upon reaching a critical level. This earlier finding has initiated research on the potential role of GLK1/2 and EX1 in SA signaling. Consistent with this view, we reveal that LESION-SIMULATING DISEASE 1 (LSD1), a transcription coregulator and negative regulator of SA-primed cell death, interacts with GLK1/2 to repress their activities in Arabidopsis (Arabidopsis thaliana). Overexpression of LSD1 repressed GLK target genes, including PhANGs, whereas loss of LSD1 enhanced their expression. Remarkably, LSD1 overexpression inhibited chloroplast biogenesis, resembling the characteristic glk1glk2 double mutant phenotype. Subsequent chromatin immunoprecipitation coupled with expression analyses further revealed that LSD1 inhibits the DNA-binding activity of GLK1 toward its target promoters. SA-induced nuclear-targeted SIB1 proteins appeared to interrupt the LSD1-GLK interaction, and the subsequent SIB1-GLK interaction activated EX1-mediated 1O2 signaling, elucidating antagonistic modules SIB1 and LSD1 in the regulation of GLK activity. Taken together, we provide a working model that SIB1 and LSD1, mutually exclusive SA-signaling components, antagonistically regulate GLK1/2 to fine-tune the expression of PhANGs, thereby modulating 1O2 homeostasis and related stress responses.

Comprehensive Genomic Analysis of G2-like Transcription Factor Genes and Their Role in Development and Abiotic Stresses in Arabidopsis

GOLDEN2-LIKE (GLK) transcription factors are a subfamily of GARP family transcription factors, which play an essential function in plant growth and development as well as stress response during abiotic and biotic stress conditions. This study reports GLK genes in the Arabidopsis thaliana genome in-depth and identified 55 AtGLK genes in the Arabidopsis genome. Phylogenetic analyses resolved these GLK gene clusters into seven groups. A Ka/Ks ratios analysis indicated that they had experienced purifying selection. Many essential cis elements are present in the promoter regions of AtGLK genes associated with plant hormones, light, and stress. The expression profile from RNA-Seq data revealed that 29.1% of them had relatively high expression in all tested tissues or organs, indicating their crucial housekeeping function in plant growth and development. However, many other GLK members were selectively expressed in particular tissues or organs. In silico study of the transcriptional regulation of AtGLKs indicated that it is strongly regulated by cold, drought, osmotic, salt, and metal ion stressors. Our research provides essential information for the functional studies of each GLK gene in different species in the future.

Genome-wide identification of Tomato Golden 2-Like transcription factors and abiotic stress related members screening

BACKGROUND

Golden 2-Like (G2-like) transcription factors play an important role in plant development. However, the roles of these G2-like regulatory genes in response to abiotic stresses in tomato are not well understood.

RESULTS

In this study, we identified 66 putative G2-like genes in tomato (Solanum lycopersicum) and classified them into 5 groups (I to V) according to gene structure, motif composition and phylogenetic analysis. The G2-like genes were unevenly distributed across all 12 chromosomes. There were nine pairs of duplicated gene segments and four tandem duplicated SlGlk genes. Analysis of the cis-regulatory elements (CREs) showed that the promoter regions of SlGlks contain many kinds of stress- and hormone-related CREs. Based on RNA-seq, SlGlks were expressed in response to three abiotic stresses. Thirty-six differentially expressed SlGlks were identified; these genes have multiple functions according to Gene Ontology (GO) analysis and are enriched mainly in the zeatin biosynthesis pathway. Further studies exhibited that silencing SlGlk16 in tomato would reduce drought stress tolerance by earlier wilted, lower superoxide dismutase (SOD), peroxidase (POD) activities, less Pro contents and more MDA contents.

CONCLUSIONS

Overall, the results of this study provide comprehensive information on G2-like transcription factors and G2-like genes that may be expressed in response to abiotic stresses.

Zinc-finger protein MdBBX7/MdCOL9, a target of MdMIEL1 E3 ligase, confers drought tolerance in apple

Water deficit is one of the main challenges for apple (Malus × domestica) growth and productivity. Breeding drought-tolerant cultivars depends on a thorough understanding of the drought responses of apple trees. Here, we identified the zinc-finger protein B-BOX 7/CONSTANS-LIKE 9 (MdBBX7/MdCOL9), which plays a positive role in apple drought tolerance. The overexpression of MdBBX7 enhanced drought tolerance, whereas knocking down MdBBX7 expression reduced it. Chromatin immunoprecipitation-sequencing (ChIP-seq) analysis identified one cis-element of MdBBX7, CCTTG, as well as its known binding motif, the T/G box. ChIP-seq and RNA-seq identified 1,197 direct targets of MdBBX7, including ETHYLENE RESPONSE FACTOR (ERF1), EARLY RESPONSIVE TO DEHYDRATION 15 (ERD15), and GOLDEN2-LIKE 1 (GLK1) and these were further verified by ChIP-qPCR and electronic mobility shift assays. Yeast two-hybrid screen identified an interacting protein of MdBBX7, RING-type E3 ligase MYB30-INTERACTING E3 LIGASE 1 (MIEL1). Further examination revealed that MdMIEL1 could mediate the ubiquitination and degradation of MdBBX7 by the 26S proteasome pathway. Genetic interaction analysis suggested that MdMIEL1 acts as an upstream factor of MdBBX7. In addition, MdMIEL1 was a negative regulator of the apple drought stress response. Taken together, our results illustrate the molecular mechanisms by which the MdMIEL1-MdBBX7 module influences the response of apple to drought stress.

Using quantitative methods to understand leaf epidermal development

As the interface between plants and the environment, the leaf epidermis provides the first layer of protection against drought, ultraviolet light, and pathogen attack. This cell layer comprises highly coordinated and specialised cells such as stomata, pavement cells and trichomes. While much has been learned from the genetic dissection of stomatal, trichome and pavement cell formation, emerging methods in quantitative measurements that monitor cellular or tissue dynamics will allow us to further investigate cell state transitions and fate determination in leaf epidermal development. In this review, we introduce the formation of epidermal cell types in Arabidopsis and provide examples of quantitative tools to describe phenotypes in leaf research. We further focus on cellular factors involved in triggering cell fates and their quantitative measurements in mechanistic studies and biological patterning. A comprehensive understanding of how a functional leaf epidermis develops will advance the breeding of crops with improved stress tolerance.

Mining Fiskeby III and Mandarin (Ottawa) Expression Profiles to Understand Iron Stress Tolerant Responses in Soybean

The soybean (Glycine max L. merr) genotype Fiskeby III is highly resistant to a multitude of abiotic stresses, including iron deficiency, incurring only mild yield loss during stress conditions. Conversely, Mandarin (Ottawa) is highly susceptible to disease and suffers severe phenotypic damage and yield loss when exposed to abiotic stresses such as iron deficiency, a major challenge to soybean production in the northern Midwestern United States. Using RNA-seq, we characterize the transcriptional response to iron deficiency in both Fiskeby III and Mandarin (Ottawa) to better understand abiotic stress tolerance. Previous work by our group identified a quantitative trait locus (QTL) on chromosome 5 associated with Fiskeby III iron efficiency, indicating Fiskeby III utilizes iron deficiency stress mechanisms not previously characterized in soybean. We targeted 10 of the potential candidate genes in the Williams 82 genome sequence associated with the QTL using virus-induced gene silencing. Coupling virus-induced gene silencing with RNA-seq, we identified a single high priority candidate gene with a significant impact on iron deficiency response pathways. Characterization of the Fiskeby III responses to iron stress and the genes underlying the chromosome 5 QTL provides novel targets for improved abiotic stress tolerance in soybean.

Mutations in a�Golden2-Like�Gene Cause Reduced Seed Weight in�Barley�albino lemma 1�Mutants

The albino lemma 1 (alm1) mutants of barley (Hordeum vulgare L.) exhibit obvious chlorophyll-deficient hulls. Hulls are seed-enclosing tissues on the spike, consisting of the lemma and palea. The alm1 phenotype is also expressed in the pericarp, culm nodes and basal leaf sheaths, but leaf blades and awns are normal green. A single recessive nuclear gene controls tissue-specific alm1 phenotypic expression. Positional cloning revealed that the ALM1 gene encodes a Golden 2-like (GLK) transcription factor, HvGLK2, belonging to the GARP subfamily of Myb transcription factors. This finding was validated by genetic evidence indicating that all 10 alm1 mutants studied had a lesion in functionally important regions of HvGLK2, including the three alpha-helix domains, an AREAEAA motif and the GCT box. Transmission electron microscopy revealed that, in lemmas of the alm1.g mutant, the chloroplasts lacked thylakoid membranes, instead of stacked thylakoid grana in wild-type chloroplasts. Compared with wild type, alm1.g plants showed similar levels of leaf photosynthesis but reduced spike photosynthesis by 34%. The alm1.g mutant and the alm1.a mutant showed a reduction in 100-grain weight by 15.8% and 23.1%, respectively. As in other plants, barley has HvGLK2 and a paralog, HvGLK1. In flag leaves and awns, HvGLK2 and HvGLK1 are expressed at moderate levels, but in hulls, HvGLK1 expression was barely detectable compared with HvGLK2. Barley alm1/Hvglk2 mutants exhibit more severe phenotypes than glk2 mutants of other plant species reported to date. The severe alm1 phenotypic expression in multiple tissues indicates that HvGLK2 plays some roles that are nonredundant with HvGLK1.

Genome-Wide Analysis of the G2-Like Transcription Factor Genes and Their Expression in Different Senescence Stages of Tobacco (Nicotiana tabacum L.)

The Golden2-like (GLK) transcription factors play important roles in regulating chloroplast growth, development, and senescence in plants. In this study, a total of 89 NtGLK genes (NtGLK1-NtGLK89) were identified in the tobacco genome and were classified into 10 subfamilies with variable numbers of exons and similar structural organizations based on the gene structure and protein motif analyses. Twelve segmental duplication pairs of NtGLK genes were identified in the genome. These NtGLK genes contain two conserved helix regions related to the HLH structure, and the sequences of the first helix region are less conserved than that of the second helix motif. Cis-regulatory elements of the NtGLK promoters were widely involved in light responsiveness, hormone treatment, and physiological stress. Moreover, a total of 206 GLK genes from tomato, tobacco, maize, rice, and Arabidopsis were retrieved and clustered into eight subgroups. Our gene expression analysis indicated that NtGLK genes showed differential expression patterns in tobacco leaves at five senescence stages. The expression levels of six NtGLK genes in group C were reduced, coinciding precisely with the increment of the degree of senescence, which might be associated with the function of leaf senescence of tobacco. Our results have revealed valuable information for further functional characterization of the GLK gene family in tobacco.

A BIN2-GLK1 Signaling Module Integrates Brassinosteroid and Light Signaling to Repress Chloroplast Development in the Dark

Arabidopsis GLYCOGEN SYNTHASE KINASE 3 (GSK3)-like kinases play various roles in plant development, including chloroplast development, but the underlying molecular mechanism is not well defined. Here, we demonstrate that transcription factors GLK1 and GLK2 interact with and are phosphorylated by the BRASSINOSTEROID insensitive2 (BIN2). The loss-of-function mutant of BIN2 and its homologs, bin2-3 bil1 bil2, displays abnormal chloroplast development, whereas the gain-of-function mutant, bin2-1, exhibits insensitivity to BR-induced de-greening and reduced numbers of thylakoids per granum, suggesting that BIN2 positively regulates chloroplast development. Furthermore, BIN2 phosphorylates GLK1 at T175, T238, T248, and T256, and mutations of these phosphorylation sites alter GLK1 protein stability and DNA binding and impair plant responses to BRs/darkness. On the other hand, BRs and darkness repress the BIN2-GLK module to enhance BR/dark-mediated de-greening and impair the formation of the photosynthetic apparatus. Our results thus provide a mechanism by which BRs modulate photomorphogenesis and chloroplast development.

AhHDA1-mediated AhGLK1 promoted chlorophyll synthesis and photosynthesis regulates recovery growth of peanut leaves after water stress

Peanut (Arachis hypogaea L.) is an important crop that is adversely affected by drought. Post-drought growth is essential for improving peanut productivity and quality. Previous studies demonstrated that AhGLK1 (Arachis hypogaea L. Golden2-like 1) activates the expression of AhPORA to stimulate chlorophyll biosynthesis, and that AhGLK1 physically interacts with AhHDA1 (Arachis hypogaea L. histone deacetylase 1). However, the roles of the AhGLK1/AhHDA1 interaction in post-drought recovery remain to be elucidated. Herein, we report that AhHDA1 binds to AhGLK1 promoter and alters histone deacetylation levels to inhibit AhGLK1 expression. RNA-seq confirms that chlorophyll synthesis and photosynthesis-related genes are induced in AhGLK1-overexpressing, but reduced in AhGLK1 RNAi hairy roots. Furthermore, ChIP-seq shows that AhCAB (Arachis hypogaea L. chlorophyll A/B binding protein) is a target of both AhHDA1 and AhGLK1. Transactivation assays reveal that AhGLK1 activates AhCAB expression, while AhHDA1 inhibits the effect of AhGLK1 on AhCAB and AhPORA transcription. ChIP-qPCR shows that AhHDA1 and AhGLK1 bind to the promoters of AhCAB and AhPORA to regulate their expression during water stress and recovery. We propose that AhHDA1 and AhGLK1 consist of an ON/OFF switch for AhCAB and AhPORA expression during water stress and recovery. AhGLK1 activates, whereas AhHDA1 suppresses the expression of AhCAB and AhPORA.

Maize GOLDEN2-LIKE genes enhance biomass and grain yields in rice by improving photosynthesis and reducing photoinhibition

Photosynthetic efficiency is a major target for improvement of crop yield potential under agricultural field conditions. Inefficiencies can occur in many steps of the photosynthetic process, from chloroplast biogenesis to functioning of the light harvesting and carbon fixation reactions. Nuclear-encoded GOLDEN2-LIKE (GLK) transcription factors regulate some of the earliest steps by activating target genes encoding chloroplast-localized and photosynthesis-related proteins. Here we show that constitutive expression of maize GLK genes in rice leads to enhanced levels of chlorophylls and pigment-protein antenna complexes, and that these increases lead to improved light harvesting efficiency via photosystem II in field-grown plants. Increased levels of xanthophylls further buffer the negative effects of photoinhibition under high or fluctuating light conditions by facilitating greater dissipation of excess absorbed energy as heat. Significantly, the enhanced photosynthetic capacity of field-grown transgenic plants resulted in increased carbohydrate levels and a 30–40% increase in both vegetative biomass and grain yield.

Li et al. improve photosynthetic efficiency in rice by constitutively expressing maize GOLDEN2-like (GLK) genes (ZmG2 and ZmGLK1). They are able to reduce photoinhibition and enhance the photosynthetic potential as well as increase the carbohydrate, biomass and grain yield.

Ectopic Expression of AhGLK1b (GOLDEN2-like Transcription Factor) in Arabidopsis Confers Dual Resistance to Fungal and Bacterial Pathogens

GOLDEN2-LIKE (GLK) is a member of the myeloblastosis (MYB) family transcription factor and it plays an important role in the regulation of plastid development and stress tolerance. In this study, a gene named AhGLK1b was identified from a cultivated peanut showing down-regulation in response to low calcium with a complete open reading frame (ORF) of 1212 bp. The AhGLK1b has 99.26% and 96.28% sequence similarities with its orthologs in Arachis ipaensis and A. duranensis, respectively. In the peanut, the AhGLK1b was localized in the nucleus and demonstrated the highest expression in the leaf, followed by the embryo. Furthermore, the expression of AhGLK1b was induced significantly in response to a bacterial pathogen, Ralstonia solanacearum infection. Ectopic expression of AhGLK1b in Arabidopsis showed stronger resistance against important phytopathogenic fungi S. sclerotiorum. It also exhibited high resistance to infection of the bacterial pathogen Pst DC3000. AhGLK1b-expressing Arabidopsis induced defense-related genes including PR10 and Phox/Bem 1 (PBI), which are involved in multiple disease resistance. Taken together, the results suggest that AhGLK1b might be useful in providing dual resistance to fungal and bacterial pathogens as well as tolerance to abiotic stresses.

GOLDEN2-LIKE Transcription Factors Regulate WRKY40 Expression in Response to Abscisic Acid

Arabidopsis (Arabidopsis thaliana) GARP (Golden2, ARR-B, Psr1) family transcription factors, GOLDEN2-LIKE1 and -2 (GLK1/2), function in different biological processes; however, whether and how these transcription factors modulate the response to abscisic acid (ABA) remain unknown. In this study, we used a glk1 glk2 double mutant to examine the role of GLK1/2 in the ABA response. The glk1 glk2 double mutant displayed ABA-hypersensitive phenotypes during seed germination and seedling development and an osmotic stress-resistant phenotype during seedling development. Genome-wide RNA sequencing analysis of the glk1 glk2 double mutant revealed that GLK1/2 regulate several ABA-responsive genes, including WRKY40, in the presence of ABA. Chromatin immunoprecipitation and gel retardation assays showed that GLK1/2 directly associate with the WRKY40 promoter via the recognition of a consensus sequence. Additionally, RNA sequencing analysis of the glk1 glk2 double mutant and wrky40 single mutant revealed that GLK1/2 and WRKY40 control a common set of downstream target genes in response to ABA. Furthermore, results of a genetic interaction test showed that the glk1 glk2 wrky40 triple mutant displayed similar ABA hypersensitivity to the wrky40 single mutant and the glk1 glk2 double mutant, while the glk1 glk2 wrky40 abi5-c (ABI5 CRISPR/Cas9 mutant) quadruple mutant displayed similar ABA hyposensitivity to the abi5-7 single mutant. Based on these results, we propose that the GLK1/2-WRKY40 transcription module plays a negative regulatory role in the ABA response.

Blue Light Regulates Phosphate Deficiency-Dependent Primary Root Growth Inhibition in Arabidopsis

Plants have evolved mechanisms to improve utilization efficiency or acquisition of inorganic phosphate (Pi) in response to Pi deficiency, such as altering root architecture, secreting acid phosphatases, and activating the expression of genes related to Pi uptake and recycling. Although many genes responsive to Pi starvation have been identified, transcription factors that affect tolerance to Pi deficiency have not been well characterized. We show here that the ectopic expression of B-BOX32 (BBX32) and the mutation of ELONGATED HYPOCOTYL 5 (HY5), whose transcriptional activity is negatively regulated by BBX32, resulted in the tolerance to Pi deficiency in Arabidopsis. The primary root lengths of 35S:BBX32 and hy5 plants were only slightly inhibited under Pi deficient condition and the fresh weights were significantly higher than those of wild type. The Pi deficiency-tolerant root phenotype of hy5 was similarly observed when grown on the medium without Pi. In addition, a double mutant, hy5 slr1, without lateral roots, also showed a long primary root phenotype under phosphate deficiency, indicating that the root phenotype of hy5 does not result from an increase of external Pi uptake. Moreover, we found that blue light may regulate Pi deficiency-dependent primary root growth inhibition through activating peroxidase gene expression, suggesting the Pi-deficiency tolerant root phenotype of hy5 may be due to blockage of blue light responses. Altogether, this study points out light quality may play an important role in the regulation of Pi deficiency responses. It may contribute to regulate plant growth under Pi deficiency through proper illumination.

Transcriptome Analysis Reveals Candidate Genes Associated with Leaf Etiolation of a Cytoplasmic Male Sterility Line in Chinese Cabbage (Brassica Rapa L. ssp. Pekinensis)

Cytoplasmic male sterility (CMS) is universally utilized in cruciferous vegetables. However, the Chinese cabbage hau CMS lines, obtained by interspecific hybridization and multiple backcrosses of the Brassica juncea (B. juncea) CMS line and Chinese cabbage, show obvious leaf etiolation, and the molecular mechanism of etiolation remains elusive. Here, the ultrastructural and phenotypic features of leaves from the Chinese cabbage CMS line 1409A and maintainer line 1409B are analyzed. The results show that chloroplasts of 1409A exhibit abnormal morphology and distribution. Next, RNA-sequencing (RNA-Seq) is used to identify 485 differentially expressed genes (DEGs) between 1409A and 1409B, and 189 up-regulated genes and 296 down-regulated genes are found. Genes that affect chloroplasts development, such as GLK1 and GLK2, and chlorophyll biosynthesis, such as PORB, are included in the down-regulated DEGs. Quantitative real-time PCR (qRT-PCR) analysis validate that the expression levels of these genes are significantly lower in 1409A than in 1409B. Taken together, these results demonstrate that leaf etiolation is markedly affected by chloroplast development and pigment biosynthesis. This study provides an effective foundation for research on the molecular mechanisms of leaf etiolation of the hau CMS line in Chinese cabbage (Brassica rapa L. ssp. pekinensis).

Blue Light Regulation of Stomatal Opening and the Plasma Membrane H+-ATPase

Recent progress of the blue light signaling pathway in guard cells highlights its regulation of H⁺-ATPase activity.

Understanding and improving global crop response to ozone pollution

Concentrations of ground-level ozone ([O3 ]) over much of the Earth's land surface have more than doubled since pre-industrial times. The air pollutant is highly variable over time and space, which makes it difficult to assess the average agronomic and economic impacts of the pollutant as well as to breed crops for O3 tolerance. Recent modeling efforts have improved quantitative understanding of the effects of current and future [O3 ] on global crop productivity, and experimental advances have improved understanding of the cellular O3 sensing, signaling and response mechanisms. This work provides the fundamental background and justification for breeding and biotechnological approaches for improving O3 tolerance in crops. There is considerable within-species variation in O3 tolerance in crops, which has been used to create mapping populations for screening. Quantitative trait loci (QTL) for O3 tolerance have been identified in model and crop species, and although none has been cloned to date, transcript profiling experiments have identified candidate genes associated with QTL. Biotechnological strategies for improving O3 tolerance are also being tested, although there is considerable research to be done before O3 -tolerant germplasm is available to growers for most crops. Strategies to improve O3 tolerance in crops have been hampered by the lack of translation of laboratory experiments to the field, and the lack of correlation between visual leaf-level O3 damage and yield loss to O3 stress. Future efforts to screen mapping populations in the field and to identify more promising phenotypes for O3 tolerance are needed.

Pollution resistance assessment of existing landscape plants on Beijing streets based on air pollution tolerance index method

Various plant species of green belt in urban traffic area help to reduce air pollution and beautify the city environment. Those plant species growing healthily under long-term atmospheric pollution environment are considered to be resilient. This study aims to identify plant species that are more tolerant to air pollution from traffic and to give recommendations for future green belt development in urban areas. Leaf samples of 47 plant species were collected from two heavy traffic roadside sites and one suburban site in Beijing during summer 2014. Four parameters in leaves were separately measured including relative water content (RWC), total chlorophyll content (TCH), leaf-extract pH (pH), and ascorbic acid (AA). The air pollution tolerance index (APTI) method was adopted to assess plants' resistance ability based on the above four parameters. The tolerant levels of plant species were classified using two methods, one by comparing the APTI value of individual plant to the average of all species and another by using fixed APTI values as standards. Tolerant species were then selected based on combination results from both methods. The results showed that different tolerance orders of species has been found at the three sampling sites due to varied air pollution and other environmental conditions. In general, plant species Magnolia denudata, Diospyros kaki, Ailanthus altissima, Fraxinus chinensis and Rosa chinensis were identified as tolerant species to air pollution environment and recommend to be planted at various location of the city, especially at heavy traffic roadside.

GOLDEN 2-LIKE Transcription Factors of Plants

Golden2-like (GLK) transcription factors are members of the GARP family of Myb transcription factors with an established relationship to chloroplast development in the plant kingdom. In the last century, Golden2 was proposed as a second golden producing factor and identified as controlling cellular differentiation in maize leaves. Then, GLKs were also found to play roles in disease defense and their function is conserved in regulating chloroplast development. Recently, research on GLKs has rapidly increased and shown that GLKs control chloroplast development in green and non-green tissues. Moreover, links between phytohormones and GLKs were verified. In this mini-review, we summarize the history, conservation, function, potential targets and degradation of GLKs.