Arabidopsis GOLDEN2-LIKE (GLK) transcription factors activate jasmonic acid (JA)-dependent disease susceptibility to the biotrophic pathogen Hyaloperonospora arabidopsidis, as well as JA-independent plant immunity against the necrotrophic pathogen Botrytis cinerea

GLKs directly regulate carotenoid biosynthesis via interacting with GBFs in plants

Carotenoids are vital photosynthetic pigments for plants. Golden2-like transcription factors (GLKs) are widely recognized as major regulators of Chl biosynthesis and chloroplast development. However, despite GLKs being subjected to intensive investigations, whether GLKs directly regulate carotenoid biosynthesis and the molecular mechanisms by which GLKs transcriptionally activate their target genes remain unclear. Here, we report that GLKs directly regulate carotenoid biosynthesis and activate their target genes in a G-box binding factor (GBF)-dependent manner in Arabidopsis. Both in vitro and in vivo studies reveal that GLKs physically interact with GBFs to activate transcription of phytoene synthase (PSY), the gene encoding a rate-limiting enzyme for carotenoid biosynthesis. While GLKs possess transactivation activity, they depend on GBFs to directly bind to the G-box motif to modulate PSY expression. Loss of GBFs impairs GLK function in regulating carotenoid and Chl biosynthesis. Since the G-box motif is an enriched motif in the promoters of GLK-regulated genes, the GLK-GBF regulatory module likely serves as a common mechanism underlying GLK-regulated photosynthetic pigment biosynthesis and chloroplast development. Our findings uncover a novel regulatory machinery of carotenoid biosynthesis, discover a molecular mechanism of transcriptional regulation by GLKs, and divulge GLKs as important regulators to coordinate photosynthetic pigment synthesis in plants.

Genome-Wide Identification of GLK Family Genes in Phoebe bournei and Their Transcriptional Analysis Under Abiotic Stresses

GOLDEN2-LIKE (GLK) transcription factors are crucial regulators of chloroplast development and stress responses in plants. In this study, we investigated the GLK gene family in Phoebe bournei (Hemsl.) Yen C. Yang, a near-threatened species important for forestry and wood utilization in China. We identified 61 PbGLK genes which were classified into seven subfamilies. Our analyses of their phylogenetic relationships, gene structures, and chromosomal distribution revealed diverse characteristics. Expression profiling under different tissues and abiotic stresses showed that PbGLK25 and PbGLK30 were particularly responsive to drought, heat, light, and shade stresses, with significant upregulation. These findings highlight the potential role of PbGLK genes in stress adaptation and provide insights for the genetic improvement of P. bournei.

Genome-wide identification and characterization of FORMIN gene family in cotton (Gossypium hirsutum L.) and their expression profiles in response to multiple abiotic stress treatments

FORMIN proteins distinguished by FH2 domain, are conserved throughout evolution and widely distributed in eukaryotic organisms. These proteins interact with various signaling molecules and cytoskeletal proteins, playing crucial roles in both biotic and abiotic stress responses. However, the functions of FORMINs in cotton (Gossypium hirsutum L.) remain uncovered. In this study, 46 FORMIN genes in G. hirsutum (referred to as GhFH) were systematically identified. The gene structures, conserved domains, and motifs of these GhFH genes were thoroughly explored. Phylogenetic and structural analysis classified these 46 GhFH genes into five distinct groups. In silico subcellular localization, prediction suggested that GhFH genes are distributed across various cellular compartments, including the nucleus, extracellular space, cytoplasm, mitochondria, cytoskeleton, plasma membrane, endoplasmic reticulum, and chloroplasts. Evolutionary and functional diversification analyses, based on on-synonymous (Ka) and synonymous (Ks) ratios and gene duplication events, indicated that GhFH genes have evolved under purifying selection. The analysis of cis-acting elements suggested that GhFH genes may be involved in plant growth, hormone regulation, light response, and stress response. Results from transcriptional factors TFs and gene ontology analysis indicate that FORMIN proteins regulate cell wall structure and cytoskeleton dynamics by reacting to hormone signals associated with environmental stress. Additionally, 45 putative ghr-miRNAs were identified from 32 families targeting 33 GhFH genes. Expression analysis revealed that GhFH1, GhFH10, GhFH20, GhFH24, and GhFH30 exhibited the highest levels of expression under red, blue, and white light conditions. Further, GhFH9, GhFH20, and GhFH30 displayed higher expression levels under heat stress, while GhFH20 and GhFH30 showed increased expression under salt stress compared to controls. The result suggests that GhFH20 and GhFH30 genes could play significant roles in the development of G. hirsutum under heat and salt stresses. Overall these findings enhance our understanding of the biological functions of the cotton FORMIN family, offering prospects for developing stress-resistant cotton varieties through manipulation of GhFH gene expression.

Plastid- and photoreceptor-dependent signaling is required for the response to photoperiod stress

Prolongation of the light period causes photoperiod stress in plants. The response to photoperiod stress includes the induction of a distinct set of stress marker genes, of reactive oxygen species (ROS), and of stress hormones. In this study, the impact of light intensity and light quality on the photoperiod stress response was investigated. A threshold light intensity of circa 50 μmol m-2 s-1 is necessary for inducing photoperiod stress, indicating the involvement of chloroplasts. Lower photoperiod stress symptoms in retrograde signaling mutants (gun4, gun5) and mutants with constrained plastid function (glk1 glk2) corroborated the role of chloroplasts. Genetic analysis revealed that the photoreceptors phyB and particularly CRY2 are important to perceive photoperiod stress. Overall, these results showed that both plastid-dependent and photoreceptor-dependent signaling pathways are involved in sensing the light conditions causing photoperiod stress and governing the response to it.

AcGLK1 promotes chloroplast division through regulating AcFtsZ1 in Actinidia chinensis

MAIN CONCLUSION

This study unravels a new regulatory member (AcGLK1) that regulates chloroplast division by affecting the expression levels of cytoskeletal filamenting temperature-sensitive Z (FtsZ) in Actinidia chinensis. GOLDEN 2-LIKE (GLK) transcription factor members of GARP subfamily play an irreplaceable role in regulating chloroplast biogenesis and development. Here we report the functional characterization of a novel GLK1 homolog (AcGLK1) isolated from kiwifruit (Actinidia chinensis cultivar 'Hongyang'). Transgenic lines overexpressing AcGLK1 (AcGLK1OE) resulted in an increase of chloroplast number, size and nutrients accumulation in a tomato variety Micro-Tom (Solanum lycopersicum). Transcriptomic data revealed a series of DEGs related to chloroplast division, in which a tomato FtsZ1 homolog (SlFtsZ1) was significantly upregulated in the transgenic lines and could be directly activated by AcGLK1. Furthermore, AcGLK1 was shown to transcriptionally activate expression of kiwifruit FtsZ1 homologous genes (Achv4p23g035689 and Achv4p19g029547) through Y1H and GUS assays. Taken together, we provide evidence showing that AcGLK1 promotes chloroplast division probably through positively regulation of the transcription of FtsZ1 homologs.

The pleiotropic functions of GOLDEN2-LIKE transcription factors in plants

The regulation of gene expression is crucial for biological plant growth and development, with transcription factors (TFs) serving as key switches in this regulatory mechanism. GOLDEN2-LIKE (GLK) TFs are a class of functionally partially redundant nuclear TFs belonging to the GARP superfamily of MYB TFs that play a key role in regulating genes related to photosynthesis and chloroplast biogenesis. Here, we summarized the current knowledge of the pleiotropic roles of GLKs in plants. In addition to their primary functions of controlling chloroplast biogenesis and function maintenance, GLKs have been proven to regulate the photomorphogenesis of seedlings, metabolite synthesis, flowering time, leaf senescence, and response to biotic and abiotic stress, ultimately contributing to crop yield. This review will provide a comprehensive understanding of the biological functions of GLKs and serve as a reference for future theoretical and applied studies of GLKs.

Genome-Wide Identification of a Maize Chitinase Gene Family and the Induction of Its Expression by Fusarium verticillioides (Sacc.) Nirenberg (1976) Infection

Maize chitinases are involved in chitin hydrolysis. Chitinases are distributed across various organisms including animals, plants, and fungi and are grouped into different glycosyl hydrolase families and classes, depending on protein structure. However, many chitinase functions and their interactions with other plant proteins remain unknown. The economic importance of maize (Zea mays L.) makes it relevant for studying the function of plant chitinases and their biological roles. This work aims to identify chitinase genes in the maize genome to study their gene structure, family/class classification, cis-related elements, and gene expression under biotic stress, such as Fusarium verticillioides infection. Thirty-nine chitinase genes were identified and found to be distributed in three glycosyl hydrolase (GH) families (18, 19 and 20). Likewise, the conserved domains and motifs were identified in each GH family member. The identified cis-regulatory elements are involved in plant development, hormone response, defense, and abiotic stress response. Chitinase protein-interaction network analysis predicted that they interact mainly with cell wall proteins. qRT-PCR analysis confirmed in silico data showing that ten different maize chitinase genes are induced in the presence of F. verticillioides, and that they could have several roles in pathogen infection depending on chitinase structure and cell wall localization.

Photosynthetic Activities, Phytohormones, and Secondary Metabolites Induction in Plants by Prevailing Compost Residue

Compost residue enriches soil health with the potential to enhance plant metabolism and hormonal balance, but has not yet been studied. A study was performed to determine how prevailing compost residue induces tomato (Solanum lycopersicum 'Scotia') plant morpho-physiology, phytohormones, and secondary metabolites. Plants were grown in soils with a previous history of annual (AN) and biennial (BI) compost amendments. The controls were soil without compost (C) amendment and municipal solid waste compost (MSWC) alone. The MSWC- and AN-plants had similar and significantly (p < 0.05) highest growth and photosynthetic activities compared to the BI- or C-plants. Total phenolics and lipid peroxidase activity were significantly (p < 0.001) high in BI-plants, while hydrogen peroxide and antioxidant capacity were significantly (p < 0.001) high in AN-plants. MSWC-plants recorded the highest cis-abscisic acid, followed by AN-, and then BI- and C-plants. Cis-zeatin, trans-zeatin, and isopentenyladenine ribosides were detected in the MSWC- and AN-plants but not in the BI- or C-plants. Furthermore, gibberellins GA53, GA19, and GA8 were high in the MSWC-plants, but only GA8 was detected in the AN plants and none in the others. Besides, MSWC plants exhibited the highest content of 1-aminocyclopropane-1-carboxylic acid. Conjugated salicylic acid was highest in the BI-plants, while jasmonic acid-isoleucine was highest in MSWC-plants and C plants. In conclusion, prevailing compost chemical residues upregulate plant growth, phytohormones, and metabolic compounds that can potentially increase plant growth and abiotic stress defense. Future work should investigate the flow of these compounds in plants under abiotic stress.

The role of the Golden2-like (GLK) transcription factor in regulating terpenoid indole alkaloid biosynthesis in Catharanthus roseus

KEY MESSAGE

A GLK homologue was identified and functionally characterized in Catharanthus roseus. Silencing CrGLK with VIGS or the chloroplast retrograde signaling inducer lincomycin increased terpenoid indole alkaloid biosynthesis. Catharanthus roseus is the sole source of the chemotherapeutic terpenoid indole alkaloids (TIAs) vinblastine and vincristine. TIA pathway genes, particularly genes in the vindoline pathway, are expressed at higher levels in immature versus mature leaves, but the molecular mechanisms responsible for this developmental regulation are unknown. We investigated the role of GOLDEN2-LIKE (GLK) transcription factors in contributing to this ontogenetic regulation since GLKs are active in seedlings upon light exposure and in the leaf's early development, but their activity is repressed as leaves age and senesce. We identified a GLK homologue in C. roseus and functionally characterized its role in regulating TIA biosynthesis, with a focus on the vindoline pathway, by transiently reducing its expression through two separate methods: virus-induced gene silencing (VIGS) and application of chloroplast retrograde signaling inducers, norflurazon and lincomycin. Reducing CrGLK levels with each method reduced chlorophyll accumulation and the expression of the light harvesting complex subunit (LHCB2.2), confirming its functional homology with GLKs in other plant species. In contrast, reducing CrGLK via VIGS or lincomycin increased TIA accumulation and TIA pathway gene expression, suggesting that CrGLK may repress TIA biosynthesis. However, norflurazon had no effect on TIA gene expression, indicating that reducing CrGLK alone is not sufficient to induce TIA biosynthesis. Future work is needed to clarify the specific molecular mechanisms leading to increased TIA biosynthesis with CrGLK silencing. This is the first identification and characterization of GLK in C. roseus and the first investigation of how chloroplast retrograde signaling might regulate TIA biosynthesis.

Evidence that Ophiostomatoid Fungal Symbionts of Mountain Pine Beetle Do Not Play a Role in Overcoming Lodgepole Pine Defenses During Mass Attack

Mountain pine beetle (MPB; Dendroctonus ponderosae Hopkins) is a devastating forest insect pest that has killed millions of hectares of pines in western North America over the past two decades. Like other bark beetles, MPB vectors ophiostomatoid fungal species, some of which are pathogenic to host pine species. The phytopathogenicity of these fungal symbionts has sparked considerable debate regarding their role in facilitating MPB attack success. We tested the hypothesis that MPB ophiostomatoid fungal associates like Grosmannia clavigera (Robinson-Jeffrey and Davidson) Zipfel, de Beer and Wingfield contribute to overwhelming host defenses during MPB mass attack. We compared responses of mature lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) trees growing in natural stands that were mass attacked by MPB with those inoculated with G. clavigera by examining host defense hormones, secondary metabolites, and gene expression profiles. The jasmonate and ethylene signatures of necrotrophic pathogen-triggered response were identified in G. clavigera-inoculated trees, but only the jasmonate signature of a herbivore-triggered response was measured in MPB-attacked trees. Several G. clavigera-induced changes in pine phenolic metabolite profiles and phenolic biosynthesis gene expression patterns were absent in MPB-attacked pines. These findings indicate that ophiostomatoid fungi like G. clavigera are not a major factor in overwhelming host defenses during MPB mass attack. Instead, fungal pathogenicity likely is more important in aiding MPB colonization and development within the host tree. Phenolics appear to play a larger role in the host response to G. clavigera than to MPB, although phenolics may also influence MPB feeding and behavior. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

PSEUDO-ETIOLATION IN LIGHT proteins reduce greening by binding GLK transcription factors

Knocking out genes encoding proteins that downregulate the accumulation of pigments may lead to increases in crop quality and yield. PSEUDO-ETIOLATION IN LIGHT 1 (PEL1) downregulates the accumulation of carotenoids in carrot and chlorophyll in Arabidopsis and rice and may inhibit GOLDEN 2-LIKE (GLK) transcription factors. PEL1 belongs to a previously unstudied gene family found only in plants. We used CRISPR/Cas9 technology to knock out each member of the 4-member PEL gene family and both GLK genes in Arabidopsis. In pel mutants, chlorophyll levels were elevated in seedlings; after flowering, chloroplasts increased in size, and anthocyanin levels increased. Although the chlorophyll-deficient phenotype of glk1 glk2 was epistatic to pel1 pel2 pel3 pel4 in most of our experiments, glk1 glk2 was not epistatic to pel1 pel2 pel3 pel4 for the accumulation of anthocyanins in most of our experiments. The pel alleles attenuated growth, altered the accumulation of nutrients in seeds, disrupted an abscisic acid-inducible inhibition of seedling growth response that promotes drought tolerance, and affected the expression of genes associated with diverse biological functions, such as stress responses, cell wall metabolism hormone responses, signaling, growth, and the accumulation of phenylpropanoids and pigments. We found that PEL proteins specifically bind 6 transcription factors that influence the accumulation of anthocyanins, GLK2, and the carboxy termini of GLK1 and Arabidopsis thaliana myeloblastosis oncogene homolog 4 (AtMYB4). Our data indicate that the PEL proteins influence the accumulation of chlorophyll and many other processes, possibly by inhibiting GLK transcription factors and via other mechanisms, and that multiple mechanisms downregulate chlorophyll content.

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.

Impact of Susceptibility on Plant Hormonal Composition during Clubroot Disease Development in Canola (Brassica napus)

Clubroot, caused by Plasmodiophora brassicae, is a soilborne disease of crucifers associated with the formation of large root galls. This root enlargement suggests modulation of plant hormonal networks by the pathogen, stimulating cell division and elongation and influencing host defense. We studied physiological changes in two Brassica napus cultivars, including plant hormone profiles-salicylic acid (SA), jasmonic acid (JA), abscisic acid (ABA), the auxin indole-3-acetic acid (IAA), and the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC)-along with their selected derivatives following inoculation with virulent and avirulent P. brassicae pathotypes. In susceptible plants, water uptake declined from the initial appearance of root galls by 21 days after inoculation, but did not have a significant effect on photosynthetic rate, stomatal conductance, or leaf chlorophyll levels. Nonetheless, a strong increase in ABA levels indicated that hormonal mechanisms were triggered to cope with water stress due to the declining water uptake. The free SA level in the roots increased strongly in resistant interactions, compared with a relatively minor increase during susceptible interactions. The ratio of conjugated SA to free SA was higher in susceptible interactions, indicating that resistant interactions are linked to the plant's ability to maintain higher levels of bioactive free SA. In contrast, JA and its biologically active form JA-Ile declined up to 7-fold in susceptible interactions, while they were maintained during resistant interactions. The ACC level increased in the roots of inoculated plants by 21 days, irrespective of clubroot susceptibility, indicating a role of ethylene in response to pathogen interactions that is independent of disease severity. IAA levels at early and later infection stages were lower only in susceptible plants, suggesting a modulation of auxin homeostasis by the pathogen relative to the host defense system.

The G2-Like gene family in Populus trichocarpa: identification, evolution and expression profiles

The Golden2-like (GLK) transcription factors are plant-specific transcription factors (TFs) that perform extensive and significant roles in regulating chloroplast development. Here, genome-wide identification, classification, conserved motifs, cis-elements, chromosomal locations, evolution and expression patterns of the PtGLK genes in the woody model plant Populus trichocarpa were analyzed in detail. In total, 55 putative PtGLKs (PtGLK1-PtGLK55) were identified and divided into 11 distinct subfamilies according to the gene structure, motif composition and phylogenetic analysis. Synteny analysis showed that 22 orthologous pairs and highly conservation between regions of GLK genes across P. trichocarpa and Arabidopsis were identified. Furthermore, analysis of the duplication events and divergence times provided insight into the evolutionary patterns of GLK genes. The previously published transcriptome data indicated that PtGLK genes exhibited distinct expression patterns in various tissues and different stages. Additionally, several PtGLKs were significantly upregulated under the responses of cold stress, osmotic stress, and methyl jasmonate (MeJA) and gibberellic acid (GA) treatments, implying that they might take part in abiotic stress and phytohormone responses. Overall, our results provide comprehensive information on the PtGLK gene family and elucidate the potential functional characterization of PtGLK genes in P. trichocarpa.

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.

The plant trans-Golgi network component ECHIDNA regulates defense, cell death, and endoplasmic reticulum stress

The trans-Golgi network (TGN) acts as a central platform for sorting and secreting various cargoes to the cell surface, thus being essential for the full execution of plant immunity. However, the fine-tuned regulation of TGN components in plant defense and stress response has been not fully elucidated. Our study revealed that despite largely compromising penetration resistance, the loss-of-function mutation of the TGN component protein ECHIDNA (ECH) induced enhanced postinvasion resistance to powdery mildew in Arabidopsis thaliana. Genetic and transcriptome analyses and hormone profiling demonstrated that ECH loss resulted in salicylic acid (SA) hyperaccumulation via the ISOCHORISMATE SYNTHASE 1 biosynthesis pathway, thereby constitutively activating SA-dependent innate immunity that was largely responsible for the enhanced postinvasion resistance. Furthermore, the ech mutant displayed accelerated SA-independent spontaneous cell death and constitutive POWDERY MILDEW RESISTANCE 4-mediated callose depositions. In addition, ECH loss led to a chronically prolonged endoplasmic reticulum stress in the ech mutant. These results provide insights into understanding the role of TGN components in the regulation of plant immunity and stress responses.

Long-lasting memory of jasmonic acid-dependent immunity requires DNA demethylation and ARGONAUTE1

Stress can have long-lasting impacts on plants. Here we report the long-term effects of the stress hormone jasmonic acid (JA) on the defence phenotype, transcriptome and DNA methylome of Arabidopsis. Three weeks after transient JA signalling, 5-week-old plants retained induced resistance (IR) against herbivory but showed increased susceptibility to pathogens. Transcriptome analysis revealed long-term priming and/or upregulation of JA-dependent defence genes but repression of ethylene- and salicylic acid-dependent genes. Long-term JA-IR was associated with shifts in glucosinolate composition and required MYC2/3/4 transcription factors, RNA-directed DNA methylation, the DNA demethylase ROS1 and the small RNA (sRNA)-binding protein AGO1. Although methylome analysis did not reveal consistent changes in DNA methylation near MYC2/3/4-controlled genes, JA-treated plants were specifically enriched with hypomethylated ATREP2 transposable elements (TEs). Epigenomic characterization of mutants and transgenic lines revealed that ATREP2 TEs are regulated by RdDM and ROS1 and produce 21 nt sRNAs that bind to nuclear AGO1. Since ATREP2 TEs are enriched with sequences from IR-related defence genes, our results suggest that AGO1-associated sRNAs from hypomethylated ATREP2 TEs trans-regulate long-lasting memory of JA-dependent immunity.

Golden 2-like transcription factor contributes to the major QTL against rice black-streaked dwarf virus disease

A major resistance QTL was identified on chromosome 6 in rice variety Wuke; both overexpression and knockdown experiments confirmed that OsGLK1 is the candidate gene for association with Rice black-streaked dwarf virus disease. Rice black-streaked dwarf virus disease is one of the most destructive rice viral diseases in China and East Asia. Progress has been limited in RBSDVD resistance breeding due to inadequate knowledge on the underlying functional genes. In this study, a major QTL for RBSDV (rice black-streaked dwarf virus) independent of SBPH (small brown planthopper) resistance was mapped in a 1.8 Mb interval on chromosome 6 by using an F_2:3 population originated from resistant rice variety Wuke. Representative transcripts within this region were analysed and three genes showing amino acid sequence variation in functional domains were selected for transformation. Overexpression experiments showed that one gene exhibited significant enhanced resistance compared to control lines, encoding protein involving Myb domain and probable transcription factor Golden 2-like1 (GLK1). Furthermore, OsGLK1 knockdown rice lines were investigated and the resistance ability was significantly declined without this gene compared to the wild type. Taken together, both overexpression and knockdown experiments strongly suggested that OsGLK1 plays an important role for RBSDV resistance and contributes to the major QTL. The study paves the way for elucidating the molecular mechanism underlying RBSDVD resistance and the molecular markers associated with OsGLK1 may be used for marker-assisted selection.

Genome-Wide Identification and Characterization of G2-Like Transcription Factor Genes in Moso Bamboo (Phyllostachys edulis)

G2-like (GLK) transcription factors contribute significantly and extensively in regulating chloroplast growth and development in plants. This study investigated the genome-wide identification, phylogenetic relationships, conserved motifs, promoter cis-elements, MCScanX, divergence times, and expression profile analysis of PeGLK genes in moso bamboo (Phyllostachys edulis). Overall, 78 putative PeGLKs (PeGLK1–PeGLK78) were identified and divided into 13 distinct subfamilies. Each subfamily contains members displaying similar gene structure and motif composition. By synteny analysis, 42 orthologous pairs and highly conserved microsynteny between regions of GLK genes across moso bamboo and maize were found. Furthermore, an analysis of the divergence times indicated that PeGLK genes had a duplication event around 15 million years ago (MYA) and a divergence happened around 38 MYA between PeGLK and ZmGLK. Tissue-specific expression analysis showed that PeGLK genes presented distinct expression profiles in various tissues, and many members were highly expressed in leaves. Additionally, several PeGLKs were significantly up-regulated under cold stress, osmotic stress, and MeJA and GA treatment, implying that they have a likelihood of affecting abiotic stress and phytohormone responses in plants. The results of this study provide a comprehensive understanding of the moso bamboo GLK gene family, as well as elucidating the potential functional characterization of PeGLK genes.

Oviposition by a Specialist Herbivore Increases Susceptibility of Canola to Herbivory by a Generalist Herbivore

Oviposition by specialist herbivores can alter the suitability of the host plant to subsequent infestation by other herbivores. In this study, we tested the effect of previous oviposition on canola, Brassica napus L., by a Brassica specialist, the diamondback moth (DBM), Plutella xylostella (L.) (Lepidoptera: Plutellidae), on subsequent herbivory by the generalist feeder, the bertha armyworm (BAW), Mamestra configurata Walker (Lepidoptera: Noctuidae). The effect of DBM oviposition on subsequent BAW oviposition and larval feeding was tested in no-choice and choice experiments. Oviposition of BAW was not altered by DBM eggs on canola plants, however, BAW had increased larval feeding on plants with DBM eggs. These results suggest that oviposition by a specialist herbivore increased the susceptibility of the host plant to generalist herbivory. In a preliminary experiment, salicylic acid, jasmonic acid, and its conjugates were not altered by DBM oviposition on canola, however, further experimentation is needed to determine if oviposition affects expression of plant defense pathways and other plant traits.

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.

Maize Golden2-like transcription factors boost rice chloroplast development, photosynthesis, and grain yield

Chloroplasts are the sites for photosynthesis, and two Golden2-like factors act as transcriptional activators of chloroplast development in rice (Oryza sativa L.) and maize (Zea mays L.). Rice OsGLK1 and OsGLK2 are orthologous to maize ZmGLK1 (ZmG1) and ZmGLK2 (ZmG2), respectively. However, while rice OsGLK1 and OsGLK2 act redundantly to regulate chloroplast development in mesophyll cells, maize ZmG1 and ZmG2 are functionally specialized and expressed in different cell-specific manners. To boost rice chloroplast development and photosynthesis, we generated transgenic rice plants overexpressing ZmG1 and ZmG2, individually or simultaneously, with constitutive promoters (pZmUbi::ZmG1 and p35S::ZmG2) or maize promoters (pZmG1::ZmG1, pZmG2::ZmG2, and pZmG1::ZmG1/pZmG2::ZmG2). Both ZmG1 and ZmG2 genes were highly expressed in transgenic rice leaves. Moreover, ZmG1 and ZmG2 showed coordinated expression in pZmG1::ZmG1/pZmG2::ZmG2 plants. All Golden2-like (GLK) transgenic plants had higher chlorophyll and protein contents, Rubisco activities and photosynthetic rates per unit leaf area in flag leaves. However, the highest grain yields occurred when maize promoters were used; pZmG1::ZmG1, pZmG2::ZmG2, and pZmG1::ZmG1/pZmG2::ZmG2 transgenic plants showed increases in grain yield by 51%, 47%, and 70%, respectively. In contrast, the pZmUbi::ZmG1 plant produced smaller seeds without yield increases. Transcriptome analysis indicated that maize GLKs act as master regulators promoting the expression of both photosynthesis-related and stress-responsive regulatory genes in both rice shoot and root. Thus, by promoting these important functions under the control of their own promoters, maize GLK1 and GLK2 genes together dramatically improved rice photosynthetic performance and productivity. A similar approach can potentially improve the productivity of many other crops.

GhGLK1 a Key Candidate Gene From GARP Family Enhances Cold and Drought Stress Tolerance in Cotton

Drought and low-temperature stresses are the most prominent abiotic stresses affecting cotton. Wild cotton being exposed to harsh environments has more potential to cope with both biotic and abiotic stresses. Exploiting wild cotton material to induce resistant germplasm would be of greater interest. The candidate gene was identified in the BC2F2 population among Gossypium tomentosum and Gossypium hirsutum as wild male donor parent noted for its drought tolerance and the recurrent parent and a high yielding but drought susceptible species by genotyping by sequencing (GBS) mapping. Golden2-like (GLK) gene, which belongs to the GARP family, is a kind of plant-specific transcription factor (TF) that was silenced by virus-induced gene silencing (VIGS). Silencing of GhGLK1 in cotton results in more damage to plants under drought and cold stress as compared with wild type (WT). The overexpression of GhGLK1 in Arabidopsis thaliana showed that the overexpressing plants showed more adaptability than the WT after drought and cold treatments. The results of trypan blue and 3,3'-diaminobenzidine (DAB) staining showed that after drought and cold treatment, the leaf damage in GhGLK1 overexpressed plants was less as compared with the WT, and the ion permeability was also lower. This study suggested that the GhGLK1 gene may be involved in the regulation of drought and cold stress response in cotton. Our current research findings add significantly to the existing knowledge of cold and drought stress tolerance in cotton.

Salicylic Acid Acts Antagonistically to Plastid Retrograde Signaling by Promoting the Accumulation of Photosynthesis-associated Proteins in Arabidopsis

Plastids are involved in phytohormone metabolism as well as photosynthesis. However, the mechanism by which plastid retrograde signals and phytohormones cooperatively regulate plastid biogenesis remains elusive. Here, we investigated the effects of an inhibitor and a mutation that generate biogenic plastid signals on phytohormones and vice versa. Inhibition of plastid biogenesis by norflurazon (NF) treatment and the plastid protein import2 (ppi2) mutation caused a decrease in salicylic acid (SA) and jasmonic acid (JA). This effect can be attributed in part to the altered expression of genes involved in the biosynthesis and the metabolism of SA and JA. However, SA-dependent induction of the PATHOGENESIS-RELATED1 gene was virtually unaffected in NF-treated plants and the ppi2 mutant. Instead, the level of chlorophyll in these plants was partially restored by the exogenous application of SA. Consistent with this observation, the levels of some photosynthesis-associated proteins increased in the ppi2 and NF-treated plants in response to SA treatment. This regulation in true leaves seems to occur at the posttranscriptional level since SA treatment did not induce the expression of photosynthesis-associated genes. In salicylic acid induction deficient 2 and lesions simulating disease resistance 1 mutants, endogenous SA regulates the accumulation of photosynthesis-associated proteins through transcriptional and posttranscriptional mechanisms. These data indicate that SA acts antagonistically to the inhibition of plastid biogenesis by promoting the accumulation of photosynthesis-associated proteins in Arabidopsis, suggesting a possible link between SA and biogenic plastid signaling.

Identification of the Golden-2-like transcription factors gene family in Gossypium hirsutum

Background

Golden2-Like (GLK) transcription factors are a type of transcriptional regulator in plants. They play a pivotal role in the plant physiological activity process and abiotic stress response.

Methods

In this study, the potential function of GLK family genes in Gossypium hirsutum was studied based on genomic identification, phylogenetic analysis, chromosome mapping and cis-regulatory elements prediction. Gene expression of nine key genes were analyzed by qRT-PCR experiments.

Results

Herein, we identified a total of 146 GhGLK genes in Gossypium hirsutum, which were unevenly distributed on each of the chromosomes. There were significant differences in the number and location of genes between the At sub-genome and the Dt sub-genome. According to the phylogenetic analysis, they were divided into ten subgroups, each of which had very similar number and structure of exons and introns. Some cis-regulatory elements were identified through promoter analysis, including five types of elements related to abiotic stress response, five types of elements related to phytohormone and five types of elements involved in growth and development. Based on public transcriptome data analysis, we identified nine key GhGLKs involved in salt, cold, and drought stress. The qRT-PCR results showed that these genes had different expression patterns under these stress conditions, suggesting that GhGLK genes played an important role in abiotic stress response. This study laid a theoretical foundation for the screening and functional verification of genes related to stress resistance of GLK gene family in cotton.

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.

Application of jasmonic acid at the stage of visible brown necrotic spots in Magnaporthe oryzae infection as a novel and environment-friendly control strategy for rice blast disease

Rice blast disease is one of the most common rice diseases worldwide. It is essential to improve disease resistance through environment-friendly methods, while maintaining yield and quality parameters. In this study, jasmonic acid (JA), a plant hormone with anti-fungal activity, was obtained, at both low (100 μmol/L) and high (400 μmol/L) concentrations in rice leaves, before, during, and after infection, respectively. JA could inhibit germination and appressorium formation of rice blast spores in a dose-dependent manner. A total of 400-μmol/L JA treatment significantly enhanced cell viability and endogenous JA level in rice leaves. Furthermore, rice leaves inoculated with Magnaporthe oryzae and sprayed with JA 72 h post-inoculation showed the maximum symptom relief and the highest endogenous JA production among all treatment approaches. The expressions of defense-related genes, OsPR10a and OsAOS2, were highly up-regulated in response to JA, whereas OsEDS1 was down-regulated. Hence, we revealed that exogenous JA could activate JA signaling to effectively control the symptoms of rice blast.

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.

The transcription factor AtGLK1 acts upstream of MYBL2 to genetically regulate sucrose-induced anthocyanin biosynthesis in Arabidopsis

BACKGROUND

The regulation of anthocyanin biosynthesis by various factors including sugars, light and abiotic stresses is mediated by numerous regulatory factors acting at the transcriptional level. Here experimental evidence was provided in order to demonstrate that the nuclear GARP transcription factor AtGLK1 plays an important role in regulating sucrose-induced anthocyanin biosynthesis in Arabidopsis.

RESULTS

The results obtained using real-time quantitative PCR and GUS staining assays revealed that AtGLK1 was mainly expressed in the green tissues of Arabidopsis seedlings and could be induced by sucrose. The loss-of-function glk1 glk2 double mutant has lower anthocyanin levels than the glk2 single mutant, although it has been determined that loss of AtGLK1 alone does not affect anthocyanin accumulation. Overexpression of AtGLK1 enhances the accumulation of anthocyanin in transgenic Arabidopsis seedlings accompanied by increased expression of anthocyanin biosynthetic and regulatory genes. Moreover, we found that AtGLK1 also participates in plastid-signaling mediated anthocyanin accumulations. Genetic, physiological, and molecular biological approaches demonstrated that AtGLK1 acts upstream of MYBL2, which is a key negative regulator of anthocyanin biosynthesis, to genetically regulate sucrose-induced anthocyanin biosynthesis.

CONCLUSION

Our results indicated that AtGLK1 positively regulates sucrose-induced anthocyanin biosynthesis in Arabidopsis via MYBL2.

Extreme Resistance to Viruses in Potato and Soybean

Plant pathogens, including viruses, negatively impact global crop production. Plants have evolved complex immune responses to pathogens. These responses are often controlled by nucleotide-binding leucine-rich repeat proteins (NLRs), which recognize intracellular, pathogen-derived proteins. Genetic resistance to plant viruses is often phenotypically characterized by programmed cell death at or near the infection site; a reaction termed the hypersensitive response. Although visualization of the hypersensitive response is often used as a hallmark of resistance, the molecular mechanisms leading to the hypersensitive response and associated cell death vary. Plants with extreme resistance to viruses rarely exhibit symptoms and have little to no detectable virus replication or spread beyond the infection site. Both extreme resistance and the hypersensitive response can be activated by the same NLR genes. In many cases, genes that normally provide an extreme resistance phenotype can be stimulated to cause a hypersensitive response by experimentally increasing cellular levels of pathogen-derived elicitor protein(s). The molecular mechanisms of extreme resistance and its relationship to the hypersensitive response are largely uncharacterized. Studies on potato and soybean cultivars that are resistant to strains of Potato virus Y (PVY), Potato virus X (PVX), and Soybean mosaic virus (SMV) indicate that abscisic acid (ABA)-mediated signaling and NLR nuclear translocation are important for the extreme resistance response. Recent research also indicates that some of the same proteins are involved in both extreme resistance and the hypersensitive response. Herein, we review and synthesize published studies on extreme resistance in potato and soybean, and describe studies in additional species, including model plant species, to highlight future research avenues that may bridge the gaps in our knowledge of plant antiviral defense mechanisms.

Generation of a Golden Leaf Triploid Poplar by Repressing the Expression of GLK Genes

Poplar trees are excellent varieties widely used for gardening and greening. However, their single color and floating fluffy seeds are major disadvantages. Plant species or varieties with variegated leaves are desperately needed to meet various demands for gardens, urban greening and landscape decoration, as they produce rich foliage colors that are aesthetically pleasing and functional. In this study, we generated a golden leaf triploid poplar (P. alba × P. berlinensis) by repressing the expression of GLK (Golden2-like) genes in leaves. The triploid golden leaf poplar had reduced chlorophyll content but almost no change in the growth rate. It has great potential in landscaping once it passes safety assessments.

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.

Quantitative Hormone Signaling Output Analyses of Arabidopsis thaliana Interactions With Virulent and Avirulent Hyaloperonospora arabidopsidis Isolates at Single-Cell Resolution

The phytohormones salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) are central regulators of biotic and abiotic stress responses in Arabidopsis thaliana. Here, we generated modular fluorescent protein-based reporter lines termed COLORFUL-PR1pro, -VSP2pro, and -PDF1.2apro. These feature hormone-controlled nucleus-targeted transcriptional output sensors and the simultaneous constitutive expression of spectrally separated nuclear reference and plasma membrane-localized reporters. This set-up allowed the study of cell-type specific hormone activities, cellular viability and microbial invasion. Moreover, we developed a software-supported high-throughput confocal microscopy imaging protocol for output quantification to resolve the spatio-temporal dynamics of respective hormonal signaling activities at single-cell resolution. Proof-of-principle analyses in A. thaliana leaves revealed distinguished hormone sensitivities in mesophyll, epidermal pavement and stomatal guard cells, suggesting cell type-specific regulatory protein activities. In plant-microbe interaction studies, we found that virulent and avirulent Hyaloperonospora arabidopsidis (Hpa) isolates exhibit different invasion dynamics and induce spatio-temporally distinct hormonal activity signatures. On the cellular level, these hormone-controlled reporter signatures demarcate the nascent sites of Hpa entry and progression, and highlight initiation, transduction and local containment of immune signals.

Infection of canola by the root pathogen Plasmodiophora brassicae increases resistance to aboveground herbivory by bertha armyworm, Mamestra configurata Walker (Lepidoptera: Noctuidae)

Infection of plants by pathogens can result in the upregulation of induced defenses; plants may be more or less susceptible to attack by insect herbivores following infection. We investigated the interaction between canola, Brassica napus L., plants infected with clubroot, Plasmodiophora brassicae Woronin, and a generalist herbivore the bertha armyworm (BAW) Mamestra configurata Walker using two canola cultivars that varied in susceptibility to clubroot disease. Volatile organic compounds released from experimental plants differed with infection and female adult BAW could discriminate between canola plants inoculated with P. brassicae and disease-free plants. Adult female moths preferentially laid eggs on disease-free plants of the susceptible cultivar to P. brassicae. Inoculation of resistant canola with P. brassicae, however, did not influence oviposition by female BAW. The fitness of BAW larvae was reduced when they were reared on susceptible canola inoculated with P. brassicae. Salicylic acid and its conjugates in susceptible canola plants were induced following P. brassicae inoculation as compared to disease-free susceptible plants. We conclude that suppression of BAW oviposition and offspring fitness may result in part from a change in the volatile profile of the plant as a result of inoculation and the induction of defenses in inoculated susceptible canola.

A DNA-Binding Bromodomain-Containing Protein Interacts with and Reduces Rx1-Mediated Immune Response to Potato Virus X

Plant NLR proteins enable the immune system to recognize and respond to pathogen attack. An early consequence of immune activation is transcriptional reprogramming. Some NLRs have been shown to act in the nucleus and interact with transcription factors. The Rx1 NLR protein of potato binds and distorts double-stranded DNA. However, the components of the chromatin-localized Rx1 complex are largely unknown. Here, we report a physical and functional interaction between Rx1 and NbDBCP, a bromodomain-containing chromatin-interacting protein. NbDBCP accumulates in the nucleoplasm and nucleolus, interacts with chromatin, and redistributes Rx1 to the nucleolus in a subpopulation of imaged cells. Rx1 overexpression reduces the interaction between NbDBCP and chromatin. NbDBCP is a negative regulator of Rx1-mediated immune responses to potato virus X (PVX), and this activity requires an intact bromodomain. Previously, Rx1 has been shown to regulate the DNA-binding activity of a Golden2-like transcription factor, NbGlk1. Rx1 and NbDBCP act synergistically to reduce NbGlk1 DNA binding, suggesting a mode of action for NbDBCP's inhibitory effect on immunity. This study provides new mechanistic insight into the mechanism by which a chromatin-localized NLR complex co-ordinates immune signaling after pathogen perception.

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.

Loss of GLK1 transcription factor function reveals new insights in chlorophyll biosynthesis and chloroplast development

Birch (Betula platyphylla × B. pendula) is an important tree for landscaping due to its attractive white bark and straight trunk. In this study, we characterized a T-DNA yellow-green leaf mutant, yl. We identified six insertion sites (ISs) in the mutant by genome resequencing and found a 40-kb deletion containing BpGLK1 around IS2 on chromosome 2. Complementation experiments with the yl mutant and repression of BpGLK1 in wild-type plants confirmed that BpGLK1 was responsible for the mutated phenotype. Physiological and ultrastructural analyses showed that the leaves of the yl mutant and BpGLK1-repression lines had decreased chlorophyll content and defective chloroplast development compared to the wild-type. Furthermore, the loss function of BpGLK1 also affected photosynthesis in leaves. Transcriptomics, proteomics, and ChIP-PCR analysis revealed that BpGLK1 directly interacted with the promoter of genes related to antenna proteins, chlorophyll biosynthesis, and photosystem subunit synthesis, and regulated their expression. Overall, our research not only provides new insights into the mechanism of chloroplast development and chlorophyll biosynthesis regulated by BpGLK1, but also provides new transgenic birch varieties with various levels of yellowing leaves by repressing BpGLK1 expression.

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.

Mimicking the Host Regulation of Salicylic Acid: A Virulence Strategy by the Clubroot Pathogen Plasmodiophora brassicae

The plant hormone salicylic acid (SA) plays a critical role in defense against biotrophic pathogens such as Plasmodiophora brassicae, which is an obligate pathogen of crucifer species and the causal agent of clubroot disease of canola (Brassica napus). P. brassicae encodes a protein, predicted to be secreted, with very limited homology to benzoic acid (BA)/SA-methyltransferase, designated PbBSMT. PbBSMT has a SA- and an indole-3-acetic acid-binding domain, which are also present in Arabidopsis thaliana BSMT1 (AtBSMT1) and, like AtBSMT1, has been shown to methylate BA and SA. In support of the hypothesis that P. brassicae uses PbBSMT to overcome SA-mediated defenses by converting SA into inactive methyl salicylate (MeSA), here, we show that PbBSMT suppresses local defense and provide evidence that PbBSMT is much more effective than AtBSMT1 at suppressing the levels of SA and its associated effects. Basal SA levels in Arabidopsis plants that constitutively overexpress PbBSMT compared with those in Arabidopsis wild-type Col-0 (WT) were reduced approximately 80% versus only a 50% reduction in plants overexpressing AtBSMT1. PbBSMT-overexpressing plants were more susceptible to P. brassicae than WT plants; they also were partially compromised in nonhost resistance to Albugo candida. In contrast, AtBSMT1-overexpressing plants were not more susceptible than WT to either P. brassicae or A. candida. Furthermore, transgenic Arabidopsis and tobacco plants overexpressing PbBSMT exhibited increased susceptibility to virulent Pseudomonas syringae pv. tomato DC3000 (DC3000) and virulent Pseudomonas syringae pv. tabaci, respectively. Gene-mediated resistance to DC3000/AvrRpt2 and tobacco mosaic virus (TMV) was also compromised in Arabidopsis and Nicotiana tabacum 'Xanthi-nc' plants overexpressing PbBSMT, respectively. Transient expression of PbBSMT or AtBSMT1 in lower leaves of N. tabacum Xanthi-nc resulted in systemic acquired resistance (SAR)-like enhanced resistance to TMV in the distal systemic leaves. Chimeric grafting experiments revealed that, similar to SAR, the development of a PbBSMT-mediated SAR-like phenotype was also dependent on the MeSA esterase activity of NtSABP2 in the systemic leaves. Collectively, these results strongly suggest that PbBSMT is a novel effector, which is secreted by P. brassicae into its host plant to deplete pathogen-induced SA accumulation.

Uncoupled Expression of Nuclear and Plastid Photosynthesis-Associated Genes Contributes to Cell Death in a Lesion Mimic Mutant

Chloroplast-to-nucleus retrograde signaling is essential for the coupled expression of photosynthesis-associated nuclear genes (PhANGs) and plastid genes (PhAPGs) to ensure the functional status of chloroplasts (Cp) in plants. Although various signaling components involved in the process have been identified in Arabidopsis (Arabidopsis thaliana), the biological relevance of such coordination remains an enigma. Here, we show that the uncoupled expression of PhANGs and PhAPGs contributes to the cell death in the lesion simulating disease1 (lsd1) mutant of Arabidopsis. A daylength-dependent increase of salicylic acid (SA) appears to rapidly up-regulate a gene encoding SIGMA FACTOR BINDING PROTEIN1 (SIB1), a transcriptional coregulator, in lsd1 before the onset of cell death. The dual targeting of SIB1 to the nucleus and the Cps leads to a simultaneous up-regulation of PhANGs and down-regulation of PhAPGs. Consequently, this disrupts the stoichiometry of photosynthetic proteins, especially in PSII, resulting in the generation of the highly reactive species singlet oxygen (¹O₂) in Cps. Accordingly, inactivation of the nuclear-encoded Cp protein EXECUTER1, a putative ¹O₂ sensor, significantly attenuates the lsd1-conferred cell death. Together, these results provide a pathway from the SA- to the ¹O₂-signaling pathway, which are intertwined via the uncoupled expression of PhANGs and PhAPGs, contributing to the lesion-mimicking cell death in lsd1.

The intracellular immune receptor Rx1 regulates the DNA-binding activity of a Golden2-like transcription factor

Plant nucleotide-binding leucine-rich repeat (NLR) proteins enable the immune system to recognize and respond to pathogen attack. An early consequence of immune activation is transcriptional reprogramming, and some NLRs have been shown to act in the nucleus and interact with transcription factors. The Rx1 NLR protein of potato is further able to bind and distort double-stranded DNA. However, Rx1 host targets that support a role for Rx1 in transcriptional reprogramming at DNA are unknown. Here, we report a functional interaction between Rx1 and NbGlk1, a Golden2-like transcription factor. Rx1 binds to NbGlk1 in vitro and in planta. NbGlk1 binds to known Golden2-like consensus DNA sequences. Rx1 reduces the binding affinity of NbGlk1 for DNA in vitro. NbGlk1 activates cellular responses to potato virus X, whereas Rx1 associates with NbGlk1 and prevents its assembly on DNA in planta unless activated by PVX. This study provides new mechanistic insight into how an NLR can coordinate an immune signaling response at DNA following pathogen perceptions.

The world according to GARP transcription factors

Plant specific GARP transcription factor family (made of ARR-B and G2-like) contains genes with very diverse in planta functions: nutrient sensing, root and shoot development, floral transition, chloroplast development, circadian clock oscillation maintenance, hormonal transport and signaling. In this work we review: first, their structural but distant relationships with MYB transcription factors, second, their role in planta, third, the diversity of their Cis-regulatory elements, fourth, their potential protein partners. We conclude that the GARP family may hold keys to understand the interactions between nutritional signaling pathways (nitrogen and phosphate at least) and development. Understanding how plant nutrition and development are coordinated is central to understand how to adapt plants to an ever-changing environment. Consequently GARPs are likely to attract increasing research attentions, as they are likely at the crossroads of these fundamental processes.

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.

PRR2, a pseudo-response regulator, promotes salicylic acid and camalexin accumulation during plant immunity

Calcium signalling mediated by Calmodulin (CaM) and calmodulin-like (CML) proteins is critical to plant immunity. CaM and CML regulate a wide range of target proteins and cellular responses. While many CaM-binding proteins have been identified, few have been characterized for their specific role in plant immunity. Here, we report new data on the biological function of a CML-interacting partner, PRR2 (PSEUDO-RESPONSE REGULATOR 2), a plant specific transcription factor. Until now, the physiological relevance of PRR2 remained largely unknown. Using a reverse genetic strategy in A. thaliana, we identified PRR2 as a positive regulator of plant immunity. We propose that PRR2 contributes to salicylic acid (SA)-dependent responses when challenged with the phytopathogenic bacterium Pseudomonas syringae. PRR2 is transcriptionally upregulated by SA and P. syringae, enhances SA biosynthesis and SA signalling responses; e.g. in response to P. syringae, PRR2 induces the production of SA and the accumulation of the defence-related protein PR1. Moreover, PRR2 overexpressing lines exhibit an enhanced production of camalexin, a phytoalexin that confers enhanced resistance against pathogens. Together, these data reveal the importance of PRR2 in plant immune responses against P. syringae and suggest a novel function for this particular plant specific transcription factor in plant physiology.

Molecular Evolution and Genetic Variation of G2-Like Transcription Factor Genes in Maize

The productivity of maize (Zea mays L.) depends on the development of chloroplasts, and G2-like transcription factors play a central role in regulating chloroplast development. In this study, we identified 59 G2-like genes in the B73 maize genome and systematically analyzed these genes at the molecular and evolutionary levels. Based on gene structure character, motif compositions and phylogenetic analysis, maize G2-like genes (ZmG1- ZmG59) were divided into seven groups (I-VII). By synteny analysis, 18 collinear gene pairs and strongly conserved microsyntny among regions hosting G2-like genes across maize and sorghum were found. Here, we showed that the vast majority of ZmG gene duplications resulted from whole genome duplication events rather than tandem duplications. After gene duplication events, some ZmG genes were silenced. The functions of G2-like genes were multifarious and most genes that are expressed in green tissues may relate to maize photosynthesis. The qRT-PCR showed that the expression of these genes was sensitive to low temperature and drought. Furthermore, we analyzed differences of ZmGs specific to cultivars in temperate and tropical regions at the population level. Interestingly, the single nucleotide polymorphism (SNP) analysis revealed that nucleotide polymorphism associated with different temperature zones. Above all, G2-like genes were highly conserved during evolution, but polymorphism could be caused due to a different geographical location. Moreover, G2-like genes might be related to cold and drought stresses.