Formation and dissociation of the BSS1 protein complex regulates plant development via brassinosteroid signaling.

Brassinosteroids (BRs) play important roles in plant development and the response to environmental cues. BIL1/BZR1 is a master transcription factor in BR signaling, but the mechanisms that lead to the finely tuned targeting of BIL1/BZR1 by BRs are unknown. Here, we identified BRZ-SENSITIVE-SHORT HYPOCOTYL1 (BSS1) as a negative regulator of BR signaling in a chemical-biological analysis involving brassinazole (Brz), a specific BR biosynthesis inhibitor. The bss1-1D mutant, which overexpresses BSS1, exhibited a Brz-hypersensitive phenotype in hypocotyl elongation. BSS1 encodes a BTB-POZ domain protein with ankyrin repeats, known as BLADE ON PETIOLE1 (BOP1), which is an important regulator of leaf morphogenesis. The bss1-1D mutant exhibited an increased accumulation of phosphorylated BIL1/BZR1 and a negative regulation of BR-responsive genes. The number of fluorescent BSS1/BOP1-GFP puncta increased in response to Brz treatment, and the puncta were diffused by BR treatment in the root and hypocotyl. We show that BSS1/BOP1 directly interacts with BIL1/BZR1 or BES1. The large protein complex formed between BSS1/BOP1 and BIL1/BZR1 was only detected in the cytosol. The nuclear BIL1/BZR1 increased in the BSS1/BOP1-deficient background and decreased in the BSS1/BOP1-overexpressing background. Our study suggests that the BSS1/BOP1 protein complex inhibits the transport of BIL1/BZR1 to the nucleus from the cytosol and negatively regulates BR signaling.

Identification of an abscisic acid transporter by functional screening using the receptor complex as a sensor.

Movement of the plant hormone abscisic acid (ABA) within plants has been documented; however, the molecular mechanisms that regulate ABA transport are not fully understood. By using a modified yeast two-hybrid system, we screened Arabidopsis cDNAs capable of inducing interactions between the ABA receptor PYR/PYL/RCAR and PP2C protein phosphatase under low ABA concentrations. By using this approach, we identified four members of the NRT1/PTR family as candidates for ABA importers. Transport assays in yeast and insect cells demonstrated that at least one of the candidates ABA-IMPORTING TRANSPORTER (AIT) 1, which had been characterized as the low-affinity nitrate transporter NRT1.2, mediates cellular ABA uptake. Compared with WT, the ait1/nrt1.2 mutants were less sensitive to exogenously applied ABA during seed germination and/or postgermination growth, whereas overexpression of AIT1/NRT1.2 resulted in ABA hypersensitivity in the same conditions. Interestingly, the inflorescence stems of ait1/nrt1.2 had a lower surface temperature than those of the WT because of excess water loss from open stomata. We detected promoter activities of AIT1/NRT1.2 around vascular tissues in inflorescence stems, leaves, and roots. These data suggest that the function of AIT1/NRT1.2 as an ABA importer at the site of ABA biosynthesis is important for the regulation of stomatal aperture in inflorescence stems.

A novel mitochondrial DnaJ/Hsp40 family protein BIL2 promotes plant growth and resistance against environmental stress in brassinosteroid signaling.

Plant steroid hormones, brassinosteroids, are essential for growth, development and responses to environmental stresses in plants. Although BR signaling proteins are localized in many organelles, i.e., the plasma membrane, nuclei, endoplasmic reticulum and vacuole, the details regarding the BR signaling pathway from perception at the cellular membrane receptor BRASSINOSTEROID INSENSITIVE 1 (BRI1) to nuclear events include several steps. Brz (Brz220) is a specific inhibitor of BR biosynthesis. In this study, we used Brz-mediated chemical genetics to identify Brz-insensitive-long hypocotyls 2-1D (bil2-1D). The BIL2 gene encodes a mitochondrial-localized DnaJ/Heat shock protein 40 (DnaJ/Hsp40) family, which is involved in protein folding. BIL2-overexpression plants (BIL2-OX) showed cell elongation under Brz treatment, increasing the growth of plant inflorescence and roots, the regulation of BR-responsive gene expression and suppression against the dwarfed BRI1-deficient mutant. BIL2-OX also showed resistance against the mitochondrial ATPase inhibitor oligomycin and higher levels of exogenous ATP compared with wild-type plants. BIL2 participates in resistance against salinity stress and strong light stress. Our results indicate that BIL2 induces cell elongation during BR signaling through the promotion of ATP synthesis in mitochondria.

The chloroplast protein BPG2 functions in brassinosteroid-mediated post-transcriptional accumulation of chloroplast rRNA.

Brassinazole (Brz) is a specific inhibitor of the biosynthesis of brassinosteroids (BRs), which regulate plant organ and chloroplast development. We identified a recessive pale green Arabidopsis mutant, bpg2-1 (Brz-insensitive-pale green 2-1) that showed reduced sensitivity to chlorophyll accumulation promoted by Brz in the light. BPG2 encodes a chloroplast-localized protein with a zinc finger motif and four GTP-binding domains that are necessary for normal chloroplast biogenesis. BPG2-homologous genes are evolutionally conserved in plants, green algae and bacteria. Expression of BPG2 is induced by light and Brz. Chloroplasts of the bpg2-1 mutant have a decreased number of stacked grana thylakoids. In bpg2-1 and bpg2-2 mutants, there was no reduction in expression of rbcL and psbA, but there was abnormal accumulation of precursors of chloroplast 16S and 23S rRNA. Chloroplast protein accumulation induced by Brz was suppressed by the bpg2 mutation. These results indicate that BPG2 plays an important role in post-transcriptional and translational regulation in the chloroplast, and is a component of BR signaling.

CNI1/ATL31, a RING-type ubiquitin ligase that functions in the carbon/nitrogen response for growth phase transition in Arabidopsis seedlings.

Plants are able to sense and respond to changes in the balance between carbon (C) and nitrogen (N) metabolite availability, known as the C/N response. During the transition to photoautotrophic growth following germination, growth of seedlings is arrested if a high external C/N ratio is detected. To clarify the mechanisms for C/N sensing and signaling during this transition period, we screened a large collection of FOX transgenic plants, overexpressing full-length cDNAs, for individuals able to continue post-germinative growth under severe C/N stress. One line, cni1-D (carbon/nitrogen insensitive 1-dominant), was shown to have a suppressed sensitivity to C/N conditions at both the physiological and molecular level. The CNI1 cDNA encoded a predicted RING-type ubiquitin ligase previously annotated as ATL31. Overexpression of ATL31 was confirmed to be responsible for the cni1-D phenotype, and a knock-out of this gene resulted in hypersensitivity to C/N conditions during post-germinative growth. The ATL31 protein was confirmed to contain ubiquitin ligase activity using an in vitro assay system. Moreover, removal of this ubiquitin ligase activity from the overexpressed protein resulted in the loss of the mutant phenotype. Taken together, these data demonstrated that CNI1/ATL31 activity is required for the plant C/N response during seedling growth transition.

Chemical genetics reveal the novel transmembrane protein BIL4, which mediates plant cell elongation in brassinosteroid signaling.

Steroid hormones are conserved between animals and plants as signaling molecules to control growth and development. Plant steroid hormones, brassinosteroids (BRs), appear to play an important role in plant cell elongation. BRs bind to leucine-rich repeat kinase BRASSINOSTEROID-INSENSITIVE 1 (BRI1) localized to the plasma membrane, activate transcription factors in collaboration with cytosolic kinases and phosphatases, and regulate BR-responsive gene expression, but the details regarding the BR signaling pathway from perception to nuclear events remain unknown. In this study we used chemical genetics to identify an evolutionarily conserved transmembrane protein, Brz-insensitive-long hypocotyls 4 (BIL4), and demonstrated its role as a critical component of plant cell elongation occurring upon BR signaling. A dominant mutation, bil4-1D, showed cell elongation in the presence of the BR-specific inhibitor Brz. Brz suppresses expression of the BIL4 gene in wild-type plants, and overexpression of BIL4 in bil4-1D suppresses the BR deficiency caused by Brz. Our results indicate that BIL4 mediates cell elongation on BR signaling.

DEAR1, a transcriptional repressor of DREB protein that mediates plant defense and freezing stress responses in Arabidopsis.

Plants have evolved intricate mechanisms to respond and adapt to a wide variety of biotic and abiotic stresses in their environment. The Arabidopsis DEAR1 (DREB and EAR motif protein 1; At3g50260) gene encodes a protein containing significant homology to the DREB1/CBF (dehydration-responsive element binding protein 1/C-repeat binding factor) domain and the EAR (ethylene response factor-associated amphiphilic repression) motif. We show here that DEAR1 mRNA accumulates in response to both pathogen infection and cold treatment. Transgenic Arabidopsis overexpressing DEAR1 (DEAR1ox) showed a dwarf phenotype and lesion-like cell death, together with constitutive expression of PR genes and accumulation of salicylic acid. DEAR1ox also showed more limited P. syringae pathogen growth compared to wild-type, consistent with an activated defense phenotype. In addition, transient expression experiments revealed that the DEAR1 protein represses DRE/CRT (dehydration-responsive element/C-repeat)-dependent transcription, which is regulated by low temperature. Furthermore, the induction of DREB1/CBF family genes by cold treatment was suppressed in DEAR1ox, leading to a reduction in freezing tolerance. These results suggest that DEAR1 has an upstream regulatory role in mediating crosstalk between signaling pathways for biotic and abiotic stress responses.

Objective evaluation measures of genetic marker selection in large-scale SNP genotyping.

High-throughput single nucleotide polymorphism (SNP) genotyping systems provide two kinds of fluorescent signals detected from different alleles. In current technologies, the process of genotype discrimination requires subjective judgments by expert operators, even when using clustering algorithms. Here, we propose two evaluation measures to manage fluorescent scatter data with nonclear plot aggregation. The first is the marker ranking measure, which provides a ranking system for the SNP markers based on the distance between the scatter plot distribution and a user-defined ideal distribution. The second measure, called individual genotype membership, uses the membership probability of each genotype related to an individual plot in the scatter data. In verification experiments, the marker ranking measure determined the ranking of SNP markers correlated with the subjective order of SNP markers judged by an expert operator. The experiment using the individual genotype membership measure clarified that the total number of unclassified individuals was remarkably reduced compared to that of manually unclassified ones. These two evaluation measures were implemented as the GTAssist software. GTAssist provides objective standards and avoids subjective biases in SNP genotyping workflows.

Identification of stress-tolerance-related transcription-factor genes via mini-scale Full-length cDNA Over-eXpressor (FOX) gene hunting system.

Recently, we developed a novel system known as Full-length cDNA Over-eXpressor (FOX) gene hunting [T. Ichikawa, M. Nakazawa, M. Kawashima, H. Iizumi, H. Kuroda, Y. Kondou, Y. Tsuhara, K. Suzuki, A. Ishikawa, M. Seki, M. Fujita, R. Motohashi, N. Nagata, T. Takagi, K. Shinozaki, M. Matsui, The FOX hunting system: an alternative gain-of-function gene hunting technique, Plant J. 48 (2006) 974-985], which involves the random overexpression of a normalized Arabidopsis full-length cDNA library. While our system allows large-scale collection of full-length cDNAs for gene discovery, we sought to downsize it to analyze a small pool of full-length cDNAs. As a model system, we focused on stress-inducible transcription factors. The full-length cDNAs of 43 stress-inducible transcription factors were mixed to create a transgenic plant library. We screened for salt-stress-resistant lines in the T1 generation and identified a number of salt-tolerant lines that harbored the same transgene (F39). F39 encodes a bZIP-type transcription factor that is identical to AtbZIP60, which is believed to be involved in the endoplasmic reticulum stress response. Microarray analysis revealed that a number of stress-inducible genes were up-regulated in the F39-overexpressing lines, suggesting that AtbZIP60 is involved in stress signal transduction. Thus, our mini-scale FOX system may be used to screen for genes with valuable functions, such as transcription factors, from a small pool of genes that show similar expression profiles.

Evolutionarily conserved BIL4 suppresses the degradation of brassinosteroid receptor BRI1 and regulates cell elongation.

Brassinosteroids (BRs), plant steroid hormones, play important roles in plant cell elongation and differentiation. To investigate the mechanisms of BR signaling, we previously used the BR biosynthesis inhibitor Brz as a chemical biology tool and identified the Brz-insensitive-long hypocotyl4 mutant (bil4). Although the BIL4 gene encodes a seven-transmembrane-domain protein that is evolutionarily conserved in plants and animals, the molecular function of BIL4 in BR signaling has not been elucidated. Here, we demonstrate that BIL4 is expressed in early elongating cells and regulates cell elongation in Arabidopsis. BIL4 also activates BR signaling and interacts with the BR receptor brassinosteroid insensitive 1 (BRI1) in endosomes. BIL4 deficiency increases the localization of BRI1 in the vacuoles. Our results demonstrate that BIL4 regulates cell elongation and BR signaling via the regulation of BRI1 localization.

In silico phenotypic screening method of mutants based on statistical modeling of genetically mixed samples.

In comprehensive functional genomics projects, systematic analysis of phenotypes is vital. However, conventional phenotypic screening is done mainly by imprecise visual observation of qualitative traits, and, therefore, in silico screening techniques for quantitative traits are required. In this report, we propose in silico phenotypic screening method that utilizes a Gaussian mixture model for the trait distribution in the offspring of a mutagenized line and the likelihood ratio test between the estimated Gaussian mixture model and the wild-type single Gaussian model. In order to evaluate the proposed method, we performed a screening experiment using real trait data of Arabidopsis. In this experiment, the proposed screening method properly distinguished the mutant line from the wild-type line. Furthermore, we conducted power analysis of the proposed method and two conventional methods under various simulated conditions of sample size and distribution of trait frequency. The result of the power analysis confirmed the effectiveness of the proposed method compared to the conventional methods.

Three-dimensional definition of leaf morphological traits of Arabidopsis in silico phenotypic analysis.

The detection of phenotypic alterations of mutants and variants is one of the bottlenecks that hinder systematic gene functional studies of the model plant Arabidopsis. In an earlier study, we have addressed this problem by proposing a novel methodology for phenome analysis based on in silico analysis of polygon models that are acquired by 3-dimensional (3D) measurement and which precisely reconstruct the actual plant shape. However, 3D quantitative descriptions of morphological traits are rare, whereas conventional 2D descriptions have already been studied but may lack the necessary precision. In this report, we focus on six major leaf morphological traits, which are commonly used in the current manual mutant screens, and propose new 3D quantitative definitions that describe these traits. In experiments to extract the traits, we found significant differences between two variants of Arabidopsis with respect to blade roundness and blade epinasty. Remarkably, the detected difference between variants in the blade roundness trait was undetectable when using conventional 2D descriptions. Thus, the result of the experiment indicates that the proposed definitions with 3D description may lead to new discoveries of phenotypic alteration in gene functional studies that would not be possible using conventional 2D descriptions.

Characterisation of hexokinase in Toxoplasma gondii tachyzoites.

We have cloned the hexokinase [E.C. 2.7.1.1] gene of Toxoplasma gondii tachyzoite and obtained an active recombinant enzyme with a calculated molecular mass of 51,465Da and an isoelectric point of 5.82. Southern blot analysis indicated that the hexokinase gene existed as a single copy in the tachyzoites of T. gondii. The sequence of T. gondii hexokinase exhibited the highest identity (44%) to that of Plasmodium falciparum hexokinase and lower identity of less than 35% to those of hexokinases from other organisms. The specific activity of the homogeneously purified recombinant enzyme was 4.04 micromol/mg protein/min at 37 degrees C under optimal conditions. The enzyme could use glucose, fructose, and mannose as substrates, though it preferred glucose. Adenosine triphosphate was exclusively the most effective phosphorus donor, and pyrophosphate did not serve as a substrate. K(m) values for glucose and adenosine triphosphate were 8.0+/-0.8 microM and 1.05+/-0.25mM, respectively. No allosteric effect of substrates was observed, and the products, glucose 6-phosphate and adenosine diphosphate, had no inhibitory effect on T. gondii hexokinase activity. Other phosphorylated hexoses, fructose 6-phosphate, trehalose 6-phosphate which is an inhibitor of yeast hexokinase, and pyrophosphate, also did not affect T. gondii hexokinase activity. Native hexokinase activity was recovered in both the cytosol and membrane fractions of the whole lysate of T. gondii tachyzoites. This result suggests that T. gondii hexokinase weakly associates with the membrane or particulate fraction of the tachyzoite cell.

FOX-superroots of Lotus corniculatus, overexpressing Arabidopsis full-length cDNA, show stable variations in morphological traits.

Using the full-length cDNA overexpressor (FOX) gene-hunting system, we have generated 130 Arabidopsis FOX-superroot lines in bird's-foot trefoil (Lotus corniculatus) for the systematic functional analysis of genes expressed in roots and for the selection of induced mutants with interesting root growth characteristics. We used the Arabidopsis-FOX Agrobacterium library (constructed by ligating pBIG2113SF) for the Agrobacterium-mediated transformation of superroots (SR) and the subsequent selection of gain-of-function mutants with ectopically expressed Arabidopsis genes. The original superroot culture of L. corniculatus is a unique host system displaying fast root growth in vitro, allowing continuous root cloning, direct somatic embryogenesis and mass regeneration of plants under entirely hormone-free culture conditions. Several of the Arabidopsis FOX-superroot lines show interesting deviations from normal growth and morphology of roots from SR-plants, such as differences in pigmentation, growth rate, length or diameter. Some of these mutations are of potential agricultural interest. Genomic PCR analysis revealed that 100 (76.9%) out of the 130 transgenic lines showed the amplification of single fragments. Sequence analysis of the PCR fragments from these 100 lines identified full-length cDNA in 74 of them. Forty-three out of 74 full-length cDNA carried known genes. The Arabidopsis FOX-superroot lines of L. corniculatus, produced in this study, expand the FOX hunting system and provide a new tool for the genetic analysis and control of root growth in a leguminous forage plant.

RETARDED GROWTH OF EMBRYO1, a new basic helix-loop-helix protein, expresses in endosperm to control embryo growth.

We have isolated two dominant mutants from screening approximately 50,000 RIKEN activation-tagging lines that have short inflorescence internodes. The activation T-DNAs were inserted near a putative basic helix-loop-helix (bHLH) gene and expression of this gene was increased in the mutant lines. Overexpression of this bHLH gene produced the original mutant phenotype, indicating it was responsible for the mutants. Specific expression was observed during seed development. The loss-of-function mutation of the RETARDED GROWTH OF EMBRYO1 (RGE1) gene caused small and shriveled seeds. The embryo of the loss-of-function mutant showed retarded growth after the heart stage although abnormal morphogenesis and pattern formation of the embryo and endosperm was not observed. We named this bHLH gene RGE1. RGE1 expression was determined in endosperm cells using the beta-glucuronidase reporter gene and reverse transcription-polymerase chain reaction. Microarray and real-time reverse transcription-polymerase chain reaction analysis showed specific down-regulation of putative GDSL motif lipase genes in the rge1-1 mutant, indicating possible involvement of these genes in seed morphology. These data suggest that RGE1 expression in the endosperm at the heart stage of embryo development plays an important role in controlling embryo growth.

Lovastatin insensitive 1, a Novel pentatricopeptide repeat protein, is a potential regulatory factor of isoprenoid biosynthesis in Arabidopsis.

Higher plants have two metabolic pathways for isoprenoid biosynthesis: the cytosolic mevalonate (MVA) pathway and the plastidal non-mevalonate (MEP) pathway. Despite the compartmentalization of these two pathways, metabolic flow occurs between them. However, little is known about the mechanisms that regulate the two pathways and the metabolic cross-talk. To identify such regulatory mechanisms, we isolated and characterized the Arabidopsis T-DNA insertion mutant lovastatin insensitive 1 (loi1), which is resistant to lovastatin and clomazone, inhibitors of the MVA and MEP pathways, respectively. The accumulation of the major products of these pathways, i.e. sterols and chlorophyll, was less affected by lovastatin and clomazone, respectively, in loi1 than in the wild type. Furthermore, the 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) activity analysis showed higher activity of HMGR in loi1-1 treated with lovastatin than that in the WT. We consider that the lovastatin-resistant phenotype of loi1-1 was derived from this post-transcriptional up-regulation of HMGR. The LOI1 gene encodes a novel pentatricopeptide repeat (PPR) protein. PPR proteins are thought to regulate the expression of genes encoded in organelle genomes by post-transcriptional regulation in mitochondria or plastids. Our results demonstrate that LOI1 is predicted to localize in mitochondria and has the ability to bind single-stranded nucleic acids. Our investigation revealed that the post-transcriptional regulation of mitochondrial RNA may be involved in isoprenoid biosynthesis in both the MVA and MEP pathways.

A genome-wide gain-of function analysis of rice genes using the FOX-hunting system.

The latest report has estimated the number of rice genes to be approximately 32,000. To elucidate the functions of a large population of rice genes and to search efficiently for agriculturally useful genes, we have been taking advantage of the Full-length cDNA Over-eXpresser (FOX) gene-hunting system. This system is very useful for analyzing various gain-of-function phenotypes from large populations of transgenic plants overexpressing cDNAs of interest and others with unknown or important functions. We collected the plasmid DNAs of 13,980 independent full-length cDNA (FL-cDNA) clones to produce a FOX library by placing individual cDNAs under the control of the maize Ubiquitin-1 promoter. The FOX library was transformed into rice by Agrobacterium-mediated high-speed transformation. So far, we have generated approximately 12,000 FOX-rice lines. Genomic PCR analysis indicated that the average number of FL-cDNAs introduced into individual lines was 1.04. Sequencing analysis of the PCR fragments carrying FL-cDNAs from 8615 FOX-rice lines identified FL-cDNAs in 8225 lines, and a database search classified the cDNAs into 5462 independent ones. Approximately 16.6% of FOX-rice lines examined showed altered growth or morphological characteristics. Three super-dwarf mutants overexpressed a novel gibberellin 2-oxidase gene,confirming the importance of this system. We also show here the other morphological alterations caused by individual FL-cDNA expression. These dominant phenotypes should be valuable indicators for gene discovery and functional analysis.

Light-dependent polyploidy control by a CUE protein variant in Arabidopsis.

Endoreduplication is a special cell cycle that increases ploidy without cell and nuclear division. In plants endoreduplication is essential for development. We isolated a dominant Arabidopsis mutant from activation tagging lines that had increased polyploidy in darkness. This mutant, ipd1-1D (increased polyploidy level in darkness 1-1D), shows longer hypocotyls and increased ploidy levels only in dark-grown seedlings. The corresponding gene encodes a protein that contains a CUE domain variant. IPD1 is specifically expressed in mitotically dividing cells. Furthermore we show that blue and far-red light can suppress the ploidy increase in ipd1-1D and also suppress the reporter expression in IPD1-promoter beta-glucuronidase transgenic plants. These results suggest that IPD1 regulates the endocycle leading to hypocotyl elongation and this function is controlled by blue and far-red light.

Increased level of polyploidy1, a conserved repressor of CYCLINA2 transcription, controls endoreduplication in Arabidopsis.

Endoreduplication is a type of cell cycle in which DNA replication continues without cell division. We have isolated several dominant mutants from Arabidopsis thaliana activation tagging lines by flow cytometry. One of the mutants, increased level of polyploidy1-1D (ilp1-1D), showed increased polyploidy in both light- and dark-grown hypocotyls. The corresponding gene of ilp1-1D encodes a protein homologous to the C-terminal region of mammalian GC binding factor. We demonstrate that this protein functions as a transcriptional repressor in vivo. The expression of all members of the CYCLINA2 (CYCA2) family was reduced in an ILP1 overexpressing line, and the mouse (Mus musculus) homolog of ILP1 repressed cyclin A2 expression in mouse NIH3T3 cells. T-DNA insertion mutants of ILP1 showed reduced polyploidy and upregulated all CYCA2 expression. Furthermore, loss of CYCA2;1 expression induces an increase in polyploidy in Arabidopsis. We demonstrate that this protein regulates endoreduplication through control of CYCA2 expression in Arabidopsis.