Inflorescence and floral development in Astragalus lagopoides Lam. (Leguminosae: Papilionoideae: Galegeae)

Inflorescence and floral organogenesis and development of the bushy perennial legume Astragalus lagopoides of the section Hymenostegis were studied by means of epi-illumination light microscopy. Based on our observations, the primordia of lanceolate racemose inflorescences are born in the axils of leaves. Each inflorescence apex initiates acropetally bracts and floral apices for some time and then eventually ceases meristematic activity and forms an oblong-shaped terminal structure. The formation of such atypical terminal protrusion on the inflorescence meristem is judged to be a diagnostic feature for well-organized cessation of meristem morphogenesis. Pentamerous perfect flowers of the plant show strong zygomorphy and marked overlap in time of initiation among different organ primordia. Unexpectedly, sepal initiation is bidirectional starting from the lateral sides of the floral apex. Other significant developmental feature includes the existence of two types of common primordia, which are formed successively. From the primary common primordia there are produced antesepalous stamens and secondary common primordia. In comparison, the five secondary common primordia subdivide into a petal and an antepetalous stamen primordia. Initiation of two different types of common primordia is possibly the result of rising overlap in time of initiation of organs and demonstrates an advanced developmental style in the genus Astragalus.     

Molecular evolution of flower development

Flowers, as reproductive structures of the most successful group of land plants, have been a central focus of study for both evolutionists and ecologists. Recent advances in unravelling the genetics of flower development have provided insight into the evolution of floral structures among angiosperms. The study of the evolution of genes that control floral morphogenesis permits us to draw inferences on the diversification of developmental systems, the origin of floral organs and the selective forces that drive evolutionary change among these plant reproductive structures.     

水稻花发育的分子生物学研究进展

水稻是世界上最重要的粮食作为之一,也是单子叶植物发育生物学研究较理想的模式植物。水稻花器官还是粮食赖以形成的基础。对水稻花发育的研究已开始成为植物分子遗传学的一个新的焦点。近年来有关水稻花发育基因调控的研究已取得了长足的进展,本文从水稻花的诱导、花分生组织的形成和花器官的发育三个方面综述近年来国内外的研究进展。 Abstract:Rice (Oryza sativa L.) is not only one of the most important food crops in the world,but also a model plant for study of molecular developmental biology in monocots.In addition,the rice floral organs provide the basis for grain formation.Study of rice floral development has become a new focus of plant molecular genetics.Recently,notable progress has been made in study of gene regulation in rice floral development.In the review,genetic and molecular mechanisms of floral induction,floral meristem formation,and floral organ development in rice are summarized.     

C类花器官特征基因AGAMOUS(AG)调控植物花分生组织维持与终止研究进展

花分生组织的维持与终止在植物花器官发生和世代交替起着至关重要的作用。成功的花分生组织决定能够确保植物正常的生殖发育和生命周期进程。诸多研究表明AGAMOUS(AG)基因作为花器官分化和开花决定的主效调节因子,能够协调花发育过程中多种细胞命运决定。然而,关于AG参与调控植物世代交替及花分生组织维持与终止的分子调控机制尚不清晰。综述了近年来AG基因参与调控植物花分生组织维持与终止的研究进展及现状,以期为深入研究植物花器官分化过程中干细胞的维持和终止,以及干细胞活动与其他发育过程之间的分子调控过程提供参考。     

Programmed cell death and tissue remodelling in plants

The use of programmed cell death (PCD) to remodel plants at the cellular, tissue, and organ levels is particularly fascinating and occurs in such processes as tracheary element differentiation, lysigenous aerenchyma formation, development of functionally unisexual flowers from bisexual floral primordia, and leaf morphogenesis. The formation of complex leaf shape through the use of PCD is a rare event across vascular plants and occurs only in a few species of Monstera and related genera, and in the lace plant (Aponogeton madagascariensis). During early development, the lace plant leaf forms a pattern of equidistantly positioned perforations across the surface of the leaf, giving it a lattice-like appearance. Due to the accessibility and predictability of this process, the lace plant provides highly suitable material for the study of developmentally regulated PCD in plants. A sterile lace plant culture system has been successfully established, providing material free of micro-organisms for experimental study. The potential role of ethylene and caspase-like activity in developmentally regulated PCD in the lace plant is currently under investigation, with preliminary results indicating that both may play a role in the cell death pathway.     

Development states associated with the floral transition.

Floral initiation can be analyzed from a developmental perspective by focusing upon how developmental fates are imprinted, remembered, and expressed. This is not an altogether new perspective, since people studying flowering have been concerned for a long time with the commitment of meristems to form flowers and the morphological, cellular, and molecular changes associated with this commitment. What is novel is the emphasis on developmental states as opposed to physiological processes. This developmental focus indicates that there appear to be at least three major developmental states that are acquired and expressed in the process of a meristem initiating floral morphogenesis. The meristem cells must first become competent to respond to a developmental signal that evokes them into a florally determined state. The leaves are the usual source of this signal and a specific leaf may or may not have the capacity to be inductively active. When a leaf does develop the capacity for inductive activity, this capacity is usually correlated with the ontogeny of the leaf. Inductive activity, however, may be continually expressed as in some day-neutral plants or may be latent as in plants where the photoperiod is the external cue for activity. Competent shoot apical meristems respond to inductive leaf signal by being evoked into a florally determined state. Under permissive conditions this florally determined state is expressed as the initiation of floral morphogenesis. Many mechanisms have evolved to regulate entry into and expression of these developmental states. As we learn more about the developmental states associated with flowering and how they are acquired and expressed, we will understand better how the various patterns of flowering are related to one another as well as which developmental processes are common to all angiosperms.     

Two RNA binding proteins,HEN4 and HUA1, act in the processing of AGAMOUS pre-mRNA in Arabidopsis thaliana

AGAMOUS, a key player in floral morphogenesis, specifies reproductive organ identities and regulates the timely termination of stem cell fates in the floral meristem. Here, we report that strains carrying mutations in three genes, HUA1, HUA2, and HUA ENHANCER4 (HEN4), exhibit floral defects similar to those in agamous mutants: reproductive-to-perianth organ transformation and loss of floral determinacy. HEN4 codes for a K homology (KH) domain-containing, putative RNA binding protein that interacts with HUA1, a CCCH zinc finger RNA binding protein in the nucleus. We show that HUA1 binds AGAMOUS pre-mRNA in vitro and that HEN4, HUA1, and HUA2 act in floral morphogenesis by specifically promoting the processing of AGAMOUS pre-mRNA. Our studies underscore the importance of RNA processing in modulating plant development.     

Genetic evidence for polarities that regulate leaf morphogenesis

Leaf morphogenesis is a fundamental process of shoot morphogenesis, since the leaf is the basic organ of the shoot. However, leaf morphogenesis is still poorly understood, in particular in dicotyledonous plants, because of the complex nature of the development of leaves. Thus, the mechanisms regulating each process of the morphogenesis, such as leaf determination, establishment of dorsoventrality, and polarity recognition, remain unknown. Developmental genetics seems to prove the most suitable approach to such processes and should allow us to dissect the relevant developmental pathways into genetically programmed, unit processes. The techniques of developmental genetics have been applied to studes of leaf morphogenesis of model plants, such asArabidopsis thaliana andAntirrhinum majus, and have recently revealed several important steps in leaf morphogenesis. The review will focus on genetic evidence for polarities that regulate leaf morphogenesis. Hypothetical mechanisms for leaf morphogenesis will be also discussed, based on the genetic data. Receipt of the Botanical Society Award of Young Scientists, 1996.     

Developmental genetics of leaf morphogenesis in dicotyledonous plants

A full understanding of the leaf is essential for a full understanding of plant morphology. However, leaf morphogenesis is still poorly understood, in particular in dicotyledonous plants, because of the complex nature of the development of leaves. Mutational analysis seems to be the most suitable strategy for investigations of such processes, and should allow us to dissect the developmental pathways into genetically programmed unit processes. The techniques of developmental genetics have been applied to the study of leaf morphogenesis in model plants, such asArabidopsis thaliana, and several key processes in leaf morphogenesis have been identified. The fundamental processes in leaf morphogenesis include the identification of leaf organs, determination of leaf primordia (occurrence of marginal meristem), and the polar or non-polar elongation of leaf cells. This review will focus on the genes that are essential for these processes and have been identified in mutational analyses. Mutational analyses of the photomorphogenesis is also briefly summarized from the perspective of the plasticity of leaf morphogenesis.     

Control of Reproductive Floral Organ Identity Specification in Arabidopsis by the C Function Regulator AGAMOUS

The floral organ identity factor AGAMOUS (AG) is a key regulator of Arabidopsis thaliana flower development, where it is involved in the formation of the reproductive floral organs as well as in the control of meristem determinacy. To obtain insights into how AG specifies organ fate, we determined the genes and processes acting downstream of this C function regulator during early flower development and distinguished between direct and indirect effects. To this end, we combined genome-wide localization studies, gene perturbation experiments, and computational analyses. Our results demonstrate that AG controls flower development to a large extent by controlling the expression of other genes with regulatory functions, which are involved in mediating a plethora of different developmental processes. One aspect of this function is the suppression of the leaf development program in emerging floral primordia. Using trichome initiation as an example, we demonstrate that AG inhibits an important aspect of leaf development through the direct control of key regulatory genes. A comparison of the gene expression programs controlled by AG and the B function regulators APETALA3 and PISTILLATA, respectively, showed that while they control many developmental processes in conjunction, they also have marked antagonistic, as well as independent activities.     

Genetic control of flower development

Flowering plants are the most highly evolved and complex organisms within the plant kingdom. The flower consists of several distinct organ systems that are responsible for higher plant reproduction. Cells within specific floral organs differentiate into spores and gametes required by the plant to complete its life cycle. Flower development represents an excellent model for understanding the molecular and physiological processes that control organ differentiation in higher plants. Rapidly emerging gene tagging procedures are facilitating the isolation of genes that control flower morphogenesis.     

NO APICAL MERISTEM (MtNAM) regulates floral organ identity and lateral organ separation in Medicago truncatula

? The CUP-SHAPED COTYLEDON (CUC)/NO APICAL MERISTEM (NAM) family of genes control boundary formation and lateral organ separation, which is critical for proper leaf and flower patterning. However, most downstream targets of CUC/NAM genes remain unclear. ? In a forward screen of the tobacco retrotransposon1 (Tnt1) insertion population in Medicago truncatula, we isolated a weak allele of the no-apical-meristem mutant mtnam-2. Meanwhile, we regenerated a mature plant from the null allele mtnam-1. These materials allowed us to extensively characterize the function of MtNAM and its downstream genes. ? MtNAM is highly expressed in vegetative shoot buds and inflorescence apices, specifically at boundaries between the shoot apical meristem and leaf/flower primordia. Mature plants of the regenerated null allele and the weak allele display remarkable floral phenotypes: floral whorls and organ numbers are reduced and the floral organ identity is compromised. Microarray and quantitative RT-PCR analyses revealed that all classes of floral homeotic genes are down-regulated in mtnam mutants. Mutations in MtNAM also lead to fused cotyledons and leaflets of the compound leaf as well as a defective shoot apical meristem. ? Our results revealed that MtNAM shares the role of CUC/NAM family genes in lateral organ separation and compound leaf development, and is also required for floral organ identity and development.     

The BLADE-ON-PETIOLE 1 gene controls leaf pattern formation through the modulation of meristematic activity in Arabidopsis

The plant leaf provides an ideal system to study the mechanisms of organ formation and morphogenesis. The key factors that control leaf morphogenesis include the timing, location and extent of meristematic activity during cell division and differentiation. We identified an Arabidopsis mutant in which the regulation of meristematic activities in leaves was aberrant. The recessive mutant allele blade-on-petiole1-1 (bop1-1) produced ectopic, lobed blades along the adaxial side of petioles of the cotyledon and rosette leaves. The ectopic organ, which has some of the characteristics of rosette leaf blades with formation of trichomes in a dorsoventrally dependent manner, was generated by prolonged and clustered cell division in the mutant petioles. Ectopic, lobed blades were also formed on the proximal part of cauline leaves that lacked a petiole. Thus, BOP1 regulates the meristematic activity of leaf cells in a proximodistally dependent manner. Manifestation of the phenotypes in the mutant leaves was dependent on the leaf position. Thus, BOP1 controls leaf morphogenesis through control of the ectopic meristematic activity but within the context of the leaf proximodistality, dorsoventrality and heteroblasty. BOP1 appears to regulate meristematic activity in organs other than leaves, since the mutation also causes some ectopic outgrowths on stem surfaces and at the base of floral organs. Three class I knox genes, i.e., KNAT1, KNAT2 and KNAT6, were expressed aberrantly in the leaves of the bop1-1 mutant. Furthermore, the bop1-1 mutation showed some synergistic effect in double mutants with as1-1 or as2-2 mutation that is known to be defective in the regulation of meristematic activity and class I knox gene expression in leaves. The bop1-1 mutation also showed a synergistic effect with the stm-1 mutation, a strong mutant allele of a class I knox gene, STM. We, thus, suggest that BOP1 promotes or maintains a developmentally determinate state in leaf cells through the regulation of class I knox genes.     

Arabidopsis thaliana (L.) Heynh. as a model object for studying genetic control of morphogenesis

A vast amount of information on the genetic control of plant development has been obtained in Arabidopsis thaliana with classical genetic and molecular biological methods. The genes involved in multistep regulation of floral morphogenesis have been identified. The formation of floral meristem is controlled by the LEAFY (LFY), UNUSUAL FLORAL ORGANS (UFO), APETALA1 (AP1), and APETALA2 (AP2) genes. Studies of the abruptus and bractea recessive monogenic mutants from the collection of the Department of Genetics and Selection, Moscow State University, showed that the ABRUPTUS (ABR) and BRACTEA (BRA) genes also play an important role in inflorescence differentiation. The ABR gene controls the early formation of organ primordia on the inflorescence and the formation of floral organ primordia after floral initiation. Further differentiation of inflorescence organ primordia in vegetative or generative organs depends on the activity of the LFY gene, and floral organ identity is determined by the homeotic genes. Presumably, the major function of the ABR gene is to determine the auxin polar transport. The BRA gene suppresses the development of bracts on the inflorescence and constrains cell division at the base of primordia of rosette and cauline leaves.     

Cleistogamy: A tool for the study of floral morphogenesis,function and evolution

Cleistogamy—the production of open (chasmogamous—CH) and closed (cleistogamous—CL) floral forms by a species—is widespread among the angiosperms. While the CL flower is autogamous, the CH flower may provide a means for outcrossing. The term “cleistogamy” has also been used to describe other phenomena. A classification of types of cleistogamy is proposed. In this review, a restricted definition of cleistogamy is used to refer to species which show real floral dimorphisms, with divergent developmental pathways leading to CL and CH as well as intermediate floral forms. Reductions in the androecium and corolla are the most common feature of the CL flowers. The structural, developmental, and functional aspects of cleistogamy are reviewed. Evidence is presented to show that the CL flowers have modifications in their development which ensure self pollination. A proposal is made for using this phenomenon of dimorphic flower production as a system for the study of floral morphogenesis, function and evolution.