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Preview: Plant Cell Advance Online Publication Papers

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.


Phosphorylation of Phosphoenolpyruvate Carboxylase is Essential for Maximal and Sustained Dark CO2 Fixation and Core Circadian Clock Operation in the Obligate Crassulacean Acid Metabolism Species Kalanchoë fedtschenkoi


Phosphoenolpyruvate carboxylase (PPC; EC catalyzes primary nocturnal CO2 fixation in Crassulacean acid metabolism (CAM) species. CAM PPC is regulated post-translationally by a circadian clock controlled protein kinase called phosphoenolpyruvate carboxylase kinase (PPCK). PPCK phosphorylates PPC during the dark period, reducing its sensitivity to feedback inhibition by malate, and thus enhancing nocturnal CO2 fixation to stored malate. Here, we report the generation and characterization of transgenic RNAi lines of the obligate CAM species Kalanchoë fedtschenkoi with reduced levels of KfPPCK1 transcripts. Plants with reduced or no detectable dark phosphorylation of PPC displayed up to a 66% reduction in total dark period CO2 fixation. These perturbations paralleled reduced malate accumulation at dawn and decreased nocturnal starch turnover. Loss of oscillations in the transcript abundance of KfPPCK1 was accompanied by a loss of oscillations in the transcript abundance of many core circadian clock genes, suggesting that perturbing the only known link between CAM and the circadian clock feeds back to perturb the central circadian clock itself. This work shows that clock control of KfPPCK1 prolongs the activity of PPC throughout the dark period in K. fedtschenkoi, optimizing CAM-associated dark CO2 fixation, malate accumulation, CAM productivity and core circadian clock robustness.

Evidence for a Role of ANAC082 as a Ribosomal Stress Response Mediator Leading to Growth Defects and Developmental Alterations in Arabidopsis


Ribosome-related mutants in Arabidopsis thaliana share several notable characteristics regarding growth and development, which implies the existence of a common pathway that responds to disorders in ribosome biogenesis. As a first step to explore this pathway genetically, we screened a mutagenized population of root initiation defective 2 (rid2), a temperature-sensitive mutant that is impaired in pre-rRNA processing, and isolated suppressor of root initiation defective two 1 (sriw1), a suppressor mutant in which the defects of cell proliferation observed in rid2 at the restrictive temperature was markedly rescued. sriw1 was identified as a missense mutation of the NAC transcription factor gene ANAC082. The sriw1 mutation greatly alleviated the developmental abnormalities of rid2 and four other tested ribosome-related mutants, including rid3. The impaired pre-rRNA processing in rid2 and rid3, however, was not relieved by sriw1. Expression of ANAC082 was localized to regions where phenotypic effects of ribosome-related mutations are readily evident, and was elevated in rid2 and rid3 compared to the wild type. These findings suggest that ANAC082 acts downstream of perturbation of biogenesis of the ribosome and may mediate a set of stress responses leading to developmental alterations and cell-proliferation defects.

Root architecture shaping by the environment is orchestrated by dynamic gene expression in space and time


Shaping of root architecture is a quintessential developmental response that involves the concerted action of many different cell types, is highly dynamic and underpins root plasticity. To determine to what extent the environmental regulation of lateral root development is a product of cell type preferential activities, we tracked transcriptomic responses to two different treatments that both change root development in Arabidopsis thaliana, at an unprecedented level of temporal detail. We found that individual transcripts are expressed with a very high degree of temporal and spatial specificity, yet biological processes are commonly regulated, in a mechanism we term response non-redundancy. Using causative gene network inference to compare the genes regulated in different cell types and during responses to nitrogen and a biotic interaction we found that common transcriptional modules often regulate the same gene families, but control different individual members of these families, specific to response and cell type. This reinforces that the activity of a gene cannot be defined simply as molecular function; rather, it is a consequence of spatial location, expression timing and environmental responsiveness.

Plant-specific Histone Deacetylases HDT1/2 Regulate GIBBERELLIN 2-OXIDASE 2 Expression to Control Arabidopsis Root Meristem Cell Number


Root growth is modulated by environmental factors and depends on cell production in the root meristem (RM). New cells in the meristem are generated by stem cells and transit-amplifying cells, which together determine RM cell number. Transcription factors and chromatin-remodelling factors have been implicated in regulating the switch from stem cells to transit-amplifying cells. Here we show that two Arabidopsis thaliana paralogs encoding plant-specific histone deacetylases, HDT1 and HDT2, regulate a second switch from transit-amplifying cells to expanding cells. Knockdown of HDT1/2 (hdt1,2i) results in an earlier switch and causes a reduced RM cell number. Our data show that HDT1/2 negatively regulate the acetylation level of the C19-GIBBERELLIN 2-OXIDASE 2 (GA2ox2) locus and repress the expression of GA2ox2 in the RM and elongation zone. Overexpression of GA2ox2 in the RM phenocopies the hdt1,2i phenotype. Conversely, knockout of GA2ox2 partially rescues the root growth defect of hdt1,2i. These results suggest that by repressing the expression of GA2ox2, HDT1/2 likely fine-tune gibberellin metabolism and they are crucial for regulating the switch from cell division to expansion to determine RM cell number. We propose that HDT1/2 function as part of a mechanism that modulates root growth in response to environmental factors.

Maize NPF6 Proteins Are Homologs of Arabidopsis CHL1 That Are Selective for both Nitrate and Chloride


Nitrate uptake by plant cells requires both high and low-affinity transport activities. Arabidopsis (Arabidopsis thaliana, At) nitrate transporter 1/peptide transporter family (NPF) 6.3 is a dual-affinity plasma membrane transport protein that has both high and low-affinity functions. At-NPF6.3 imports and senses nitrate and is regulated by phosphorylation at Thr-101 (T101). A detailed functional analysis of two maize (Zea mays, Zm) homologs of At-NPF6.3 (Zm-NPF6.6 and Zm-NPF6.4) showed that Zm-NPF6.6 was a pH dependent non-biphasic high-affinity nitrate-specific transport protein. By contrast, maize NPF6.4 was a low-affinity nitrate transporter with efflux activity. When supplied chloride, NPF6.4 switched to a high-affinity chloride selective transporter, while NPF6.6 had only a low-affinity chloride transport activity. Structural predictions identified a nitrate binding His (H362) in NPF6.6 but not in NPF6.4. Mutation of NPF6.4 Tyr-370 to His (Y370H) resulted in saturable high-affinity nitrate transport activity and nitrate selectivity. Loss of H362 in NPF6.6 (H362Y) eliminated both nitrate and chloride transport. Furthermore, alterations to Thr-104, a conserved phosphorylation site in NPF6.6, resulted in a similar high-affinity nitrate transport activity with increased Km whereas equivalent changes in NPF6.4 (T106) disrupted high-affinity chloride transport activity. NPF6 proteins exhibit different substrate specificity in plants and regulate nitrate transport affinity/selectivity using a conserved His residue.

The Maize Imprinted Gene Floury3 Encodes a PLATZ Protein Required for tRNA and 5S rRNA Transcription Through Interaction with RNA Polymerase III


Maize (Zea mays) floury3 (fl3) is a classic semi-dominant negative mutant that exhibits severe defects in the endosperm but fl3 plants otherwise appear normal. We cloned the fl3 gene and determined that it encodes a PLATZ (plant AT-rich sequence- and zinc-binding) protein. The mutation in fl3 resulted in an Asn to His replacement in the conserved PLATZ domain, creating a dominant allele. Fl3 is specifically expressed in starchy endosperm cells and regulated by genomic imprinting, which leads to the suppressed expression of fl3 when transmitted through the male, perhaps as a consequence the semi-dominant behavior. Yeast two-hybrid screening and bimolecular luciferase complementation (BiLC) experiments revealed that FL3 interacts with the RNA polymerase III subunit 53 (RPC53) and transcription factor class C 1 (TFC1), two critical factors of the RNA polymerase III (RNAPIII) transcription complex. In the fl3 endosperm, the levels of many tRNAs and 5S rRNA that are transcribed by RNAPIII are significantly reduced, suggesting that the incorrectly folded fl3 protein may impair the function of RNAPIII. The transcriptome is dramatically altered in fl3 mutants, in which the down-regulated genes are primarily enriched in pathways related to translation, ribosome, misfolded protein responses and nutrient reservoir activity. Collectively, these changes may lead to defects in endosperm development and storage reserve filling in fl3 seeds.

ABA-Induced Stomatal Closure Involves ALMT4, a Phosphorylation-Dependent Vacuolar Anion Channel of Arabidopsis


Stomatal pores are formed between a pair of guard cells, and allow plant uptake of CO2 and water evaporation. Their aperture depends on changes in osmolyte concentration of guard cell vacuoles, specifically of K+ and Mal2-. Efflux of Mal2- from the vacuole is required for stomatal closure; however, it is not clear how the anion is released. Here we report the identification of ALMT4 (ALUMINIUM ACTIVATED MALATE TRANSPORTER 4) as an Arabidopsis thaliana ion channel that can mediate Mal2- release from the vacuole and is required for stomatal closure in response to abscisic acid (ABA). Knock-out mutants showed impaired stomatal closure in response to the drought-stress hormone ABA and increased whole-plant wilting in response to drought and ABA. Electrophysiological data show that ALMT4 can mediate Mal2- efflux and that the channel activity is dependent on a phosphorylatable C-terminal serine. Dephosphomimetic mutants of ALMT4 S382 showed increased channel activity and Mal2- efflux. Reconstituting the active channel in almt4 mutants impaired growth and stomatal opening. Phosphomimetic mutants were electrically inactive and phenocopied the almt4 mutants. Surprisingly, S382 can be phosphorylated by mitogen-activated protein (MAP) kinases in vitro. In brief, ALMT4 likely mediates Mal2- efflux during ABA-induced stomatal closure and its activity depends on phosphorylation.

A Specific Gibberellin 20-oxidase Dictates the Flowering-Runnering Decision in Diploid Strawberry


Asexual and sexual reproduction occur jointly in many angiosperms. Stolons (elongated stems) are used for asexual reproduction in the crop species potato (Solanum tuberosum) and strawberry (Fragaria spp.), where they produce tubers and clonal plants, respectively. In strawberry, stolon production is essential for vegetative propagation at the expense of fruit yield, but the underlying molecular mechanisms are unknown. Here, we show that the stolon-deficiency trait of the runnerless (r) natural mutant in woodland diploid strawberry (Fragaria vesca) is due to a deletion in the active site of a gibberellin (GA) 20-oxidase (GA20ox) gene, which is expressed primarily in the axillary meristem dome and primordia and in developing stolons. This mutation, which is found in all r mutants, goes back more than three centuries. When FveGA20ox4 is mutated, axillary meristems remain dormant or produce secondary shoots terminated by inflorescences, thus increasing the number of inflorescences in the plant. The application of bioactive GA restored the runnering phenotype in the r mutant, indicating that GA biosynthesis in the axillary meristem is essential for inducing stolon differentiation. The possibility of regulating the runnering-flowering decision in strawberry via FveGA20ox4 provides a path for improving productivity in strawberry by controlling the trade-off between sexual reproduction and vegetative propagation.

Plant signaling and metabolic pathways enabling arbuscular mycorrhizal symbiosis


Plants have lived in close association with arbuscular mycorrhizal (AM) fungi for over 400 million years and today this endosymbiosis occurs broadly in the plant kingdom where it has a pronounced impact on plant mineral nutrition. The symbiosis develops deep within the root cortex with minimal alterations in the external appearance of the colonized root; however, the absence of macroscopic alterations belies the extensive signaling, cellular remodeling and metabolic alterations that occur within each colonized cortical cell to enable accommodation of the fungal endosymbiont. Research over the past few years has identified missing calcium channels and potential second messengers in the symbiosis signaling pathway and established connections between this signaling pathway and key transcriptional regulators that direct AM-specific gene expression. Phylogenomics has revealed the existence of genes conserved for AM symbiosis which provide clues as to how plant cells fine-tune their biology to enable symbiosis, and an exciting coalescence of genome mining, lipid profiling and tracer studies collectively led to the conclusion that AM fungi are fatty acid auxotrophs and that plants provide their fungal endosymbionts with fatty acids. Here we provide an overview of the molecular program for AM symbiosis and discuss these recent advances.

General control non-repressible-4 (GCN4) degrades 14-3-3 and RIN4 complex to regulate stomatal aperture with implications on nonhost resistance and drought tolerance


Plants have complex and adaptive innate immune responses against pathogen infections. Stomata are key entry points for many plant pathogens. Both pathogens and plants regulate stomatal aperture for pathogen entry and defense, respectively. Not all plant proteins involved in stomatal aperture regulation have been identified. Here we report general control non-repressible-4 (GCN4), an AAA+-ATPase family protein, as one of the key proteins regulating stomatal aperture during biotic and abiotic stress. Silencing of GCN4 in Nicotiana benthamiana and Arabidopsis compromises host and nonhost disease resistance due to open stomata during pathogen infection. AtGCN4 overexpression plants have reduced H+-ATPase activity, less responsive stomata to coronatine or fusicoccin, have reduced pathogen entry, and confers drought tolerance. This study also demonstrates that AtGCN4 interacts with RIN4 and 14-3-3 proteins and suggest that GCN4 may degrade RIN4 and 14-3-3 proteins via a proteasome mediated pathway to reduce the activity of plasma membrane H+-ATPase complex thus reducing proton pump activity to close stomata.

Journal Impact: Brave New World


The Arabidopsis Leucine-rich Repeat Receptor Kinase BIR3 Negatively Regulates BAK1 Receptor Complex Formation and Stabilizes BAK1


BAK1 is a co-receptor and positive regulator of multiple ligand-binding leucine-rich-repeat receptor kinases (LRR-RKs) and is involved in brassinosteroid (BR)-dependent growth and development, innate immunity and cell death control. The BAK1-interacting LRR-RKs BIR2 and BIR3 were previously identified by proteomics analyses of in vivo BAK1 complexes. Here we show that BAK1-related pathways such as innate immunity and cell death control are affected by BIR3 in Arabidopsis thaliana. BIR3 also has a strong negative impact on BR signaling. BIR3 directly interacts with the BR receptor BRI1 and other ligand-binding receptors and negatively regulates BR signaling by competitive inhibition of BRI1. BIR3 is released from BAK1 and BRI1 after ligand exposure and directly affects the formation of BAK1 complexes with BRI1 or FLAGELLIN SENSING2. Double mutants of bak1 and bir3 show spontaneous cell death and constitutive activation of defense responses. BAK1 and its closest homolog BKK1 interact with and are stabilized by BIR3, suggesting that bak1 bir3 double mutants mimic the spontaneous cell death phenotype observed in bak1 bkk1 mutants via destabilization of BIR3 target proteins. Our results provide evidence for a negative regulatory mechanism for BAK1 receptor complexes in which BIR3 interacts with BAK1 and inhibits ligand-binding receptors to prevent BAK1 receptor complex formation.

Light and Ethylene Coordinately Regulate the Phosphate Starvation Response through Transcriptional Regulation of PHOSPHATE STARVATION RESPONSE1


Plants have evolved an array of adaptive responses to low phosphate (Pi) availability, a process modulated by various external stimuli and endogenous growth regulatory signals. Little is known about how these signaling processes interact to produce an integrated response. Arabidopsis thaliana PHOSPHATE STARVATION RESPONSE1 (PHR1) encodes a conserved MYB type transcription factor that is essential for programming Pi starvation-induced (PSI) gene expression and downstream Pi starvation responses (PSRs). Here, we show that loss-of-function mutations in FHY3 and FAR1, encoding two positive regulators of phytochrome signaling, and in EIN3, encoding a master regulator of ethylene responses, cause attenuated PHR1 expression, whereas mutation in HY5, encoding another positive regulator of light signaling, causes increased PHR1 expression. FHY3, FAR1, HY5 and EIN3 directly bind to the PHR1 promoter through distinct cis-elements. FHY3, FAR1 and EIN3 activate, while HY5 represses, PHR1 expression. FHY3 directly interacts with EIN3, and HY5 suppresses the transcriptional activation activity of FHY3 and EIN3 on PHR1. Finally, both light and ethylene promote FHY3 protein accumulation, and ethylene blocks the light-promoted stabilization of HY5. Our results suggest that light and ethylene coordinately regulate PHR1 expression and PSRs through signaling convergence at the PHR1 promoter.

Genomic analysis of the DNA replication timing program during mitotic S phase in maize (Zea mays L.) root tips


All plants and animals must replicate their DNA, using a regulated process to ensure that their genomes are completely and accurately replicated. DNA replication timing programs have been extensively studied in yeast and animal systems, but much less is known about the replication programs of plants. We report a novel adaptation of the "Repli-seq" assay for use in intact root tips of maize (Zea mays) that include several different cell lineages, and present replication profiles from cells in early, mid, and late S phase of the mitotic cell cycle. This represents the first genomic analysis of replication timing in a crop species, and the first in any plant species to characterize replication at the whole-genome level. Maize root tips have a complex replication timing program, including regions of distinct early, mid and late-S replication that each constitute between 20-24% of the genome, as well as other loci corresponding to ~32% of the genome that exhibit replication activity in two different time windows. Analyses of genomic, transcriptional, and chromatin features of the euchromatic portion of the maize genome provide evidence for a gradient of early replicating, open chromatin that transitions gradually to less open and less transcriptionally active chromatin replicating in mid S phase. Our genomic level analysis also demonstrated that the centromere core replicates in mid S, before heavily compacted classical heterochromatin, including pericentromeres and knobs, which replicate during late S phase.

Architecture and Dynamics of the Jasmonic Acid Gene Regulatory Network


Jasmonic acid (JA) is a critical hormonal regulator of plant growth and defense. To advance our understanding of the architecture and dynamic regulation of the JA gene regulatory network, we performed a high-resolution RNA-Seq time series of methyl JA-treated Arabidopsis thaliana at 15 time points over a 16-h period. Computational analysis showed that MeJA induces a burst of transcriptional activity, generating diverse expression patterns over time that partition into distinct sectors of the JA response targeting specific biological processes. The presence of transcription factor (TF) DNA-binding motifs correlated with specific TF activity during temporal MeJA-induced transcriptional reprogramming. Insight into the underlying dynamic transcriptional regulation mechanisms was captured in a chronological model of the JA gene regulatory network. Several TFs, including MYB59 and bHLH27, were uncovered as early network components with a role in pathogen and insect resistance. Analysis of subnetworks surrounding the TFs ORA47, RAP2.6L, MYB59 and ANAC055, using transcriptome profiling of overexpressors and mutants, provided insights into their regulatory role in defined modules of the JA network. Collectively, our work illuminates the complexity of the JA gene regulatory network, pinpoints and validates previously unknown regulators, and provides a valuable resource for functional studies on JA signaling components in plant defense and development.

An InDel in the Promoter of Al-activated malate transporter 9 Selected During Tomato Domestication Determines Fruit Malate Contents and Aluminum Tolerance


Deciphering the mechanism of malate accumulation in plants would contribute to a greater understanding of plant chemistry, which has implications for improving flavor quality in crop species and enhancing human health benefits. However, the regulation of malate metabolism is poorly understood in crops such as tomato (Solanum lycopersicum). Here, we integrated a metabolite-based genome-wide association study (mGWAS) with linkage mapping and gene functional studies to characterize the genetics of malate accumulation in a global collection of tomato accessions with broad genetic diversity. We report that TFM6 (tomato fruit malate 6), which corresponds to Al-Activated Malate Transporter 9 (Sl-ALMT9 in tomato), is the major quantitative trait locus responsible for variation in fruit malate accumulation among tomato genotypes. A 3-bp indel in the promoter region of Sl-ALMT9 was linked to high fruit malate content. Further analysis indicated that this indel disrupts a W-box binding site in the Sl-ALMT9 promoter, which prevents binding of the WRKY transcription repressor Sl-WRKY42, thereby alleviating the repression of Sl-ALMT9 expression and promoting high fruit malate accumulation. Evolutionary analysis revealed that this highly expressed Sl-ALMT9 allele was selected for during tomato domestication. Furthermore, vacuole membrane-localized Sl-ALMT9 increases in abundance following Al treatment, thereby elevating malate transport and enhancing Al resistance.

The Intracellular Immune Receptor Sw-5b Confers Broad-spectrum Resistance to Tospoviruses through Recognition of a Conserved 21-amino-acid Viral Effector Epitope


Plants use both cell surface-resident pattern-recognition receptors (PRRs) and intracellular nucleotide-binding leucine-rich repeat (NLR) receptors to detect various pathogens. Plant PRRs typically recognize conserved pathogen-associated molecular patterns (PAMPs) to provide broad-spectrum resistance. By contrast, plant NLRs generally detect pathogen strain-specific effectors and confer race-specific resistance. Here, we demonstrate that the tomato NLR Sw-5b confers broad-spectrum resistance against American type tospoviruses by recognizing a conserved 21-amino-acid peptide region within viral movement protein NSm (NSm21). Sw-5b NB-ARC-LRR domains directly associate with NSm21 in vitro and in planta. Domain swap, site-directed mutagenesis and structure modeling analyses identified four polymorphic sites in the Sw-5b LRR domain that are critical for the recognition of NSm21. Furthermore, recognition of NSm21 by Sw-5b likely disturbs the residues adjacent to R927 in the LRR domain to weaken the intramolecular interaction between LRR and NB-ARC domains, thus translating recognition of NSm21 into activation of Sw-5b. Natural variation analysis of Sw-5b homologs from wild tomato species of South America revealed that the four polymorphic sites in the Sw-5b LRR domain were positively selected during evolution and are all necessary to confer resistance to tospovirus. The results described here provide a new example of a plant NLR mediating broad-spectrum resistance through recognition of a small conserved PAMP-like region within the pathogen effector.

Zygotic Genome Activation Occurs Shortly After Fertilization in Maize


The formation of a zygote via the fusion of an egg and sperm cell and its subsequent asymmetric division (ACD) herald the start of the plant's life cycle. Zygotic genome activation (ZGA) is thought to occur gradually, with the initial steps of zygote and embryo development being primarily maternally controlled, and subsequent steps being governed by the zygotic genome. Here, using maize (Zea mays) as a model plant system, we determined the timing of zygote development and generated RNA-Seq transcriptome profiles of gametes, zygotes, and apical and basal daughter cells. ZGA occurs shortly after fertilization and involves about 10% of the genome being activated in a highly dynamic pattern. In particular, genes encoding transcriptional regulators of various families are activated shortly after fertilization. Further analyses suggested that chromatin assembly is strongly modified after fertilization, that the egg cell is primed to activate the translational machinery, and that hormones likely play a minor role in the initial steps of early embryo development in maize. Our findings provide important insights into gamete and zygote activity in plants, and our RNA-Seq transcriptome profiles represent a comprehensive, unique RNA-Seq dataset that can be used by the research community.

Subgenome dominance in an interspecific hybrid, synthetic allopolyploid, and a 140-year- old naturally established neo-allopolyploid monkeyflower


Recent studies have shown that one of the parental subgenomes in ancient polyploids is generally more dominant - having retained more genes and being more highly expressed - a phenomenon termed subgenome dominance. The genomic features that determine how quickly and which subgenome dominates within a newly formed polyploid remain poorly understood. To investigate the rate of emergence of subgenome dominance, we examined gene expression, gene methylation, and transposable element (TE) methylation in a natural, less than 140 year old allopolyploid (Mimulus peregrinus), a resynthesized inter-species triploid hybrid (M. robertsii), a resynthesized allopolyploid (M. peregrinus), and progenitor species (M. guttatus and M. luteus). We show that subgenome expression dominance occurs instantly following the hybridization of divergent genomes and significantly increases over generations. Additionally, CHH methylation levels are reduced in regions near genes and within TEs in the first-generation hybrid, intermediate in the resynthesized allopolyploid, and are repatterned differently between the dominant and recessive subgenomes in the natural allopolyploid. Subgenome differences in levels of TE methylation mirror the increase in expression bias observed over the generations following hybridization. These findings provide important insights into genomic and epigenomic shock that occurs following hybridization and polyploid events, and may also contribute to uncovering the mechanistic basis of heterosis and subgenome dominance.

Phosphorylation of Histone H2A at Serine 95: A Plant-specific Mark Involved in Flowering Time Regulation and H2A.Z Deposition


Phosphorylation of histone H3 affects transcription, chromatin condensation, and chromosome segregation. However, the role of phosphorylation of histone H2A remains unclear. Here, we found that Arabidopsis thaliana MUT9P-LIKE-KINASE (MLK4) phosphorylates histone H2A on serine 95, a plant-specific modification in the histone core domain. Mutations in MLK4 caused late flowering under long-day conditions but no notable phenotype under short days. MLK4 interacts with CIRCADIAN CLOCK ASSOCIATED1 (CCA1), which allows MLK4 to bind to the GIGANTEA (GI) promoter. CCA1 interacts with YAF9a, a co-subunit of the Swi2/Snf2-related ATPase (SWR1) and NuA4 complexes, which are responsible for incorporating the histone variant H2A.Z into chromatin and histone H4 acetylase activity, respectively. Importantly, loss of MLK4 function led to delayed flowering by decreasing phosphorylation of H2A serine 95, along with attenuated accumulation of H2A.Z and the acetylation of H4 at GI, thus reducing GI expression. Together, our results provide insight into how phosphorylation of H2A serine 95 promotes flowering and suggest that phosphorylation of H2A serine 95 modulated by MLK4 is required for the regulation of flowering time and is involved in deposition of the histone variant H2A.Z and H4 acetylation in Arabidopsis.

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.