Abnormal Inflorescence Research Articles

The origin and evolution of salicylic acid signaling and biosynthesis in plants

Salicylic acid (SA) plays a pivotal role in plant response to biotic and abiotic stress. Several core SA signaling regulators and key proteins in SA biosynthesis have been well characterized. However, much remains unknown about the origin, evolution, and early diversification of core elements in plant SA signaling and biosynthesis. In this study, we identified 10 core protein families in SA signaling and biosynthesis across green plant lineages. We found that the key SA signaling receptors, the nonexpresser of pathogenesis-related (NPR) proteins, originated in the most recent common ancestor (MRCA) of land plants and formed divergent groups in the ancestor of seed plants. However, key transcription factors for SA signaling, TGACG motif-binding proteins (TGAs), originated in the MRCA of streptophytes, arguing for the stepwise evolution of core SA signaling in plants. Different from the assembly of the core SA signaling pathway in the ancestor of seed plants, SA exists extensively in green plants, including chlorophytes and streptophyte algae. However, the full isochorismate synthase (ICS)-based SA synthesis pathway was first assembled in the MRCA of land plants. We further revealed that the ancient abnormal inflorescence meristem 1 (AIM1)-based β-oxidation pathway is crucial for the biosynthesis of SA in chlorophyte algae, and this biosynthesis pathway may have facilitated the adaptation of early-diverging green algae to the high-light-intensity environment on land. Taken together, our findings provide significant insights into the early evolution and diversification of plant SA signaling and biosynthesis pathways, highlighting a crucial role of SA in stress tolerance during plant terrestrialization.

The nuclear export receptor OsXPO1 is required for rice development and involved in abiotic stress responses

The transport of proteins to and from the nucleus is necessary for many cellular processes and is one of the ways plants respond to developmental signals and environmental stresses. Nucleocytoplasmic trafficking of proteins is mediated by the nuclear transport receptor (NTR). Although NTR has been extensively studied in humans and Arabidopsis, it has rarely been identified and functionally characterized in rice. In this study, we identified exportin 1 in rice (OsXPO1) as a nuclear export receptor. OsXPO1 shares high protein identity with its functional homologs in Arabidopsis and other organisms. OsXPO1 localized to both the nucleus and the cytoplasm, directly interacted with the small GTPases OsRAN1 and OsRAN2 in the nucleus, and mediated their nuclear export. Loss-of-function osxpo1 mutations were lethal at the seedling stage. Suppression of OsXPO1 expression in RNA interference lines produced multifaceted developmental defects, including arrested growth, premature senescence, abnormal inflorescence, and brown and mouth-opened spikelets. Overexpression of OsXPO1 in rice reduced plant height and seed-setting rate, but increased plant tolerance in response to PEG-mimicked drought stress and salt stress. These results indicate that OsXPO1 is a nuclear export receptor and acts in regulating plant development and abiotic stress responses.

Chemical Genetics Approach Identifies Abnormal Inflorescence Meristem 1 as a Putative Target of a Novel Sulfonamide That Protects Catalase2-Deficient Arabidopsis against Photorespiratory Stress.

Alterations of hydrogen peroxide (H2O2) levels have a profound impact on numerous signaling cascades orchestrating plant growth, development, and stress signaling, including programmed cell death. To expand the repertoire of known molecular mechanisms implicated in H2O2 signaling, we performed a forward chemical screen to identify small molecules that could alleviate the photorespiratory-induced cell death phenotype of Arabidopsis thaliana mutants lacking H2O2-scavenging capacity by peroxisomal catalase2. Here, we report the characterization of pakerine, an m-sulfamoyl benzamide from the sulfonamide family. Pakerine alleviates the cell death phenotype of cat2 mutants exposed to photorespiration-promoting conditions and delays dark-induced senescence in wild-type Arabidopsis leaves. By using a combination of transcriptomics, metabolomics, and affinity purification, we identified abnormal inflorescence meristem 1 (AIM1) as a putative protein target of pakerine. AIM1 is a 3-hydroxyacyl-CoA dehydrogenase involved in fatty acid β-oxidation that contributes to jasmonic acid (JA) and salicylic acid (SA) biosynthesis. Whereas intact JA biosynthesis was not required for pakerine bioactivity, our results point toward a role for β-oxidation-dependent SA production in the execution of H2O2-mediated cell death.

In vitro physical mutagenesis of giant reed (Arundo donax L.)

AbstractGiant reed (Arundo donax L.) is a C3 perennial, warm‐season, rhizomatous grass of emerging interest for bioenergy and biomass derivatives production, and for phytoremediation. It only propagates vegetatively and very little genetic variation is found among ecotypes, basically precluding breeding efforts. With the objective to increase the genetic variation in this species, we developed and applied a mutagenesis protocol based on γ‐irradiation of in vitro cell cultures from which regenerants were obtained. Based on a radiosensitivity test, the irradiation dose reducing to 50% the number of regenerants per callus (RD50) was estimated at 35 Gy. A large mutagenic experiment was carried out by irradiating a total of 3120 calli with approx. 1×, 1.5× and 2× RD50. A total of 1004 regenerants from irradiated calli were hardened in pots and transplanted to the field. Initial phenotypic characterization of the collection showed correlated responses of biomass‐related quantitative traits to irradiation doses. Approx. 10% of field‐grown clones showed remarkable morphological aberrations including dwarfism, altered tillering, abnormal inflorescence, leaf variegation and others, which were tested for stability over generations. Clone lethality reached 0.4%. Our results show for the first time that physical mutagenesis can efficiently induce new genetic and phenotypic variation of agronomic and prospective industrial value in giant reed. The methodology and the plant materials described here may contribute to the domestication and the genetic improvement of this important biomass species.

ABNORMAL INFLORESCENCE MERISTEM1 Functions in Salicylic Acid Biosynthesis to Maintain Proper Reactive Oxygen Species Levels for Root Meristem Activity in Rice.

Root meristem activity determines root growth and root architecture and consequently affects water and nutrient uptake in plants. However, our knowledge about the regulation of root meristem activity in crop plants is very limited. Here, we report the isolation and characterization of a short root mutant in rice (Oryza sativa) with reduced root meristem activity. This root growth defect is caused by a mutation in ABNORMAL INFLORESCENCE MERISTEM1 (AIM1), which encodes a 3-hydroxyacyl-CoA dehydrogenase, an enzyme involved in β-oxidation. The reduced root meristem activity of aim1 results from reduced salicylic acid (SA) levels and can be rescued by SA application. Furthermore, reduced SA levels are associated with reduced levels of reactive oxygen species (ROS) in aim1, likely due to increased expression of redox and ROS-scavenging-related genes, whose increased expression is (at least in part) caused by reduced expression of the SA-inducible transcriptional repressors WRKY62 and WRKY76. Like SA, ROS application substantially increased root length and root meristem activity in aim1 These results suggest that AIM1 is required for root growth in rice due to its critical role in SA biosynthesis: SA maintains root meristem activity through promoting ROS accumulation by inducing the activity of WRKY transcriptional repressors, which repress the expression of redox and ROS-scavenging genes.

生育段階の異なるシュートに対する温度条件がシュッコンカスミソウ‘アルタイル’の形態異常花序発生に及ぼす影響

生育段階の異なるシュートに対する温度条件がシュッコンカスミソウ‘アルタイル’の形態異常花序発生に及ぼす影響を検討した．シュート長20 cmから2週間15°Cに加温することで，8週間15°Cに加温した場合と同様に，形態異常花序発生が軽減された．2週間加温した個体の切り花長と切り花重は，8週間加温した個体よりも大きくなった．形態異常花序が発生するシュート長と頂芽における花芽分化段階との関係を調べたところ，頂芽のステージが栄養成長からがく片形成期に当たるシュート長が約1～20 cmから15日間の15°C加温で最も形態異常花序が抑制できた．形態異常花序に及ぼす低温の影響を明確にするため，異なる生育段階に対する低温遭遇（7°C）が形態異常花序発生に及ぼす影響を調査した．異なる生育段階に高温（15°C）に遭遇させた実験と同様に，頂芽のステージが栄養成長からがく片形成期までの低温遭遇が形態異常花序発生に大きく関与していた．以上のことから，摘心直後からがく片形成期の期間，株を低温に遭遇させないように温度管理することで，形態異常花序の発生を抑制でき，切り花形質も改善できると考えられた．

Knockout of the two evolutionarily conserved peroxisomal 3-ketoacyl-CoA thiolases in Arabidopsis recapitulates the abnormal inflorescence meristem 1 phenotype.

A specific function for peroxisomal β-oxidation in inflorescence development in Arabidopsis thaliana is suggested by the mutation of the abnormal inflorescence meristem 1 gene, which encodes one of two peroxisomal multifunctional proteins. Therefore, it should be possible to identify other β-oxidation mutants that recapitulate the aim1 phenotype. Three genes encode peroxisomal 3-ketoacyl-CoA thiolase (KAT) in Arabidopsis. KAT2 and KAT5 are present throughout angiosperms whereas KAT1 is a Brassicaceae-specific duplication of KAT2 expressed at low levels in Arabidopsis. KAT2 plays a dominant role in all known aspects of peroxisomal β-oxidation, including that of fatty acids, pro-auxins, jasmonate precursor oxophytodienoic acid, and trans-cinnamic acid. The functions of KAT1 and KAT5 are unknown. Since KAT5 is conserved throughout vascular plants and expressed strongly in flowers, kat2 kat5 double mutants were generated. These were slow growing, had abnormally branched inflorescences, and ectopic organ growth. They made viable pollen, but produced no seed indicating that infertility was due to defective gynaecium function. These phenotypes are strikingly similar to those of aim1. KAT5 in the Brassicaceae encodes both cytosolic and peroxisomal proteins and kat2 kat5 defects could be complemented by the re-introduction of peroxisomal (but not cytosolic) KAT5. It is concluded that peroxisomal KAT2 and KAT5 have partially redundant functions and operate downstream of AIM1 to provide β-oxidation functions essential for inflorescence development and fertility.

最低気温がシュッコンカスミソウ‘アルタイル’の形態異常花序発生および切り花形質に及ぼす影響

最低気温がシュッコンカスミソウ‘アルタイル’の形態異常花序発生および切り花形質に及ぼす影響を調査した．形態異常花序を異常の特徴と程度に基づき3つのタイプ（1：茎が短いもの，2：2本の茎が癒着，3：ひどく湾曲し変形したもの）に分類した．形態異常花序の発生は初冬から早春にかけて増加した．軽度なタイプ1とタイプ2による形態異常は，開花時期に関係なくほぼ一定の割合で発生が認められたのに対して，切り花の外観を大きく損なうタイプ3は3月開花の個体で大幅に増加した．最低気温（7°C, 11°C, 15°C）が形態異常発生に及ぼす影響を調査したところ，タイプ3は最低気温が低いほど発生率が高かった．一方，切り花長，切り花重は最低気温が高いほど劣ることが明らかになり，形態異常花序発生を抑制したうえで，十分なボリュームの切り花を得るためには11°Cの加温が有効であると考えられた．栽培期間中の低温への積算遭遇時間とタイプ3の発生割合の関係を分析したところ，発蕾から開花までの積算低温遭遇時間と形態異常花序発生の間に相関は認められず，摘心から発蕾までの積算低温遭遇時間と形態異常花序発生との間には有意な相関が認められたことから，摘心から発蕾の期間における9°C以下の低温遭遇が重度の形態異常花序（タイプ3）の発生に関与することが示唆された．

Peroxisomal ATP-Binding Cassette Transporter COMATOSE and the Multifunctional Protein ABNORMAL INFLORESCENCE MERISTEM Are Required for the Production of Benzoylated Metabolites in Arabidopsis Seeds

Secondary metabolites derived from benzoic acid (BA) are of central importance in the interactions of plants with pests, pathogens, and symbionts and are potentially important in plant development. Peroxisomal β-oxidation has recently been shown to contribute to BA biosynthesis in plants, but not all of the enzymes involved have been defined. In this report, we demonstrate that the peroxisomal ATP-binding cassette transporter COMATOSE is required for the accumulation of benzoylated secondary metabolites in Arabidopsis (Arabidopsis thaliana) seeds, including benzoylated glucosinolates and substituted hydroxybenzoylcholines. The ABNORMAL INFLORESCENCE MERISTEM protein, one of two multifunctional proteins encoded by Arabidopsis, is essential for the accumulation of these compounds, and MULTIFUNCTIONAL PROTEIN2 contributes to the synthesis of substituted hydroxybenzoylcholines. Of the two major 3-ketoacyl coenzyme A thiolases, KAT2 plays the primary role in BA synthesis. Thus, BA biosynthesis in Arabidopsis employs the same core set of β-oxidation enzymes as in the synthesis of indole-3-acetic acid from indole-3-butyric acid.

A fluorometric assay for trehalose in the picomole range.

BackgroundTrehalose is a non-reducing disaccharide that is used as an osmolyte, transport sugar, carbon reserve and stress protectant in a wide range of organisms. In plants, trehalose 6-phosphate (Tre6P), the intermediate of trehalose biosynthesis, is thought to be a signal of sucrose status. Trehalose itself may play a role in pathogenic and symbiotic plant-microbe interactions, in responses to abiotic stress and in developmental signalling, but its precise functions are unknown. A major obstacle to investigating its function is the technical difficulty of measuring the very low levels of trehalose usually found in plant tissues, as most of the established trehalose assays lack sufficient specificity and/or sensitivity.ResultsA kinetic assay for trehalose was established using recombinant Escherichia coli cytoplasmic trehalase (treF), which was shown to be highly specific for trehalose. Hydrolysis of trehalose to glucose is monitored fluorometrically and the trehalose content of the tissue extract is determined from an internal calibration curve. The assay is linear for 0.2-40 pmol trehalose, and recoveries of trehalose were ≥88%. A. thaliana Col-0 rosettes contain about 20–30 nmol g-1FW of trehalose, increasing to about 50–60 nmol g-1FW in plants grown at 8°C. Trehalose is not correlated with sucrose content, whereas a strong correlation between Tre6P and sucrose was confirmed. The trehalose contents of ear inflorescence primordia from the maize ramosa3 mutant and wild type plants were 6.6±2.6 nmol g-1FW and 19.0±12.7 nmol g-1FW, respectively. The trehalose:Tre6P ratios in the ramosa3 and wild-type primordia were 2.43±0.85 and 6.16±3.45, respectively.ConclusionThe fluorometric assay is highly specific for trehalose and sensitive enough to measure the trehalose content of very small amounts of plant tissue. Chilling induced a 2-fold accumulation of trehalose in A. thaliana rosettes, but the levels were too low to make a substantial quantitative contribution to osmoregulation. Trehalose is unlikely to function as a signal of sucrose status. The abnormal inflorescence branching phenotype of the maize ramosa3 mutant might be linked to a decrease in trehalose levels in the inflorescence primordia or a downward shift in the trehalose:Tre6P ratio.

Arabidopsis proline-rich protein important for development and abiotic stress tolerance is involved in microRNA biogenesis

MicroRNAs (miRNAs) are important for plant development and stress responses. However, factors regulating miRNA metabolism are not completely understood. SICKLE (SIC), a proline-rich protein critical for development and abiotic stress tolerance of Arabidopsis, was identified in this study. Loss-of-function sic-1 mutant plants exhibited a serrated, sickle-like leaf margin, reduced height, delayed flowering, and abnormal inflorescence phyllotaxy, which are common characteristics of mutants involved in miRNA biogenesis. The sic-1 mutant plants accumulated lower levels of a subset of miRNAs and transacting siRNAs but higher levels of corresponding primary miRNAs than the WT. The SIC protein colocalizes with the miRNA biogenesis component HYL1 in distinct subnuclear bodies. sic-1 mutant plants also accumulated higher levels of introns from hundreds of loci. In addition, sic-1 mutant plants are hypersensitive to chilling and salt stresses. These results suggest that SIC is a unique factor required for the biogenesis of some miRNAs and degradation of some spliced introns and important for plant development and abiotic stress responses.

The Arabidopsis thaliana Homeobox Gene ATHB12 Is Involved in Symptom Development Caused by Geminivirus Infection

BackgroundGeminiviruses are single-stranded DNA viruses that infect a number of monocotyledonous and dicotyledonous plants. Arabidopsis is susceptible to infection with the Curtovirus, Beet severe curly top virus (BSCTV). Infection of Arabidopsis with BSCTV causes severe symptoms characterized by stunting, leaf curling, and the development of abnormal inflorescence and root structures. BSCTV-induced symptom development requires the virus-encoded C4 protein which is thought to interact with specific plant-host proteins and disrupt signaling pathways important for controlling cell division and development. Very little is known about the specific plant regulatory factors that participate in BSCTV-induced symptom development. This study was conducted to identify specific transcription factors that are induced by BSCTV infection.Methodology/Principal Findings Arabidopsis plants were inoculated with BSCTV and the induction of specific transcription factors was monitored using quantitative real-time polymerase chain reaction assays. We found that the ATHB12 and ATHB7 genes, members of the homeodomain-leucine zipper family of transcription factors previously shown to be induced by abscisic acid and water stress, are induced in symptomatic tissues of Arabidopsis inoculated with BSCTV. ATHB12 expression is correlated with an array of morphological abnormalities including leaf curling, stunting, and callus-like structures in infected Arabidopsis. Inoculation of plants with a BSCTV mutant with a defective c4 gene failed to induce ATHB12. Transgenic plants expressing the BSCTV C4 gene exhibited increased ATHB12 expression whereas BSCTV-infected ATHB12 knock-down plants developed milder symptoms and had lower ATHB12 expression compared to the wild-type plants. Reporter gene studies demonstrated that the ATHB12 promoter was responsive to BSCTV infection and the highest expression levels were observed in symptomatic tissues where cell cycle genes also were induced.Conclusions/SignificanceThese results suggest that ATHB7 and ATHB12 may play an important role in the activation of the abnormal cell division associated with symptom development during geminivirus infection.

Conserved and Novel Functions for Arabidopsis thaliana MIA40 in Assembly of Proteins in Mitochondria and Peroxisomes

The disulfide relay system of the mitochondrial intermembrane space has been extensively characterized in Saccharomyces cerevisiae. It contains two essential components, Mia40 and Erv1. The genome of Arabidopsis thaliana contains a single gene for each of these components. Although insertional inactivation of Erv1 leads to a lethal phenotype, inactivation of Mia40 results in no detectable deleterious phenotype. A. thaliana Mia40 is targeted to and accumulates in mitochondria and peroxisomes. Inactivation of Mia40 results in an alteration of several proteins in mitochondria, an absence of copper/zinc superoxide dismutase (CSD1), the chaperone for superoxide dismutase (Ccs1) that inserts copper into CSD1, and a decrease in capacity and amount of complex I. In peroxisomes the absence of Mia40 leads to an absence of CSD3 and a decrease in abnormal inflorescence meristem 1 (Aim1), a β-oxidation pathway enzyme. Inactivation of Mia40 leads to an alteration of the transcriptome of A. thaliana, with genes encoding peroxisomal proteins, redox functions, and biotic stress significantly changing in abundance. Thus, the mechanistic operation of the mitochondrial disulfide relay system is different in A. thaliana compared with other systems, and Mia40 has taken on new roles in peroxisomes and mitochondria.

Dense Panicle Related Gene Cloning from Rice Mutant A989 and Transgenic Plant Analysis

Flowering time and inflorescence architecture are important and inter-relating traits in rice ( Oryza sativa L.). Overexpression of each of RCN1 to RCN3 in rice could result in delayed flowering and abnormal inflorescence architecture. From a transferred DNA (T-DNA) insertion population constructed from japonica rice cv. Zhonghua 11, a mutant, A989, exhibiting dense panicle and late flowering was isolated. Genetic and molecular analysis showed that a single copy of T-DNA was inserted into the genome of the mutant, and the T-DNA was inserted into the 330 bp position after RCN2 poly A. Overexpression of RCN2 was observed in the mutant as revealed by reverse transcription-polymerase chain reaction (RT-PCR) test. When RCN2 gene was transferred into wild type driven by double 35S promoters, the transformants showed no panicle due to the suspensions of differentiation and growth of the secondary-branch primordium; however, the transition from vegetative growth to productive growth was normal in the transgenic plant.

The Arabidopsis thaliana multifunctional protein gene (MFP2) of peroxisomal β‐oxidation is essential for seedling establishment

The multifunctional protein (MFP) of peroxisomal beta-oxidation catalyses four separate reactions, two of which (2-trans enoyl-CoA hydratase and L-3-hydroxyacyl-CoA dehydrogenase) are core activities required for the catabolism of all fatty acids. We have isolated and characterized five Arabidopsis thaliana mutants in the MFP2 gene that is expressed predominantly in germinating seeds. Seedlings of mfp2 require an exogenous supply of sucrose for seedling establishment to occur. Analysis of mfp2-1 seedlings revealed that seed storage lipid was catabolized more slowly, long-chain acyl-CoA substrates accumulated and there was an increase in peroxisome size. Despite a reduction in the rate of beta-oxidation, mfp2 seedlings are not resistant to the herbicide 2,4-dichlorophenoxybutyric acid, which is catabolized to the auxin 2,4-dichlorophenoxyacetic acid by beta-oxidation. Acyl-CoA feeding experiments show that the MFP2 2-trans enoyl-CoA hydratase only exhibits activity against long chain (C18:0) substrates, whereas the MFP2 L-3-hydroxyacyl-CoA dehydrogenase is active on C6:0, C12:0 and C18:0 substrates. A mutation in the abnormal inflorescence meristem gene AIM1, the only homologue of MFP2, results in an abnormal inflorescence meristem phenotype in mature plants (Richmond and Bleecker, Plant Cell 11, 1999, 1911) demonstrating that the role of these genes is very different. The mfp2-1 aim1double mutant aborted during the early stages of embryo development showing that these two proteins share a common function that is essential for this key stage in the life cycle.

フリージアの促成栽培における低温処理後の高温遭遇が花芽の形態的変化と開花時の障害発生に及ぼす影響

Corms of the freesia 'Blue Heaven' were chilled wet at 10°C for 5 or 6 weeks and the resulting sprouting young plants were immediately exposed to 24°C for 1, 2, or 3 weeks and then to 10°C until they were planted in a plastic house maintained at 5°C or above on 13 Nov. Flower buds, exposed to the high temperature, were prepared and photographed on a scanning electron microscope (SEM). Based on their morphology, the types at flowering were classified into 6 categories. When flower buds were exposed to high temperature during the differentiation of floral primordia, the which formed, varied depending on the developmental stages of flower buds at the start of the high temperature treatment and its duration. These inflorescences were classified as gladiolus bloom or abnormal inflorescence according to the degree of heat injury. The angle between the horizontal and the crossing lines from the base of the first to that of the last floret in an is a practical indicator to measure the degree of gladiolus bloom. Visually normal inflorescence, appeared in plants which received the most severe high temperature treatment. These plants initiated leaves and flower buds after abortion of the pre-differentiated flower buds. Normal and pseudo-normal inflorescences are easily distinguished by the difference in the number of leaves or days from chilling to flowering. Heat injury was less severe if the plants were exposed to high temperatures at a more advanced stage of flower bud development.

A defect in beta-oxidation causes abnormal inflorescence development in Arabidopsis.

The abnormal inflorescence meristem1 (aim1) mutation affects inflorescence and floral development in Arabidopsis. After the transition to reproductive growth, the aim1 inflorescence meristem becomes disorganized, producing abnormal floral meristems and resulting in plants with severely reduced fertility. The derived amino acid sequence of AIM1 shows extensive similarity to the cucumber multifunctional protein involved in beta-oxidation of fatty acids, which possesses l-3-hydroxyacyl-CoA hydrolyase, l-3-hydroxyacyl-dehydrogenase, d-3-hydroxyacyl-CoA epimerase, and Delta(3), Delta(2)-enoyl-CoA isomerase activities. A defect in beta-oxidation has been confirmed by demonstrating the resistance of the aim1 mutant to 2,4-diphenoxybutyric acid, which is converted to the herbicide 2,4-D by the beta-oxidation pathway. In addition, the loss of AIM1 alters the fatty acid composition of the mature adult plant.

A Defect in b-Oxidation Causes Abnormal Inflorescence Development in Arabidopsis

A Defect in b-Oxidation Causes Abnormal Inflorescence Development in Arabidopsis

AGO1 defines a novel locus of Arabidopsis controlling leaf development.

An allelic series of the novel argonaute mutant (ago1-1 to ago1-6) of the herbaceous plant Arabidopsis thaliana has been isolated. The ago1 mutation pleotropically affects general plant architecture. The apical shoot meristem generates rosette leaves and a single stem, but axillary meristems rarely develop. Rosette leaves lack a leaf blade but still show adaxial/abaxial differentiation. Instead of cauline leaves, filamentous structures without adaxial/abaxial differentiation develop along the stem and an abnormal inflorescence bearing infertile flowers with filamentous organs is produced. Two independent T-DNA insertions into the AGO1 locus led to the isolation of two corresponding genomic sequences as well as a complete cDNA. The AGO1 locus was mapped close to the marker mi291a on chromosome 1. Antisense expression of the cDNA resulted in a partial mutant phenotype. Sense expression caused some transgenic lines to develop goblet-like leaves and petals. The cDNA encodes a putative 115 kDa protein with sequence similarity to translation products of a novel gene family present in nematodes as well as humans. No specific function has been assigned to these genes. Similar proteins are not encoded by the genomes of yeast or bacteria, suggesting that AGO1 belongs to a novel class of genes with a function specific to multicellular organisms.

Early-Flowering Mutants of Arabidopsis thaliana

The transition from vegetative to reproductive growth represents a major change in angiosperm development. During the floral transition, shoot meristem activity progresses through the overlapping developmental phases: vegetative→inflorescence→floral. The transition to flowering is regulated by graft-transmissible substances, and the arrival of these substances at the shoot apex is correlated with the establishment of the inflorescence meristem. To identify the genes involved in regulating the floral transition, we have screened for mutants of Arabidopsis thaliana that display altered timing of flowering. We expect that such heterochronic mutants are defective either in the production or transport of floral signals, or in the response of the shoot meristem to these signals. We have isolated four early-flowering mutants: terminal flower 1 (tfl1) and early-flowering (elf) 1, 2 and 3. Three phenotypic classes are represented by these mutations: (I) early-flowering, photoperiod-sensitive, and displaying abnormal inflorescence development (tfl1); (II) early-flowering photoperiod-sensitive (elf1 and elf2); and (III) early-flowering, photoperiod-insensitive (elf3). This paper describes the initial phenotypic and genetic analysis of the mutants, and discusses how the genes represented by these mutations may fit into the signal/response pathway that leads to reproductive development.