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Plant Cell Advance Publication Papers

The Plant Cell, published by the American Society of Plant Biologists, has the highest impact factor of primary research journals in plant biology.


Expression of the Nitrate Transporter Gene OsNRT1.1A/OsNPF6.3 Confers High Yield and Early Maturation in Rice


Nitrogen (N) is a major driving force for crop yield improvement, but application of high levels of N delays flowering, prolonging maturation and thus increasing the risk of yield losses. Therefore, traits that enable utilization of high levels of N without delaying maturation will be highly desirable for crop breeding. Here we show that OsNRT1.1A (OsNPF6.3), a member of the rice (Oryza sativa L.) nitrate transporter 1/peptide transporter family, is involved in regulating N utilization and flowering, providing a target to produce high yield and early maturation simultaneously. OsNRT.1A has functionally diverged from previously reported NRT1.1 genes in plants, and functions in up-regulating the expression of N utilization-related genes not only for nitrate but also for ammonium, as well as flowering-related genes. Relative to wild type, osnrt1.1a mutants exhibited reduced N utilization and late flowering. By contrast, overexpression of OsNRT1.1A in rice greatly improved N utilization and grain yield, and maturation time was also significantly shortened. These effects were further confirmed in different rice backgrounds and also in Arabidopsis thaliana. Our study paves a path for the use of a single gene to dramatically increase yield and shorten maturation time for crops, outcomes that promise to substantially increase world food security.

HSI2/VAL1 Silences AGL15 to Regulate the Developmental Transition from Seed Maturation to Vegetative Growth in Arabidopsis


Gene expression during seed development in Arabidopsis thaliana is controlled by transcription factors including LEAFY COTYLEDON 1 and 2 (LEC1 and LEC2), ABA INSENSITIVE 3 (ABI3), FUSCA3 (FUS3), known as LAFL proteins, and AGAMOUS-LIKE 15 (AGL15). The transition from seed maturation to germination and seedling growth requires the transcriptional silencing of these seed maturation-specific factors leading to down-regulation of structural genes including those that encode seed storage proteins, oleosins, and dehydrins. During seed germination and vegetative growth, B3-domain protein HSI2/VAL1 is required for the transcriptional silencing of LAFL genes. Here, we report chromatin immunoprecipitation analysis indicating that HSI2/VAL1 binds to the upstream sequences of the AGL15 gene but not at LEC1, ABI3, FUS3, or LEC2 loci. Functional analysis indicates that the HSI2/VAL1 B3 domain interacts with two RY elements upstream of the AGL15 coding region and at least one of them is required for HSI2/VAL1-dependent AGL15 repression. Expression analysis of the major seed maturation regulatory genes LEC1, ABI3, FUS3 and LEC2 in different genetic backgrounds demonstrates that HSI2/VAL1 is epistatic to AGL15 and represses the seed maturation regulatory program through downregulation of AGL15 by deposition of H3K27me3 at this locus. This hypothesis is further supported by results that show that HSI2/VAL1 physically interacts with the Polycomb Repressive Complex 2 component protein MSI1, which is also enriched at the AGL15 locus.

Crystal structure of plant legumain reveals a unique two-chain state with pH-dependent activity regulation


The vacuolar cysteine protease legumain can cleave and selectively rebuild peptide bonds, thereby vastly expanding the sequential repertoire of biomolecules. In this context, plant legumains have recently at-tracted particular interest. Furthermore, legumains have important roles in many physiological pro-cesses, including programmed cell death. Their efficient peptide bond ligase activity has gained tremen-dous interest in the design of cyclic peptides for drug design. However, the mechanistic understanding of these dual activities is incomplete and partly conflicting. Here we present the crystal structure of a plant legumain, Arabidopsis thaliana isoform- (AtLEG). Employing a conserved legumain fold, the plant legumain AtLEG revealed unique mechanisms of auto-activation, including a plant-specific two-chain activation state, which remains conformationally stable at neutral pH, which is a prerequisite for full ligase activity and survival in different cell compartments. The charge distribution around the α6-helix mediates the pH-dependent dimerization and serves as a gatekeeper for the active site, thus regulating its protease and ligase activity.

LLM-domain B-GATA Transcription Factors Play Multifaceted Roles in Controlling Greening in Arabidopsis


Chlorophyll accumulation and chloroplast development are regulated at multiple levels during plant development. The paralogous LLM-domain B-GATA transcription factors GNC and GNL contribute to chlorophyll biosynthesis and chloroplast formation in light-grown Arabidopsis thaliana seedlings. Whereas there is already ample knowledge about the transcriptional regulation of GNC and GNL, the identity of their downstream targets is largely unclear. Here, we identified genes controlling greening directly downstream of the GATAs by integrating data from RNA-sequencing and microarray datasets. We found that genes encoding subunits of the Mg-chelatase complex and 3,8-divinyl protochlorophyllide a 8-vinyl reductase (DVR) likely function directly downstream of the GATAs and that DVR expression is limiting in the pale-green gnc gnl mutants. The GATAs also regulate the nucleus-encoded SIGMA (SIG) factor genes, which control transcription in the chloroplast and suppress the greening defects of sig mutants. Furthermore, GNC and GNL act, at the gene expression level, in an additive manner with the GLK1 (GOLDEN2-LIKE1) and GLK2 transcription factor genes, which are also important for proper chlorophyll accumulation. We thus reveal that chlorophyll biosynthesis genes are directly controlled by LLM-domain B-GATAs and demonstrate that these transcription factors play an indirect role in the control of greening through regulating SIGMA factor genes.

Targeted profiling of A. thaliana sub-proteomes illuminates new co- and post-translationally N-terminal Myristoylated proteins


N-terminal myristoylation (MYR) is a biologically important co-translational protein lipidation. MYR is difficult to detect in vivo and challenging to predict in silico. We developed a targeted proteomics strategy to identify the Arabidopsis thaliana myristoylated (MYRed) proteome in specific sub-cellular compartments. This global profiling approach allowed us to: (i) identify one third of all open reading frames of the A. thaliana proteome including 54% of the predicted myristoylome; and (ii) establish the first comprehensive plant myristoylome, featuring direct evidence of MYR in 72 proteins. Eighteen MYRed proteins were unexpected, indicating that the in vivo A. thaliana myristoylome extends beyond current predicted sets. A MYR site was also identified downstream of a predicted initiation codon, indicating that post-translational MYR occurs in plants. Over half of the identified proteins could be quantified and assigned to a subcellular compartment. Hierarchical clustering of protein accumulation profiles combined with MYR data and the S-acylated (PALed) proteome revealed that N-terminal double acylation drastically influences protein redirection to the plasma membrane. In a few cases, MYR function extended beyond simple membrane association. This study identified hundreds of N-acylated proteins for which these modifications have the potential to define new protein localization control mechanisms and expand protein function.

Pathogen Trojan horse delivers bioactive host protein to alter maize (Zea mays) anther cell behavior in situ


Small proteins are crucial signals during development, host defense, and physiology. Spatiotemporal restricted functions of signaling proteins remain challenging to study in planta. The several month span required to assess transgene expression, particularly in flowers, combined with the uncertainties from transgene position effects and ubiquitous or overexpression, makes monitoring of spatiotemporally restricted signaling proteins lengthy and difficult. This situation could be rectified with a transient assay in which protein deployment is controlled spatially and temporally in planta to assess protein functions, timing, and cellular targets as well as to facilitate rapid mutagenesis to define functional protein domains. In maize (Zea mays), secreted ZmMAC1 (MULTIPLE ARCHESPORIAL CELLS1) was proposed to trigger somatic niche formation during anther development by participating in a ligand-receptor module. We engineered a protein-delivery system that exploits the secretory capabilities of the corn smut fungus Ustilago maydis, to allow protein delivery to individual cells in certain cell layers at precise time points. Pathogen-supplied ZmMAC1 cell-autonomously corrected somatic cell division and differentiation defects in mutant Zmmac1-1 anthers. These results suggest that exploiting host-pathogen interactions may become a useful method for targeting host proteins to cell and tissue types to clarify cellular autonomy and to analyze steps in cell responses.

TEOSINTE BRANCHED1 Regulates Inflorescence Architecture and Development in Bread Wheat (Triticum aestivum L.)


The flowers of major cereals are arranged on reproductive branches known as spikelets, which group together to form an inflorescence. Diversity for inflorescence architecture has been exploited during domestication to increase crop yields, and genetic variation for this trait has potential to further boost grain production. Multiple genes that regulate inflorescence architecture have been identified by studying alleles that modify gene activity or dosage; however, little is known in wheat. Here, we show TEOSINTE BRANCHED1 (TB1) regulates inflorescence architecture in bread wheat (Triticum aestivum L.) by investigating lines that display a form of inflorescence branching known as 'paired spikelets'. We show that TB1 interacts with FLOWERING LOCUS T1, and that increased dosage of TB1 alters inflorescence architecture and growth rate in a process that includes reduced expression of meristem identity genes, with allelic diversity for TB1 found to associate genetically with paired spikelet development in modern cultivars. We propose TB1 coordinates formation of axillary spikelets during the vegetative to floral transition, and that alleles known to modify dosage or function of TB1 could help increase wheat yields.

Opaque11 Is a Central Hub of the Regulatory Network for Maize Endosperm Development and Nutrient Metabolism


Maize (Zea mays) endosperm is a primary tissue for nutrient storage and is highly differentiated during development. However, the regulatory networks of endosperm development and nutrient metabolism remain largely unknown. Maize opaque11 (o11) is a classic seed mutant with a small and opaque endosperm showing decreased starch and protein accumulation. We cloned O11 and found that it encodes an endosperm-specific bHLH transcription factor (TF). Loss-of-function of O11 significantly affected transcription of carbohydrate/amino acid metabolism and stress-response genes. Genome-wide binding site analysis revealed 9,885 O11-binding sites distributed over 6,033 genes. Using chromatin immunoprecipitation sequencing (ChIP-seq) coupled with RNA sequencing (RNA-seq) assays, we identified 259 O11-modulated target genes. O11 was found to directly regulate key TFs in endosperm development (NKD2 and ZmDof3) and nutrient metabolism (O2 and PBF). Moreover, O11 directly regulates cyPPDKs and multiple carbohydrate metabolic enzymes. O11 is an activator of ZmYODA, suggesting its regulatory function through the MAPK pathway in endosperm development. Many stress-response genes are also direct targets of O11. Moreover, eleven O11-interacting proteins were identified, including ZmICE1, which co-regulates stress-response targets and ZmYODA with O11. Therefore, this study reveals an endosperm regulatory network centered around O11, which coordinates endosperm development, metabolism and stress responses.

Joseph J. Kieber


MAC3A and MAC3B, Two Core Subunits of the MOS4-Associated Complex, Positively Influence miRNA Biogenesis


MAC3A and MAC3B are conserved U-box containing proteins in eukaryotes. They are subunits of the MOS4-associated complex (MAC) that plays essential roles in plant immunity and development in Arabidopsis. However, their functional mechanisms remain elusive. Here we show that Arabidopsis thaliana MAC3A and MAC3B act redundantly in microRNA (miRNA) biogenesis. Lack of both MAC3A and MAC3B in the mac3b mac3b double mutant reduces the accumulation of miRNAs, causing elevated transcript levels of miRNA targets. mac3a mac3b also decreases the levels of primary miRNA transcripts (pri-miRNAs). However, MAC3A and MAC3B do not affect the promoter activity of genes encoding miRNAs (MIR genes), suggesting that they may not affect MIR transcription. This result together with the fact that MAC3A associates with pri-miRNAs in vivo indicates that MAC3A and MAC3B may stabilize pri-miRNAs. Furthermore, we find that MAC3A and MAC3B interact with the DCL1 complex that catalyzes miRNA maturation, promote DCL1 activity and are required for the localization of HYL1, a component of the DCL1 complex. Besides MAC3A and MAC3B, two other MAC subunits, CDC5 and PRL1, also function in miRNA biogenesis. Based on these results, we propose that MAC functions as a complex to control miRNA levels through modulating pri-miRNA transcription, processing and stability.

GRF-interacting factor1 (gif1) Regulates Shoot Architecture and Meristem Determinacy in Maize


Plant architecture results from a balance of indeterminate and determinate cell fates. Cells with indeterminate fates are located in meristems, comprising groups of pluripotent cells that produce lateral organs. Meristematic cells are also found in intercalary stem tissue, which provides cells for internodes, and at leaf margins to contribute to leaf width. We identified a maize (Zea mays) mutant that has a defect in balancing determinacy and indeterminacy. The mutant has narrow leaves and short internodes, suggesting a reduction in indeterminate cells in the leaf and stem. In contrast, the mutants fail to control indeterminacy in shoot meristems. Inflorescence meristems are fasciated, and determinate axillary meristems become indeterminate. Positional cloning identified growth regulating factor-interacting factor1 (gif1) as the responsible gene. gif1 mRNA accumulates in distinct domains of shoot meristems, consistent with tissues affected by the mutation. We determined which GROWTH REGULATING FACTORs (GRFs) interact with GIF1 and carried out RNA-seq analysis. Many genes known to play roles in inflorescence architecture were differentially expressed in gif1. Chromatin immunoprecipitation identified some differentially expressed genes as direct targets of GIF1. The interactions with these diverse direct and indirect targets help explain the paradoxical phenotypes of maize GIF1. These results provide insights into the biological functions of gif1.

Galactoglycerolipid Lipase PGD1 Is Involved in Thylakoid Membrane Remodeling in Response to Adverse Environmental Conditions in Chlamydomonas


Photosynthesis occurs in the thylakoid membrane, where the predominant lipid is monogalactosyldiacylglycerol (MGDG). As environmental conditions change, photosynthetic membranes have to adjust. In this study, we used a loss-of-function Chlamydomonas reinhardtii mutant deficient in the MGDG-specific lipase PGD1 (PLASTID GALACTOGLYCEROLIPID DEGRADATION1) to investigate the link between MGDG turnover, chloroplast ultrastructure, and the production of reactive oxygen species (ROS) in response to different adverse environmental conditions. The pgd1 mutant showed altered MGDG abundance and acyl composition and altered abundance of photosynthesis complexes, with an increased PSII/PSI ratio. Transmission electron microscopy showed hyperstacking of the thylakoid grana in the pgd1 mutant. The mutant also exhibited increased ROS production during N deprivation and high light exposure. Supplementation with bicarbonate or treatment with the photosynthetic electron transport blocker DCMU protected the cells against oxidative stress in the light and reverted chlorosis of pgd1 cells during N deprivation. Furthermore, exposure to stress conditions such as cold and high osmolarity induced the expression of PGD1, and loss of PGD1 in the mutant led to increased ROS production and inhibited cell growth. These findings suggest that PGD1 plays essential roles in maintaining appropriate thylakoid membrane composition and structure, thereby affecting growth and stress tolerance when cells are challenged under adverse conditions.

Defended to the Nines: 25 years of Resistance Gene Cloning Identifies Nine Mechanisms for R Protein Function


Plants display extensive genetic variation at resistance (R) gene loci for resistance to a variety of pathogens. The first R gene, Hm1, was cloned over 25 years ago, and many different R genes have since been identified and isolated. The encoded proteins have provided clues to diverse molecular mechanisms underlying immunity. The majority encode either cell-surface or intracellular receptors, and we present here a meta- analysis of 314 cloned R genes. We distinguish nine molecular mechanisms by which R proteins can elevate or trigger disease resistance. These mechanisms include direct (1) and indirect (2) perception of pathogen-derived molecules on the cell surface by receptor-like proteins and -kinases; intracellular detection of pathogen-derived molecules by nucleotide-binding, leucine-rich repeat receptors (NLRs), either directly (3), indirectly (4) or through integrated domains (5); perception of Transcription Activator-like (TAL) effectors through activation of Executor genes (6); and loss-of-susceptibility, either active (7), passive (8), or by host reprogramming (9). Although the molecular mechanisms underlying the function of R genes are only understood for a small proportion of these, a clearer understanding of mechanisms is emerging and will be crucial for rational engineering and deployment of novel R genes.

A Novel Class of Histone Readers


The biotrophic development of Ustilago maydis studied by RNAseq analysis


The corn smut fungus Ustilago maydis is a model organism for elucidating host colonization strategies of biotrophic fungi. Here we performed an in depth transcriptional profiling of the entire plant-associated development of U. maydis wild-type strains. In our analysis we focused on fungal metabolism, nutritional strategies, secreted effectors and regulatory networks. Secreted proteins were enriched in three distinct expression modules corresponding to stages on the plant surface, establishment of biotrophy and induction of tumors. These modules are likely the key determinants for U. maydis virulence. With respect to nutrient utilization, we observed that expression of several nutrient transporters was tied to these virulence modules rather than being controlled by nutrient availability. We show that oligopeptide transporters likely involved in nitrogen assimilation are important virulence factors. By measuring the intramodular connectivity of transcription factors, we identified the potential drivers for the virulence modules. While known components of the b-mating type cascade emerged as inducers for the plant surface and biotrophy module, we identified a set of yet uncharacterized transcription factors as likely responsible for expression of the tumor module. We demonstrate a crucial role for leaf tumor formation and effector gene expression for one of these transcription factors.

AUXIN RESPONSE FACTOR3 Regulates Floral Meristem Determinacy By Repressing Cytokinin Biosynthesis and Signaling


Successful floral meristem (FM) determinacy is critical for subsequent reproductive development and the plant life cycle. Although the phytohormones cytokinin and auxin interact to coregulate many aspects of plant development, whether and how cytokinin and auxin function in FM determinacy remain unclear. Here we show that in Arabidopsis thaliana, cytokinin homeostasis is critical for FM determinacy. In this developmental context, auxin promotes the expression of AUXIN RESPONSE FACTOR3 (ARF3) to repress cytokinin activity. ARF3 directly represses the expression of ISOPENTENYLTRANSFERASE (IPT) family genes and indirectly represses LONELY GUY (LOG) family genes, both of which encode enzymes required for cytokinin biosynthesis. ARF3 also directly inhibits the expression of ARABIDOPSIS HISTIDINE KINASE4 (AHK4), a cytokinin receptor gene, resulting in reduced cytokinin activity. Consequently, ARF3 controls cell division by regulating cell cycle gene expression through cytokinin. In flowers, we show that AGAMOUS (AG) dynamically regulates the expression of ARF3 and IPTs, resulting in coordinated regulation of FM maintenance and termination through cell division. Moreover, genome-wide transcriptional profiling revealed both repressive and active roles for ARF3 in early flower development. Our findings establish a molecular link between AG and auxin/cytokinin and shed light on the mechanisms of stem cell maintenance and termination in the FM.

Inter-Regulation of CDKA/CDK1 and the Plant-Specific Cyclin-Dependent Kinase CDKB in Control of the Chlamydomonas Cell Cycle


The cyclin-dependent kinase CDK1 is essential for mitosis in fungi and animals. Plant genomes contain the CDK1 ortholog CDKA, and a plant kingdom-specific relative, CDKB. The green alga Chlamydomonas reinhardtii has a long G1 growth period followed by rapid cycles of DNA replication and cell division. We show that null alleles of CDKA extend the growth period prior to the first division cycle and modestly extend the subsequent division cycles, but do not prevent cell division, indicating at most a minor role for the CDK1 ortholog in mitosis in Chlamydomonas. A null allele of cyclin A has a similar though less extreme phenotype. In contrast, both CDKB and cyclin B are essential for mitosis. CDK kinase activity measurements imply that the predominant in vivo complexes are probably cyclin A-CDKA and cyclin B-CDKB. We propose a negative feedback loop: CDKA activates cyclin B-CDKB. Cyclin B-CDKB in turn promotes mitotic entry and inactivates cyclin A-CDKA. Cyclin A-CDKA and cyclin B-CDKB may redundantly promote DNA replication. We show that the anaphase-promoting complex is required for inactivation of both CDKA and CDKB and is essential for anaphase. These results are consistent with findings in Arabidopsis and may delineate the core of plant kingdom cell cycle control which, compared to the well-studied yeast and animal systems, exhibits deep conservation in some respects and striking divergence in others.

Role of the Nod Factor Hydrolase MtNFH1 in Regulating Nod Factor Levels during Rhizobial Infection and in Mature Nodules of Medicago truncatula


Establishment of symbiosis between legumes and nitrogen-fixing rhizobia depends on bacterial Nod factors (NFs) that trigger symbiosis-related NF signaling in host plants. NFs are modified oligosaccharides of chitin with a fatty acid moiety. NFs can be cleaved and inactivated by host enzymes, such as MtNFH1 (MEDICAGO TRUNCATULA NOD FACTOR HYDROLASE 1). In contrast to related chitinases, MtNFH1 hydrolyzes neither chitin nor chitin fragments, indicating a high cleavage preference for NFs. Here, we provide evidence for a role of MtNFH1 in the symbiosis with Sinorhizobium meliloti. Upon rhizobial inoculation, MtNFH1 accumulated at the curled tip of root hairs, in the so-called infection chamber. Mutant analysis revealed that lack of MtNFH1 delayed rhizobial root hair infection, suggesting that excess amounts of NFs negatively affect the initiation of infection threads. MtNFH1 deficiency resulted in nodule hypertrophy and abnormal nodule branching of young nodules. Nodule branching was also stimulated in plants expressing MtNFH1 driven by a tandem CaMV 35S promoter and plants inoculated by a NF-overproducing S. meliloti strain. We suggest that fine-tuning of NF levels by MtNFH1 is necessary for optimal root hair infection as well as for NF-regulated growth of mature nodules.

Arabidopsis thaliana FANCD2 Promotes Meiotic Crossover Formation


Fanconi anemia (FA) is a human autosomal recessive disorder characterized by chromosomal instability, developmental pathologies, predisposition to cancer and reduced fertility. So far, nineteen genes have been implicated in FA, most of them involved in DNA repair. Some are conserved across higher eukaryotes, including plants. The Arabidopsis thaliana genome encodes a homologue of the Fanconi anemia D2 gene (FANCD2) whose function in DNA repair is not yet fully understood. Here we provide evidence that AtFANCD2 is required for meiotic homologous recombination. Meiosis is a specialized cell division that ensures reduction of genomic content by half and DNA exchange between homologous chromosomes via crossovers (COs) prior to gamete formation. In plants, a mutation in AtFANCD2 results in a 14% reduction of CO numbers. Genetic analysis demonstrated that AtFANCD2 acts in parallel to both MUTS HOMOLOG 4 (AtMSH4), known for its role in promoting interfering COs and MMS AND UV SENSITIVE 81 (AtMUS81), known for its role in the formation of non-interfering COs. AtFANCD2 promotes non-interfering COs in a MUS81-independent manner and is therefore part of an uncharted meiotic CO-promoting mechanism, in addition to those described previously.

GIF Transcriptional Co-regulators Control Root Meristem Homeostasis


In the root meristem, the quiescent center (QC) is surrounded by stem cells, which in turn generate the different cell types of the root. QC cells rarely divide under normal conditions but can replenish damaged stem cells. In the proximal meristem, the daughters of stem cells, which are referred to as transit amplifying cells, undergo additional rounds of cell division prior to differentiation. Here, we describe the functions of GRF-INTERACTING FACTORs (GIFs), including ANGUSTIFOLIA3 (AN3), in Arabidopsis thaliana roots. GIFs have been shown to interact with GRF transcription factors and SWI/SNF chromatin remodeling complexes. We found that combinations of GIF mutants cause the loss of QC identity. However, despite their QC impairment, GIF mutants have a significantly enlarged root meristem with additional lateral root cap layers. We show that the increased expression of PLETHORA1 (PLT1) is at least partially responsible for the large root meristems of an3 mutants. Furthermore, we found that GIFs are necessary for maintaining the precise expression patterns of key developmental regulators and that AN3 complexes bind directly to the promoter regions of PLT1 as well as SCARECROW. We propose that AN3/GIFs participate in different pathways that control QC organization and the size of the meristem.

Snf1-RELATED KINASE1-Controlled C/S1-bZIP Signaling Activates Alternative Mitochondrial Metabolic Pathways to Ensure Plant Survival in Extended Darkness


Sustaining energy homeostasis is of pivotal importance for all living organisms. In Arabidopsis thaliana, evolutionarily conserved SnRK1 kinases (Snf1-RELATED KINASE1) control metabolic adaptation during low energy stress. To unravel starvation-induced transcriptional mechanisms, we performed transcriptome studies of inducible knock-down lines and found that S1-basic leucine Zipper transcription factors (S1-bZIPs) control a defined subset of genes downstream of SnRK1. For example, S1-bZIPs coordinate the expression of genes involved in branched-chain amino acid catabolism, which constitutes an alternative mitochondrial respiratory pathway that is crucial for plant survival during starvation. Molecular analyses defined S1-bZIPs as SnRK1-dependent regulators that directly control transcription via binding to G-box promoter elements. Moreover, SnRK1 triggers phosphorylation of group C-bZIPs and the formation of C/S1-heterodimers and thus, the recruitment of SnRK1 directly to target promoters. Subsequently, the C/S1-bZIP-SnRK1 complex interacts with the histone acetylation machinery to remodel chromatin and facilitate transcription. Taken together, this work reveals molecular mechanisms underlying how energy deprivation is transduced to reprogram gene expression, leading to metabolic adaptation upon stress.

A Role for MINIYO and QUATRE-QUART 2 in the Assembly of RNA Polymerases II, IV and V in Arabidopsis


RNA polymerases IV and V (Pol IV and Pol V) are required for the generation of noncoding RNAs in RNA-directed DNA methylation (RdDM). Their subunit compositions resemble that of Pol II. The mechanism and accessory factors involved in their assembly remain largely unknown. In this study, we identified mutant alleles of MINIYO (IYO), QUATRE-QUART 2 (QQT2) and NUCLEAR RNA POLYMERASE B11/D11/E11 (NRPB/D/E11) that cause defects in RdDM in Arabidopsis. We found that Pol IV-dependent small interfering RNAs (siRNAs) and Pol V-dependent transcripts were greatly reduced in the mutants. NRPE1, the largest subunit of Pol V, failed to associate with other Pol V subunits in the iyo and qqt2 mutants, suggesting the involvement of IYO and QQT2 in Pol V assembly. In addition, we found that IYO and QQT2 were mutually dependent for their association with the NRPE3 subassembly prior to the assembly of Pol V holoenzyme. Finally, we show that IYO and QQT2 are similarly required for the assembly of Pol II and Pol IV. Our findings reveal IYO and QQT2 as cofactors for the assembly of Pol II, Pol IV and Pol V and provide mechanistic insights into how RNA polymerases are assembled in plants.

S-type Anion Channels SLAC1 and SLAH3 Function as Essential Negative Regulators of Inward K+ Channels and Stomatal Opening in Arabidopsis


Drought stress induces stomatal closure and inhibits stomatal opening simultaneously. However, the underlying molecular mechanism is still largely unknown. Here we show that S-type anion channels SLAC1 and SLAH3 mainly inhibit inward K+ (K+in) channel KAT1 by protein-protein interaction, and consequently prevent stomatal opening in Arabidopsis. Voltage-clamp results demonstrated that SLAC1 inhibited KAT1 dramatically, but did not inhibit KAT2. SLAH3 inhibited KAT1 to a weaker degree relative to SLAC1. Both the N terminus and the C terminuses of SLAC1 inhibited KAT1, but the inhibition by the N terminus was stronger. The C terminus was essential for the inhibition of KAT1 by SLAC1. Furthermore, drought stress strongly up-regulated the expression of SLAC1 and SLAH3 in Arabidopsis guard cells, and the over-expression of wild type and truncated SLAC1 dramatically impaired K+in currents of guard cells and light-induced stomatal opening. Additionally, the inhibition of KAT1 by SLAC1 and KC1 only partially overlapped, suggesting that SLAC1 and KC1 inhibited K+in channels using different molecular mechanisms. Taken together, we discovered a novel regulatory mechanism for stomatal movement, in which singling pathways for stomatal closure and opening are directly coupled together by protein-protein interaction between SLAC1/SLAH3 and KAT1 in Arabidopsis.