Imaging the Cytoskeleton in Living Plant Roots.

For the past two decades, genetically encoded fluorescent proteins have emerged as the most popular method to image the plant cytoskeleton. Because fluorescent protein technology involves handling living plant cells, it is important to implement protocols that enable these delicate plant specimens to maintain optimal growth for the entire duration of the imaging experiment. To this end, we rely on a system that consists of a large coverslip coated with nutrient-supplemented agar. This agar-coverslip system is planted with surface-sterilized Arabidopsis thaliana seeds expressing cytoskeletal fluorescent protein reporters. The agar-coverslip system with planted seeds is then maintained in an environmentally controlled growth chamber. The entire setup is transferred onto the stage of a confocal microscope for imaging when roots of germinated seedlings reach a desired length. For plants with larger roots such as Medicago truncatula, the polymerized nutrient-supplemented agar is gently lifted or cut and used to secure pre-germinated seeds on the coverslip prior to root imaging. The agar-coverslip system we use for imaging the cytoskeleton in living roots along with general methods for expressing green fluorescent protein (GFP)-based cytoskeletal reporters in hairy roots of Medicago truncatula is described here.

An Efficient Brome mosaic virus-Based Gene Silencing Protocol for Hexaploid Wheat (Triticum aestivum L.).

Virus-induced gene silencing (VIGS) is a rapid and powerful method to evaluate gene function, especially for species like hexaploid wheat that have large, redundant genomes and are difficult and time-consuming to transform. The Brome mosaic virus (BMV)-based VIGS vector is widely used in monocotyledonous species but not wheat. Here we report the establishment of a simple and effective VIGS procedure in bread wheat using BMVCP5, the most recently improved BMV silencing vector, and wheat genes PHYTOENE DESATURASE (TaPDS) and PHOSPHATE2 (TaPHO2) as targets. Time-course experiments revealed that smaller inserts (~100 nucleotides, nt) were more stable in BMVCP5 and conferred higher silencing efficiency and longer silencing duration, compared with larger inserts. When using a 100-nt insert and a novel coleoptile inoculation method, BMVCP5 induced extensive silencing of TaPDS transcript and a visible bleaching phenotype in the 2nd to 5th systemically-infected leaves from nine to at least 28 days post inoculation (dpi). For TaPHO2, the ability of BMVCP5 to simultaneously silence all three homoeologs was demonstrated. To investigate the feasibility of BMV VIGS in wheat roots, ectopically expressed enhanced GREEN FLUORESCENT PROTEIN (eGFP) in a transgenic wheat line was targeted for silencing. Silencing of eGFP fluorescence was observed in both the maturation and elongation zones of roots. BMVCP5 mediated significant silencing of eGFP and TaPHO2 mRNA expression in roots at 14 and 21 dpi, and TaPHO2 silencing led to the doubling of inorganic phosphate concentration in the 2nd through 4th systemic leaves. All 54 wheat cultivars screened were susceptible to BMV infection. BMVCP5-mediated TaPDS silencing resulted in the expected bleaching phenotype in all eight cultivars examined, and decreased TaPDS transcript was detected in all three cultivars examined. This BMVCP5 VIGS technology may serve as a rapid and effective functional genomics tool for high-throughput gene function studies in aerial and root tissues and in many wheat cultivars.

Genome-wide discovery of DNA polymorphisms by whole genome sequencing differentiates weedy and cultivated rice.

Analyzing the genome level DNA polymorphisms between weedy and cultivated rice is crucial to elucidate the molecular basis of weedy and agronomic traits, which in turn can enhance our ability to control weedy rice and its utilization for rice improvement. Here, we presented the genome-wide genetic variations between a weedy rice accession PSRR-1 and two cultivated rice accessions, Bengal and Nona Bokra, belonging to japonica and indica subspecies, respectively. The total number of SNPs and InDels in PSRR/Bengal was similar to that of Nona Bokra/Bengal, but was three times greater than that of PSRR/Nona Bokra. There were 11546 large-effect SNPs/InDels affecting 5673 genes, which most likely differentiated weedy rice from cultivated rice. These large effect DNA polymorphisms were mostly resulted in stop codon gain and least by start codon loss. Analysis of the molecular functions and biological processes of weedy rice specific SNPs/InDels indicated that most of these genes were involved in protein modification/phosphorylation, protein kinase activity, and protein/nucleotide binding. By integrating previous QTL mapping results with the DNA polymorphisms data, the candidate genes for seed dormancy and seed shattering were narrowed down. The genomic resource generated in this study will facilitate discovery of functional variants for weedy and agronomic traits.

Genetic interaction involving photoperiod-responsive Hd1 promotes early flowering under long-day conditions in rice.

Although flowering in rice has been extensively investigated, few studies focused on genetic interactions. Flowering evaluation of two recombinant inbred line (RIL) populations involving photo-insensitive rice cultivars, Bengal and Cypress, and a weedy rice accession, PSRR-1, under natural long-day (LD) conditions, revealed six to ten quantitative trait loci (QTLs) and a major QTL interaction. In addition to the validation of several previously cloned genes using an introgression lines (IL) population of PSRR-1, a few novel QTLs were also discovered. Analysis of the marker profiles of the advanced backcross lines revealed that Hd1 allele of PSRR-1 was responsible for the photoperiodic response in the near-isogenic lines (NILs) developed in both cultivar backgrounds. Based on the phenotypic and genotypic data of the NILs, and NIL mapping population and the transcript abundance of key flowering pathway genes, we conclude that Hd1 and its interaction with a novel gene other than Ghd7 play an important role in controlling flowering under LD conditions. Our study demonstrates the important role of genetic interaction that regulates flowering time in rice and the need for further investigation to exploit it for breeding adaptable rice varieties.

Expression Profiling Coupled with In-silico Mapping Identifies Candidate Genes for Reducing Aflatoxin Accumulation in Maize.

Aflatoxin, produced by Aspergillus flavus, is hazardous to health of humans and livestock. The lack of information about large effect QTL for resistance to aflatoxin accumulation is a major obstacle to employ marker-assisted selection for maize improvement. The understanding of resistance mechanisms of the host plant and the associated genes is necessary for improving resistance to A. flavus infection. A suppression subtraction hybridization (SSH) cDNA library was made using the developing kernels of Mp715 (resistant inbred) and B73 (susceptible inbred) and 480 randomly selected cDNA clones were sequenced to identify differentially expressed genes (DEGs) in response to A. flavus infection and map these clones onto the corn genome by in-silico mapping. A total of 267 unigenes were identified and majority of genes were related to metabolism, stress response, and disease resistance. Based on the reverse northern hybridization experiment, 26 DEGs were selected for semi-quantitative RT-PCR analysis in seven inbreds with variable resistance to aflatoxin accumulation at two time points after A. flavus inoculation. Most of these genes were highly expressed in resistant inbreds. Quantitative RT-PCR analysis validated upregulation of PR-4, DEAD-box RNA helicase, and leucine rich repeat family protein in resistant inbreds. Fifty-six unigenes, which were placed on linkage map through in-silico mapping, overlapped the QTL regions for resistance to aflatoxin accumulation identified in a mapping population derived from the cross between B73 and Mp715. Since majority of these mapped genes were related to disease resistance, stress response, and metabolism, these should be ideal candidates to investigate host pathogen interaction and to reduce aflatoxin accumulation in maize.

Comprehensive Analysis and Expression Profiling of the OsLAX and OsABCB Auxin Transporter Gene Families in Rice (Oryza sativa) under Phytohormone Stimuli and Abiotic Stresses.

The plant hormone auxin regulates many aspects of plant growth and developmental processes. Auxin gradient is formed in plant as a result of polar auxin transportation by three types of auxin transporters such as OsLAX, OsPIN, and OsABCB. We report here the analysis of two rice auxin transporter gene families, OsLAX and OsABCB, using bioinformatics tools, publicly accessible microarray data, and quantitative RT-PCR. There are 5 putative OsLAXs and 22 putative OsABCBs in rice genome, which were mapped on 8 chromosomes. The exon-intron structure of OsLAX genes and properties of deduced proteins were relatively conserved within grass family, while that of OsABCB genes varied greatly. Both constitutive and organ/tissue specific expression patterns were observed in OsLAXs and OsABCBs. Analysis of evolutionarily closely related "gene pairs" together with organ/tissue specific expression revealed possible "function gaining" and "function losing" events during rice evolution. Most OsLAX and OsABCB genes were regulated by drought and salt stress, as well as hormonal stimuli [auxin and Abscisic Acid (ABA)], which suggests extensive crosstalk between abiotic stresses and hormone signaling pathways. The existence of large number of auxin and stress related cis-regulatory elements in promoter regions might account for their massive responsiveness of these genes to these environmental stimuli, indicating complexity of regulatory networks involved in various developmental and physiological processes. The comprehensive analysis of OsLAX and OsABCB auxin transporter genes in this study would be helpful for understanding the biological significance of these gene families in hormone signaling and adaptation of rice plants to unfavorable environments.

Comprehensive Analysis of the Soybean (Glycine max) GmLAX Auxin Transporter Gene Family.

The phytohormone auxin plays a critical role in regulation of plant growth and development as well as plant responses to abiotic stresses. This is mainly achieved through its uneven distribution in plant via a polar auxin transport process. Auxin transporters are major players in polar auxin transport. The AUXIN RESISTENT 1/LIKE AUX1 (AUX/LAX) auxin influx carriers belong to the amino acid permease family of proton-driven transporters and function in the uptake of indole-3-acetic acid (IAA). In this study, genome-wide comprehensive analysis of the soybean AUX/LAX (GmLAX) gene family, including phylogenic relationships, chromosome localization, and gene structure, was carried out. A total of 15 GmLAX genes, including seven duplicated gene pairs, were identified in the soybean genome. They were distributed on 10 chromosomes. Despite their higher percentage identities at the protein level, GmLAXs exhibited versatile tissue-specific expression patterns, indicating coordinated functioning during plant growth and development. Most GmLAXs were responsive to drought and dehydration stresses and auxin and abscisic acid (ABA) stimuli, in a tissue- and/or time point- sensitive mode. Several GmLAX members were involved in responding to salt stress. Sequence analysis revealed that promoters of GmLAXs contained different combinations of stress-related cis-regulatory elements. These studies suggest that the soybean GmLAXs were under control of a very complex regulatory network, responding to various internal and external signals. This study helps to identity candidate GmLAXs for further analysis of their roles in soybean development and adaption to adverse environments.

Genome-wide analysis and expression profiling of the PIN auxin transporter gene family in soybean (Glycine max).

BACKGROUND: The plant phytohormone auxin controls many aspects of plant growth and development, which largely depends on its uneven distribution in plant tissues. Transmembrane proteins of the PIN family are auxin efflux facilitators. They play a key role in polar auxin transport and are associated with auxin asymmetrical distribution in plants. PIN genes have been characterized in several plant species, while comprehensive analysis of this gene family in soybean has not been reported yet. RESULTS: In this study, twenty-three members of the PIN gene family were identified in the soybean genome through homology searches. Analysis of chromosome distribution and phylogenetic relationships of the soybean PIN genes indicated nine pairs of duplicated genes and a legume specific subfamily. Organ/tissue expression patterns and promoter activity assays of the soybean PINs suggested redundant functions for most duplicated genes and complementary and tissue-specific functions during development for non-duplicated genes. The soybean PIN genes were differentially regulated by various abiotic stresses and phytohormone stimuli, implying crosstalk between auxin and abiotic stress signaling pathways. This was further supported by the altered auxin distribution under these conditions as revealed by DR5::GUS transgenic soybean hairy root. Our data indicates that GmPIN9, a legume-specific PIN gene, which was responsive to several abiotic stresses, might play a role in auxin re-distribution in soybean root under abiotic stress conditions. CONCLUSIONS: This study provided the first comprehensive analysis of the soybean PIN gene family. Information on phylogenetic relationships, gene structure, protein profiles and expression profiles of the soybean PIN genes in different tissues and under various abiotic stress treatments helps to identity candidates with potential roles in specific developmental processes and/or environmental stress conditions. Our study advances our understanding of plant responses to abiotic stresses and serves as a basis for uncovering the biological role of PIN genes in soybean development and adaption to adverse environments.

Soybean transcription factor ORFeome associated with drought resistance: a valuable resource to accelerate research on abiotic stress resistance.

BACKGROUND: Whole genome sequencing provides the most comprehensive collection of an organism's genetic information. The availability of complete genome sequences is expected to dramatically deliver a high impact on biology. However, to achieve this impact in the area of crop improvement, significant efforts are still required on functional genomics, including the areas of gene annotation, cloning, expression profiling, and functional validation. RESULTS: Here we report our efforts in generating the first transcription factor (TF) open reading frame (ORF)eome resource associated with drought resistance in soybean (Glycine max), a major oil/protein crop grown worldwide. This study provides a highly annotated soybean TF-ORFeome associated with drought resistance. It contains information from experimentally verified protein-coding sequences (CDS), expression profiling under several abiotic stresses (drought, salinity, dehydration and ABA), and computationally predicted protein subcellular localization and cis-regulatory elements (CREs) analysis. All the information is available to plant researchers through a freely accessible and user-friendly database, Soybean Knowledge Base (SoyKB). CONCLUSIONS: The soybean TF-ORFeome provides a valuable public resource for functional genomics studies, especially in the area of plant abiotic stresses. It will accelerate findings in the areas of abiotic stresses and lead to the generation of crops with enhanced resistance to multiple stresses.

Genetic variants in root architecture-related genes in a Glycine soja accession, a potential resource to improve cultivated soybean.

BACKGROUND: Root system architecture is important for water acquisition and nutrient acquisition for all crops. In soybean breeding programs, wild soybean alleles have been used successfully to enhance yield and seed composition traits, but have never been investigated to improve root system architecture. Therefore, in this study, high-density single-feature polymorphic markers and simple sequence repeats were used to map quantitative trait loci (QTLs) governing root system architecture in an inter-specific soybean mapping population developed from a cross between Glycine max and Glycine soja. RESULTS: Wild and cultivated soybean both contributed alleles towards significant additive large effect QTLs on chromosome 6 and 7 for a longer total root length and root distribution, respectively. Epistatic effect QTLs were also identified for taproot length, average diameter, and root distribution. These root traits will influence the water and nutrient uptake in soybean. Two cell division-related genes (D type cyclin and auxin efflux carrier protein) with insertion/deletion variations might contribute to the shorter root phenotypes observed in G. soja compared with cultivated soybean. Based on the location of the QTLs and sequence information from a second G. soja accession, three genes (slow anion channel associated 1 like, Auxin responsive NEDD8-activating complex and peroxidase), each with a non-synonymous single nucleotide polymorphism mutation were identified, which may also contribute to changes in root architecture in the cultivated soybean. In addition, Apoptosis inhibitor 5-like on chromosome 7 and slow anion channel associated 1-like on chromosome 15 had epistatic interactions for taproot length QTLs in soybean. CONCLUSION: Rare alleles from a G. soja accession are expected to enhance our understanding of the genetic components involved in root architecture traits, and could be combined to improve root system and drought adaptation in soybean.

A DEAD box RNA helicase is critical for pre-mRNA splicing, cold-responsive gene regulation, and cold tolerance in Arabidopsis.

Cold stress resulting from chilling and freezing temperatures substantially reduces crop production worldwide. To identify genes critical for cold tolerance in plants, we screened Arabidopsis thaliana mutants for deregulated expression of a firefly luciferase reporter gene under the control of the C-REPEAT BINDING FACTOR2 (CBF2) promoter (CBF2:LUC). A regulator of CBF gene expression1 (rcf1-1) mutant that is hypersensitive to cold stress was chosen for in-depth characterization. RCF1 encodes a cold-inducible DEAD (Asp-Glu-Ala-Asp) box RNA helicase. Unlike a previously reported DEAD box RNA helicase (LOW EXPRESSION OF OSMOTICALLY RESPONSIVE GENES4 [LOS4]) that regulates mRNA export, RCF1 does not play a role in mRNA export. Instead, RCF1 functions to maintain proper splicing of pre-mRNAs; many cold-responsive genes are mis-spliced in rcf1-1 mutant plants under cold stress. Functional characterization of four genes (PSEUDO-RESPONSE REGULATOR5 [PRR5], SHAGGY-LIKE SERINE/THREONINE KINASE12 [SK12], MYB FAMILY TRANSCRIPTION FACTOR CIRCADIAN1 [CIR1], and SPFH/PHB DOMAIN-CONTAINING MEMBRANE-ASSOCIATED PROTEIN [SPFH]) that are mis-spliced in rcf1-1 revealed that these genes are cold-inducible positive (CIR1 and SPFH) and negative (PRR5 and SK12) regulators of cold-responsive genes and cold tolerance. Together, our results suggest that the cold-inducible RNA helicase RCF1 is essential for pre-mRNA splicing and is important for cold-responsive gene regulation and cold tolerance in plants.

Expression of flavonoid 3'-hydroxylase is controlled by P1, the regulator of 3-deoxyflavonoid biosynthesis in maize.

BACKGROUND: The maize (Zea mays) red aleurone1 (pr1) encodes a CYP450-dependent flavonoid 3'-hydroxylase (ZmF3'H1) required for the biosynthesis of purple and red anthocyanin pigments. We previously showed that Zmf3'h1 is regulated by C1 (Colorless1) and R1 (Red1) transcription factors. The current study demonstrates that, in addition to its role in anthocyanin biosynthesis, the Zmf3'h1 gene also participates in the biosynthesis of 3-deoxyflavonoids and phlobaphenes that accumulate in maize pericarps, cob glumes, and silks. Biosynthesis of 3-deoxyflavonoids is regulated by P1 (Pericarp color1) and is independent from the action of C1 and R1 transcription factors. RESULTS: In maize, apiforol and luteoforol are the precursors of condensed phlobaphenes. Maize lines with functional alleles of pr1 and p1 (Pr1;P1) accumulate luteoforol, while null pr1 lines with a functional or non-functional p1 allele (pr1;P1 or pr1;p1) accumulate apiforol. Apiforol lacks a hydroxyl group at the 3'-position of the flavylium B-ring, while luteoforol has this hydroxyl group. Our biochemical analysis of accumulated compounds in different pr1 genotypes showed that the pr1 encoded ZmF3'H1 has a role in the conversion of mono-hydroxylated to bi-hydroxylated compounds in the B-ring. Steady state RNA analyses demonstrated that Zmf3'h1 mRNA accumulation requires a functional p1 allele. Using a combination of EMSA and ChIP experiments, we established that the Zmf3'h1 gene is a direct target of P1. Highlighting the significance of the Zmf3'h1 gene for resistance against biotic stress, we also show here that the p1 controlled 3-deoxyanthocyanidin and C-glycosyl flavone (maysin) defence compounds accumulate at significantly higher levels in Pr1 silks as compared to pr1 silks. By virtue of increased maysin synthesis in Pr1 plants, corn ear worm larvae fed on Pr1; P1 silks showed slower growth as compared to pr1; P1 silks. CONCLUSIONS: Our results show that the Zmf3'h1 gene participates in the biosynthesis of phlobaphenes and agronomically important 3-deoxyflavonoid compounds under the regulatory control of P1.

Regulatory switch enforced by basic helix-loop-helix and ACT-domain mediated dimerizations of the maize transcription factor R.

The maize R2R3-MYB regulator C1 cooperates with the basic helix-loop-helix (bHLH) factor R to activate the expression of anthocyanin biosynthetic genes coordinately. As is the case for other bHLH factors, R harbors several protein-protein interaction domains. Here we show that not the classical but rather a briefly extended R bHLH region forms homodimers that bind canonical G-box DNA motifs. This bHLH DNA-binding activity is abolished if the C-terminal ACT (aspartokinase, chorismate, and TyrA) domain is licensed to homodimerize. Then the bHLH remains in the monomeric form, allowing it to interact with R-interacting factor 1 (RIF1). In this configuration, the R-RIF1 complex is recruited to the promoters of a subset of anthocyanin biosynthetic genes, such as A1, through the interaction with its MYB partner C1. If, however, the ACT domain remains monomeric, the bHLH region dimerizes and binds to G-boxes present in several anthocyanin genes, such as Bz1. Our results provide a mechanism by which a dimerization domain in a bHLH factor behaves as a switch that permits distinct configurations of a regulatory complex to be tethered to different promoters. Such a combinatorial gene regulatory framework provides one mechanism by which genes lacking obviously conserved cis-regulatory elements are regulated coordinately.

SELEX (Systematic Evolution of Ligands by EXponential Enrichment), as a powerful tool for deciphering the protein-DNA interaction space.

DNA-binding proteins, including transcription factors, play essential roles in many biological processes. The identification of the DNA sequences to which these proteins bind is a first, yet still challenging, step for determining their functions. SELEX provides an excellent tool for deciphering protein DNA-binding sequence specificity, and it has been widely adopted for addressing fundamental biological questions. SELEX is an experimental procedure that involves the progressive selection, from a large combinatorial double-stranded oligonucleotide library, of DNA ligands with variable DNA-binding affinities and specificities by repeated rounds of partition and amplification. In this chapter, we describe a SELEX protocol that we have successfully applied to both plant and animal MYB transcription factors.

ZEBRA2, encoding a carotenoid isomerase, is involved in photoprotection in rice.

"zebra" mutants have alternating green and chlorotic crossbands on leaf blades and are widely distributed in monocotyledonous crops. Most recently, we cloned the first responsible gene from rice, ZEBRA2, which also leads to the phenotype of rice preharvest sprouting. ZEBRA2, a single-copy gene in the rice genome, encodes a carotenoid isomerase (CRTISO), the key enzyme catalyzing the conversion of cis-lycopene to all-trans lycopene. ZEBRA2 shares high identity with known CRTISOs from other species. Expression analysis via both RT-PCR and ZEBRA2-promoter-ß-glucuronidase (GUS) transgenic rice indicates that ZEBRA2 is predominantly expressed in mesophyll cells of mature leaves where active photosynthesis occurs. Consistent with the alteration in agronomic traits, the zebra2 mutant exhibits decreased photosynthetic rate and chlorophyll content. Mutation of the ZEBRA2 gene results in the accumulation of all-trans-lycopene precursor, prolycopene (7Z,9Z,7'Z,9'Z tetra cis-lycopene), in dark-grown zebra2 tissues. Light-grown zebra2 mutant exhibits the characteristic "zebra" phenotype and decreased level of lutein, the xanthophyll that is essential for efficient chl triplet quenching. More severe phenotype of the zebra2 mutant under high light intensity indicates that "zebra" phenotype might be caused by photooxidative damages. We conclude that ZEBRA2 is involved in photoprotection in rice.

ZmMYB31 directly represses maize lignin genes and redirects the phenylpropanoid metabolic flux.

Few regulators of phenylpropanoids have been identified in monocots having potential as biofuel crops. Here we demonstrate the role of the maize (Zea mays) R2R3-MYB factor ZmMYB31 in the control of the phenylpropanoid pathway. We determined its in vitro consensus DNA-binding sequence as ACC(T)/(A) ACC, and chromatin immunoprecipitation (ChIP) established that it interacts with two lignin gene promoters in vivo. To explore the potential of ZmMYB31 as a regulator of phenylpropanoids in other plants, its role in the regulation of the phenylpropanoid pathway was further investigated in Arabidopsis thaliana. ZmMYB31 downregulates several genes involved in the synthesis of monolignols and transgenic plants are dwarf and show a significantly reduced lignin content with unaltered polymer composition. We demonstrate that these changes increase cell wall degradability of the transgenic plants. In addition, ZmMYB31 represses the synthesis of sinapoylmalate, resulting in plants that are more sensitive to UV irradiation, and induces several stress-related proteins. Our results suggest that, as an indirect effect of repression of lignin biosynthesis, transgenic plants redirect carbon flux towards the biosynthesis of anthocyanins. Thus, ZmMYB31 can be considered a good candidate for the manipulation of lignin biosynthesis in biotechnological applications.

Cell-wall invertases from rice are differentially expressed in Caryopsis during the grain filling stage.

Cell-wall invertase plays an important role in sucrose partitioning between source and sink organs in higher plants. To investigate the role of cell-wall invertases for seed development in rice (Oryza sativa L.), cDNAs of three putative cell-wall invertase genes OsCIN1, OsCIN2 and OsCIN3 were isolated. Semi-quantitative reverse transcription-polymerase chain reaction analysis revealed different expression patterns of the three genes in various rice tissues/organs. In developing caryopses, they exhibited similar temporal expression patterns, expressed highly at the early and middle grain filling stages and gradually declined to low levels afterward. However, the spatial expression patterns of them were very different, with OsCIN1 primarily expressed in the caryopsis coat, OsCIN2 in embryo and endosperm, and OsCIN3 in embryo. Further RNA in situ hybridization analysis revealed that a strong signal of OsCIN2 mRNA was detected in the vascular parenchyma surrounding the xylem of the chalazal vein and the aleurone layer, whereas OsCIN3 transcript was strongly detected in the vascular parenchyma surrounding the phloem of the chalazal vein, cross-cells, the aleurone layer and the nucellar tissue. These data indicate that the three cell-wall invertase genes play complementary/synergetic roles in assimilate unloading during the grain filling stage. In addition, the cell type-specific expression patterns of OsCIN3 in source leaf blades and anthers were also investigated, and its corresponding physiological roles were discussed.

Molecular cloning, functional characterization and expression analysis of a novel monosaccharide transporter gene OsMST6 from rice (Oryza sativa L.).

Monosaccharides transporters play important roles in assimilate supply for sink tissue development. In this study, a new monosaccharide transporter gene OsMST6 was identified from rice (Oryza sativa L.). The predicted OsMST6 protein shows typical features of sugar transporters and shares 79.6% identity with the rice monosaccharide transporter OsMST3. Heterologous expression in yeast (Saccharomyces cerevisiae) demonstrated that OsMST6 is a broad-spectrum monosaccharide transporter, with a K (m) of 266.1 muMu for glucose. OsMST6-green fluorescent protein fusion protein is localized to the plasma membrane in plant. Semi-quantitative RT-PCR analysis exhibited that OsMST6 is expressed in all tested organs/tissues. In developing seeds, OsMST6 expression level is high at the early and middle grain filling stages and gradually declines later. Further analysis detected its expression in both maternal and filial tissues. RNA in situ hybridization analysis indicated that OsMST6 is predominantly expressed in the vascular parenchyma of the chalazal vein, cross-cells, nucellar tissue and endosperm of young seeds, in mesophyll cells of source leaf blades, and in pollens and the connective vein of anthers. In addition, OsMST6 expression is up-regulated by salt stress and sugars. The physiological role of OsMST6 for seed development and its roles in other sink and source tissues are discussed.

Mutations of genes in synthesis of the carotenoid precursors of ABA lead to pre-harvest sprouting and photo-oxidation in rice.

Pre-harvest sprouting (PHS) or vivipary in cereals is an important agronomic trait that results in significant economic loss. A considerable number of mutations that cause PHS have been identified in several species. However, relatively few viviparous mutants in rice (Oryza sativa L.) have been reported. To explore the mechanism of PHS in rice, we carried out an extensive genetic screening and identified 12 PHS mutants (phs). Based on their phenotypes, these phs mutants were classified into three groups. Here we characterize in detail one of these groups, which contains mutations in genes encoding major enzymes of the carotenoid biosynthesis pathway, including phytoene desaturase (OsPDS), zeta-carotene desaturase (OsZDS), carotenoid isomerase (OsCRTISO) and lycopene beta-cyclase (beta-OsLCY), which are essential for the biosynthesis of carotenoid precursors of ABA. As expected, the amount of ABA was reduced in all four phs mutants compared with that in the wild type. Chlorophyll fluorescence analysis revealed the occurrence of photoinhibition in the photosystem and decreased capacity for eliminating excess energy by thermal dissipation. The greatly increased activities of reactive oxygen species (ROS) scavenging enzymes, and reduced photosystem (PS) II core proteins CP43, CP47 and D1 in leaves of the Oscrtiso/phs3-1mutant and OsLCY RNAi transgenic rice indicated that photo-oxidative damage occurred in PS II, consistent with the accumulation of ROS in these plants. These results suggest that the impairment of carotenoid biosynthesis causes photo-oxidation and ABA-deficiency phenotypes, of which the latter is a major factor controlling the PHS trait in rice.

Molecular cloning and expression analysis of a monosaccharide transporter gene OsMST4 from rice (Oryza sativa L.).

Monosaccharide transporters mediate the membrane transport of a variable range of monosaccharides, which plays a crucial role in sugar distribution throughout the plant. To investigate the significance of monosaccharide transporters for rice (Oryza sativa L.) seed development, cDNA of a new putative monosaccharide transporter gene OsMST4 was isolated. The deduced OsMST4 protein shows typical features of monosaccharide transporters, and shares high homology with other plant homologues. Heterologous expression in yeast (Saccharomyces cerevisiae) showed that OsMST4 is a functional monosaccharide transporter capable of transporting glucose, fructose, mannose and galactose. Transcriptional analysis revealed that OsMST4 is expressed in all tested organs/tissues. In developing caryopses, its expression is high at the early and middle grain filling stages, and declines gradually to low levels after that. Further analysis revealed that it is expressed in both the maternal tissue and the filial tissue, with its highest expression in embryo. Cellular location in young caryopses through RNA in situ hybridization showed that OsMST4 mRNA mainly accumulates in the vascular parenchyma of the chalazal vein, cross-cells, nucellar tissue and endosperm. The expression pattern of OsMST4 was further confirmed by histochemical analysis of the OsMST4-promoter-beta-glucuronidase (GUS) transgenic rice plants. These data indicate that OsMST4 is actively involved in monosaccharides supply for seed development during the course of grain filling. In addition, the cell type-specific expression patterns of OsMST4 in other sink and source tissues were also investigated, and its corresponding physiological roles were discussed.