Esr genes show different levels of expression in the same region of maize endosperm

Decoding the gene regulatory network of endosperm differentiation in maize

The persistent cereal endosperm constitutes the majority of the grain volume. Dissecting the gene regulatory network underlying cereal endosperm development will facilitate yield and quality improvement of cereal crops. Here, we use single-cell transcriptomics to analyze the developing maize (Zea mays) endosperm during cell differentiation. After obtaining transcriptomic data from 17,022 single cells, we identify 12 cell clusters corresponding to five endosperm cell types and revealing complex transcriptional heterogeneity. We delineate the temporal gene-expression pattern from 6 to 7 days after pollination. We profile the genomic DNA-binding sites of 161 transcription factors differentially expressed between cell clusters and constructed a gene regulatory network by combining the single-cell transcriptomic data with the direct DNA-binding profiles, identifying 181 regulons containing genes encoding transcription factors along with their high-confidence targets, Furthermore, we map the regulons to endosperm cell clusters, identify cell-cluster-specific essential regulators, and experimentally validated three predicted key regulators. This study provides a framework for understanding cereal endosperm development and function at single-cell resolution.

Characterization and Transcriptome Analysis of Maize Small-Kernel Mutant smk7a in Different Development Stages

The kernel serves as a storage organ for various nutrients and determines the yield and quality of maize. Understanding the mechanisms regulating kernel development is important for maize production. In this study, a small-kernel mutant smk7a of maize was characterized. Cytological observation suggested that the development of the endosperm and embryo was arrested in smk7a in the early development stage. Biochemical tests revealed that the starch, zein protein, and indole-3-acetic acid (IAA) contents were significantly lower in smk7a compared with wild-type (WT). Consistent with the defective development phenotype, transcriptome analysis of the kernels 12 and 20 days after pollination (DAP) revealed that the starch, zein, and auxin biosynthesis-related genes were dramatically downregulated in smk7a. Genetic mapping indicated that the mutant was controlled by a recessive gene located on chromosome 2. Our results suggest that disrupted nutrition accumulation and auxin synthesis cause the defective endosperm and embryo development of smk7a.

Genome-wide identification and comparative analysis of CLE family in rapeseed and its diploid progenitors

Crop genomics and breeding CLAVATA3/EMBRYO SURROUNDING REGION-RELATED (CLE) proteins belong to a small peptide family in plants. During plant development, CLE gene family members play a pivotal role in regulating cell-to-cell communication and stem cell maintenance. However, the evolutionary process and functional importance of CLEs are unclear in Brassicaceae. In this study, a total of 70 BnCLEs were identified in Brassica napus (2n = 4x = 38, A_nC_n): 32 from the A_n subgenome, 36 from the C_n subgenome, and 2 from the unanchored subgenome. Meanwhile, 29 BrCLE and 32 BoCLE genes were explored in Brassica rapa (2n = 2x = 20, A_r) and Brassica oleracea (2n = 2x = 18, C_o). Phylogenetic analysis revealed that 163 CLEs derived from three Brassica species and Arabidopsis thaliana can be divided into seven subfamilies. Homology and synteny analyses indicated whole-genome triplication (WGT) and segmental duplication may be the major contributors to the expansion of CLE family. In addition, RNA-seq and qPCR analysis indicated that 19 and 16 BnCLEs were more highly expressed in immature seeds and roots than in other tissues. Some CLE gene pairs exhibited different expression patterns in the same tissue, which indicated possible functional divergence. Furthermore, genetic variations and regional association mapping analysis indicated that 12 BnCLEs were potential genes for regulating important agronomic traits. This study provided valuable information to understand the molecular evolution and biological function of CLEs in B. napus and its diploid progenitors, which will be helpful for genetic improvement of high-yield breeding in B. napus.

Maize Endosperm Development: Tissues, Cells, Molecular Regulation and Grain Quality Improvement

Maize endosperm plays important roles in human diet, animal feed and industrial applications. Knowing the mechanisms that regulate maize endosperm development could facilitate the improvement of grain quality. This review provides a detailed account of maize endosperm development at the cellular and histological levels. It features the stages of early development as well as developmental patterns of the various individual tissues and cell types. It then covers molecular genetics, gene expression networks, and current understanding of key regulators as they affect the development of each tissue. The article then briefly considers key changes that have occurred in endosperm development during maize domestication. Finally, it considers prospects for how knowledge of the regulation of endosperm development could be utilized to enhance maize grain quality to improve agronomic performance, nutrition and economic value.

Molecular Traits and Functional Analysis of the CLAVATA3/Endosperm Surrounding Region-Related Small Signaling Peptides in Three Species of Gossypium Genus

The CLAVATA3/endosperm surrounding region-related (CLE) small peptides are a group of C-terminally encoded and post-translationally modified signal molecules involved in regulating the growth and development of various plants. However, the function and evolution of these peptides have so far remained elusive in cotton. In this study, 55, 56, and 86 CLE genes were identified in the Gossypium raimondii, Gossypium arboreum, and Gossypium hirsutum genomes, respectively, and all members were divided into seven groups. These groups were distinctly different in their protein characteristics, gene structures, conserved motifs, and multiple sequence alignment. Whole genome or segmental duplications played a significant role in the expansion of the CLE family in cotton, and experienced purifying selection during the long evolutionary process in cotton. Cis-acting regulatory elements and transcript profiling revealed that the CLE genes of cotton exist in different tissues, developmental stages, and respond to abiotic stresses. Protein properties, structure prediction, protein interaction network prediction of GhCLE2, GhCLE33.2, and GhCLE28.1 peptides were, respectively, analyzed. In addition, the overexpression of GhCLE2, GhCLE33.2, or GhCLE28.1 in Arabidopsis, respectively, resulted in a distinctive shrub-like dwarf plant, slightly purple leaves, large rosettes with large malformed leaves, and lack of reproductive growth. This study provides important insights into the evolution of cotton CLEs and delineates the functional conservatism and divergence of CLE genes in the growth and development of cotton.

Maize endosperm development

Recent breakthroughs in transcriptome analysis and gene characterization have provided valuable resources and information about the maize endosperm developmental program. The high temporal-resolution transcriptome analysis has yielded unprecedented access to information about the genetic control of seed development. Detailed spatial transcriptome analysis using laser-capture microdissection has revealed the expression patterns of specific populations of genes in the four major endosperm compartments: the basal endosperm transfer layer (BETL), aleurone layer (AL), starchy endosperm (SE), and embryo-surrounding region (ESR). Although the overall picture of the transcriptional regulatory network of endosperm development remains fragmentary, there have been some exciting advances, such as the identification of OPAQUE11 (O11) as a central hub of the maize endosperm regulatory network connecting endosperm development, nutrient metabolism, and stress responses, and the discovery that the endosperm adjacent to scutellum (EAS) serves as a dynamic interface for endosperm-embryo crosstalk. In addition, several genes that function in BETL development, AL differentiation, and the endosperm cell cycle have been identified, such as ZmSWEET4c, Thk1, and Dek15, respectively. Here, we focus on current advances in understanding the molecular factors involved in BETL, AL, SE, ESR, and EAS development, including the specific transcriptional regulatory networks that function in each compartment during endosperm development.

Genetic and Molecular Regulation of Seed Storage Proteins (SSPs) to Improve Protein Nutritional Value of Oilseed Rape (Brassica napus L.) Seeds

The world-wide demand for additional protein sources for human nutrition and animal feed keeps rising due to rapidly growing world population. Oilseed rape is a second important oil producing crop and the by-product of the oil production is a protein rich meal. The protein in rapeseed meal finds its application in animal feed and various industrial purposes, but its improvement is of great interest, especially for non-ruminants and poultry feed. To be able to manipulate the quality and quantity of seed protein in oilseed rape, understanding genetic architecture of seed storage protein (SSPs) synthesis and accumulation in this crop species is of great interest. For this, application of modern molecular breeding tools such as whole genome sequencing, genotyping, association mapping, and genome editing methods implemented in oilseed rape seed protein improvement would be of great interest. This review examines current knowledge and opportunities to manipulate of SSPs in oilseed rape to improve its quality, quantity and digestibility.

Signaling in Early Maize Kernel Development

Developing the next plant generation within the seed requires the coordination of complex programs driving pattern formation, growth, and differentiation of the three main seed compartments: the embryo (future plant), the endosperm (storage compartment), representing the two filial tissues, and the surrounding maternal tissues. This review focuses on the signaling pathways and molecular players involved in early maize kernel development. In the 2 weeks following pollination, functional tissues are shaped from single cells, readying the kernel for filling with storage compounds. Although the overall picture of the signaling pathways regulating embryo and endosperm development remains fragmentary, several types of molecular actors, such as hormones, sugars, or peptides, have been shown to be involved in particular aspects of these developmental processes. These molecular actors are likely to be components of signaling pathways that lead to transcriptional programming mediated by transcriptional factors. Through the integrated action of these components, multiple types of information received by cells or tissues lead to the correct differentiation and patterning of kernel compartments. In this review, recent advances regarding the four types of molecular actors (hormones, sugars, peptides/receptors, and transcription factors) involved in early maize development are presented.

ZmZHOUPI, an endosperm-specific basic helix-loop-helix transcription factor involved in maize seed development

In angiosperm seeds the embryo is embedded within the endosperm, which is in turn enveloped by the seed coat, making inter-compartmental communication essential for coordinated seed growth. In this context the basic helix-loop-helix domain transcription factor AtZHOUPI (AtZOU) fulfils a key role in both the lysis of the transient endosperm and in embryo cuticle formation in Arabidopsis thaliana. In maize (Zea mays), a cereal with a persistent endosperm, a single gene, ZmZOU, falls into the same phylogenetic clade as AtZOU. Its expression is limited to the endosperm where it peaks during the filling stage. In ZmZOU-RNA interference knock-down lines embryo size is slightly reduced and the embryonic suspensor and the adjacent embryo surrounding region show retarded breakdown. Ectopic expression of ZmZOU reduces stomatal number, possibly due to inappropriate protein interactions. ZmZOU forms functional heterodimers with AtICE/AtSCREAM and the closely related maize proteins ZmICEb and ZmICEc, but its interaction is more efficient with the ZmICEa protein, which shows sequence divergence and only has close homologues in other monocotyledonous species. Consistent with the observation that these complexes can trans-activate target gene promoters from Arabidopsis, ZmZOU partially complements the Atzou-4 mutant. However, structural, trans-activation and gene expression data support the hypothesis that ZmZOU and ZmICEa may have coevolved to form a functional complex unique to monocot seeds. This divergence may explain the reduced functionality of ZmZOU in Arabidopsis, and reflect functional specificities which are unique to the monocotyledon lineage.

Barley (Hordeum vulgare L.) low phytic acid 1-1: an endosperm-specific, filial determinant of seed total phosphorus

Inositol hexaphosphate (Ins P6 or "phytic acid") typically accounts for 75 (± 10%) of seed total phosphorus (P). In some cases, genetic blocks in seed Ins P6 accumulation can also alter the distribution or total amount of seed P. In nonmutant barley (Hordeum vulgare L.) caryopses, ~80% of Ins P6 and total P accumulate in the aleurone layer, the outer layer of the endosperm, with the remainder in the germ. In barley low phytic acid 1-1 (Hvlpa1-1) seed, both endosperm Ins P6 and total P are reduced (~45% and ~25%, respectively), but germs are phenotypically wild type. This translates into a net reduction in whole-seed total P of ~15%. Nutrient culture studies demonstrate that the reduction in endosperm total P is not due to a reduction in the uptake of P into the maternal plant. Genetic tests (analyses of testcross and F2 seed) reveal that the Hvlpa1-1 genotype of the filial seed conditions the seed total P reduction; sibling seed in the same head of barley that differ in their Hvlpa1-1 genotype (heterozygous vs. homozygous recessive) differ in their total P (normal vs. reduced, respectively). Therefore, Hvlpa1 functions as a seed-specific or filial determinant of barley endosperm total P.

A PCR-based forward genetics screening, using expression domain-specific markers, identifies mutants in endosperm transfer cell development

Mutant collections are an invaluable source of material on which forward genetic approaches allow the identification of genes affecting a wide variety of biological processes. However, some particular developmental stages and morphological structures may resist analysis due to their physical inaccessibility or to deleterious effects associated to their modification. Furthermore, lethal mutations acting early in development may escape detection. We have approached the characterization of 101 maize seed mutants, selected from a collection of 27,500 visually screened Mu-insertion lines, using a molecular marker approach based on a set of genes previously ascribed to different tissue compartments within the early developing kernel. A streamlined combination of qRT-PCR assays has allowed us to preliminary pinpoint the affected compartment, establish developmental comparisons to WT siblings and select mutant lines with alterations in the different compartments. Furthermore, clusters of markers co-affected by the underlying mutation were identified. We have analyzed more extensively a set of lines presenting significant variation in transfer cell-associated expression markers, and have performed morphological observations, and immunolocalization experiments to confirm the results, validating this approach as an efficient mutant description tool.

Functional characterization of the HD-ZIP IV transcription factor OCL1 from maize

OCL1 (OUTER CELL LAYER1) encodes a maize HD-ZIP class IV transcription factor (TF) characterized by the presence of a homeo DNA-binding domain (HD), a dimerization leucine zipper domain (ZIP), and a steroidogenic acute regulatory protein (StAR)-related lipid transfer domain (START) involved in lipid transport in animals but the function of which is still unknown in plants. By combining yeast and plant trans-activation assays, the transcriptional activation domain of OCL1 was localized to 85 amino acids in the N-terminal part of the START domain. Full-length OCL1 devoid of this activation domain is unable to trans-activate a reporter gene under the control of a minimal promoter fused to six repeats of the L1 box, a cis-element present in target genes of HD-ZIP IV TFs in Arabidopsis. In addition, ectopic expression of OCL1 leads to pleiotropic phenotypic aberrations in transgenic maize plants, the most conspicuous one being a strong delay in flowering time which is correlated with the misexpression of molecular markers for floral transition such as ZMM4 (Zea Mays MADS-box4) or DLF1 (DELAYED FLOWERING1). As suggested by the interaction in planta between OCL1 and SWI3C1, a bona fide subunit of the SWI/SNF complex, OCL1 may modulate transcriptional activity of its target genes by interaction with a chromatin remodelling complex.

Family life at close quarters: communication and constraint in angiosperm seed development

The formation of viable angiosperm seeds involves the co-ordinated growth and development of three genetically distinct organisms, the maternally derived seed coat and the zygotic embryo and endosperm. The physical relationships of these tissues are initially established during the specification and differentiation of the female gametophyte within the tissues of the developing ovule. The molecular programmes implicated in both ovule and seed development involve elements of globally important pathways (such as auxin signalling), as well as ovule- and seed-specific pathways. Recurrent themes, such as the precisely controlled death of specific cell types and the regulation of cell-cell communication and nutrition by the selective establishment of symplastic and apoplastic barriers, appear to play key roles in both pre- and post-fertilization seed development. Much of post-fertilization seed growth occurs during a key developmental window shortly after fertilization and involves the dramatic expansion of the young endosperm, constrained by surrounding maternal tissues. The complex tissue-specific regulation of carbohydrate metabolism in specific seed compartments has been shown to provide a driving force for this early seed expansion. The embryo, which is arguably the most important component of the seed, appears to be only minimally involved in early seed development. Given the evolutionary and agronomic importance of angiosperm seeds, the complex combination of communication pathways which co-ordinate their growth and development remains remarkably poorly understood.

ZmPIN1-mediated auxin transport is related to cellular differentiation during maize embryogenesis and endosperm development

To study the influence of PINFORMED1 (PIN1)-mediated auxin transport during embryogenesis and endosperm development in monocots, the expression pattern of the three identified ZmPIN1 genes was determined at the transcript level. Localization of the corresponding proteins was also analyzed during maize (Zea mays) kernel development. An anti-indole-3-acetic acid (IAA) monoclonal antibody was used to visualize IAA distribution and correlate the direction of auxin active transport, mediated by ZmPIN1 proteins, with the actual amount of auxin present in maize kernels at different developmental stages. ZmPIN1 genes are expressed in the endosperm soon after double fertilization occurs; however, unlike other tissues, the ZmPIN1 proteins were never polarly localized in the plasma membrane of endosperm cells. ZmPIN1 transcripts and proteins also colocalize in developing embryos, and the ZmPIN1 proteins are polarly localized in the embryo cell plasma membrane from the first developmental stages, indicating the existence of ZmPIN1-mediated auxin fluxes. Auxin distribution visualization indicates that the aleurone, the basal endosperm transfer layer, and the embryo-surrounding region accumulate free auxin, which also has a maximum in the kernel maternal chalaza. During embryogenesis, polar auxin transport always correlates with the differentiation of embryo tissues and the definition of the embryo organs. On the basis of these reports and of the observations on tissue differentiation and IAA distribution in defective endosperm-B18 mutant and in N-1-naphthylphthalamic acid-treated kernels, a model for ZmPIN1-mediated transport of auxin and the related auxin fluxes during maize kernel development is proposed. Common features between this model and the model previously proposed for Arabidopsis (Arabidopsis thaliana) are discussed.

Bioinformatic analysis of the CLE signaling peptide family

BACKGROUND

Plants encode a large number of leucine-rich repeat receptor-like kinases. Legumes encode several LRR-RLK linked to the process of root nodule formation, the ligands of which are unknown. To identify ligands for these receptors, we used a combination of profile hidden Markov models and position-specific iterative BLAST, allowing us to detect new members of the CLV3/ESR (CLE) protein family from publicly available sequence databases.

RESULTS

We identified 114 new members of the CLE protein family from various plant species, as well as five protein sequences containing multiple CLE domains. We were able to cluster the CLE domain proteins into 13 distinct groups based on their pairwise similarities in the primary CLE motif. In addition, we identified secondary motifs that coincide with our sequence clusters. The groupings based on the CLE motifs correlate with known biological functions of CLE signaling peptides and are analogous to groupings based on phylogenetic analysis and ectopic overexpression studies. We tested the biological function of two of the predicted CLE signaling peptides in the legume Medicago truncatula. These peptides inhibit the activity of the root apical and lateral root meristems in a manner consistent with our functional predictions based on other CLE signaling peptides clustering in the same groups.

CONCLUSION

Our analysis provides an identification and classification of a large number of novel potential CLE signaling peptides. The additional motifs we found could lead to future discovery of recognition sites for processing peptidases as well as predictions for receptor binding specificity.

Gain-of-function phenotypes of many CLAVATA3/ESR genes, including four new family members, correlate with tandem variations in the conserved CLAVATA3/ESR domain

Secreted peptide ligands are known to play key roles in the regulation of plant growth, development, and environmental responses. However, phenotypes for surprisingly few such genes have been identified via loss-of-function mutant screens. To begin to understand the processes regulated by the CLAVATA3 (CLV3)/ESR (CLE) ligand gene family, we took a systems approach to gene identification and gain-of-function phenotype screens in transgenic plants. We identified four new CLE family members in the Arabidopsis (Arabidopsis thaliana) genome sequence and determined their relative transcript levels in various organs. Overexpression of CLV3 and the 17 CLE genes we tested resulted in premature mortality and/or developmental timing delays in transgenic Arabidopsis plants. Overexpression of 10 CLE genes and the CLV3 positive control resulted in arrest of growth from the shoot apical meristem (SAM). Overexpression of nearly all the CLE genes and CLV3 resulted in either inhibition or stimulation of root growth. CLE4 expression reversed the SAM proliferation phenotype of a clv3 mutant to one of SAM arrest. Dwarf plants resulted from overexpression of five CLE genes. Overexpression of new family members CLE42 and CLE44 resulted in distinctive shrub-like dwarf plants lacking apical dominance. Our results indicate the capacity for functional redundancy of many of the CLE ligands. Additionally, overexpression phenotypes of various CLE family members suggest roles in organ size regulation, apical dominance, and root growth. Similarities among overexpression phenotypes of many CLE genes correlate with similarities in their CLE domain sequences, suggesting that the CLE domain is responsible for interaction with cognate receptors.

Multicellular structures developing during maize microspore culture express endosperm and embryo-specific genes and show different embryogenic potentialities

During maize pollen embryogenesis, a range of multicellular structures are formed. Using different approaches, the "nature" of these structures has been determined in terms of their embryogenic potential. In situ molecular identification techniques for gene transcripts and products, and a novel cell tracking system indicated the presence of embryogenic (embryo-like structures, ELS) and non-embryogenic (callus-like structures, CLS) structures that occurred for short periods within the cultures. Some multicellular structures with a compact appearance generated embryos. RT-PCR and fluorescence in situ hybridization (FISH) with confocal microscopy techniques using specific gene markers of the endosperm (ZmESR2, ZmAE3) and embryo (LTP2 and ZmOCL1, ZmOCL3) revealed "embryo" and "endosperm" potentialities in these various multicellular structures present in the cultures. The results presented here showed distinct and specific patterns of gene expression. Altogether, the results demonstrate the presence of different molecules on both embryonic and non-embryonic structures. Their possible roles are discussed in the context of a parallel between embryo/endosperm interactions in planta and embryonic and non-embryonic structure interrelations under in vitro conditions.

Genetic analysis as a tool to investigate the molecular mechanisms underlying seed development in maize

BACKGROUND

In angiosperms the seed is the outcome of double fertilization, a process leading to the formation of the embryo and the endosperm. The development of the two seed compartments goes through three main phases: polarization, differentiation of the main tissues and organs and maturation.

SCOPE

This review focuses on the maize kernel as a model system for developmental and genetic studies of seed development in angiosperms. An overview of what is known about the genetic and molecular aspects underlying embryo and endosperm formation and maturation is presented. The role played by embryonic meristems in laying down the plant architecture is discussed. The acquisition of the different endosperm domains are presented together with the use of molecular markers available for the detection of these domains. Finally the role of programmed cell death in embryo and endosperm development is considered.

CONCLUSIONS

The sequence of events occurring in the developing maize seed appears to be strictly regulated. Proper seed development requires the co-ordinated expression of embryo and endosperm genes and relies on the interaction between the two seed components and between the seed and the maternal tissues. Mutant analysis is instrumental in unravelling the genetic control underlying the formation of each compartment as well as the molecular signals interplaying between the two compartments.

Expression of the promoter of HyPRP, an embryo-specific gene from Zea mays in maize and tobacco transgenic plants

zmHyPRP is a gene specifically expressed in maize immature embryos where its transcripts are mainly observed in the scutellum. It has been shown that zmHyPRP expression in the embryo is arrested when ABA levels increase at the beginning of the maturation stage. Here we report the ability of 2 Kb zmHyPRP promoter to reproduce the zmHyPRP gene specific expression pattern in the maize embryo and its repression by ABA at the end of the morphogenetic process. Three different approaches have been used, transient particle bombardment of maize immature excised embryos and stable transformation of maize and tobacco plants with a construct containing 2 Kb of zmHyPRP promoter fused to the GUS gene. This construct has shown to confer specific expression to maize and tobacco embryos but in tobacco expression in the embryo was very low. The same construct was also negatively regulated by ABA in embryos of both species. This suggests that 2 Kb of the zmHyPRP promoter contain all regulatory elements sufficient to confer the developmental expression patterns of the gene characterized to date.

Expression patterns of duplicate genes in the developing root in Arabidopsis thaliana

Data on gene expression in the development of the root in Arabidopsis thaliana were used to test for expression profile differences among multi-gene families and to examine the extent to which expression differences accompanied coding sequences divergence within families. Significant differences among families were observed on two principal axes, accounting for over 80% of the variance in the expression data. The number of synonymous nucleotide substitutions per synonymous site (d(S)) and the number of nonsynonymous nucleotide substitutions per nonsynonymous site (d(N)) were estimated between the members of two-member families (N = 428) and between phylogenetically independent sister pairs (N = 190) of sequences within larger families. Ribosomal proteins and a few other proteins were exceptional in showing highly divergent expression patterns in spite of very low levels of amino acid sequence divergence, as indicated by the low d(N) relative to d(S). However, the majority of gene duplicates showed relatively high levels of amino acid sequence divergence without appreciable change in expression pattern in the cell types analyzed.

maternally expressed gene1 Is a novel maize endosperm transfer cell-specific gene with a maternal parent-of-origin pattern of expression

Growth of the maize (Zea mays) endosperm is tightly regulated by maternal zygotic and sporophytic genes, some of which are subject to a parent-of-origin effect. We report here a novel gene, maternally expressed gene1 (meg1), which shows a maternal parent-of-origin expression pattern during early stages of endosperm development but biallelic expression at later stages. Interestingly, a stable reporter fusion containing the meg1 promoter exhibits a similar pattern of expression. meg1 is exclusively expressed in the basal transfer region of the endosperm. Further, we show that the putatively processed MEG1 protein is glycosylated and subsequently localized to the labyrinthine ingrowths of the transfer cell walls. Hence, the discovery of a parent-of-origin gene expressed solely in the basal transfer region opens the door to epigenetic mechanisms operating in the endosperm to regulate certain aspects of nutrient trafficking from the maternal tissue into the developing seed.

An invertase inhibitor from maize localizes to the embryo surrounding region during early kernel development

Invertase activity is thought to play a regulatory role during early kernel development by converting sucrose originating from source leaves into hexoses to support cell division in the endosperm and embryo. Invertases are regulated at the posttranslational level by small protein inhibitors, INVINHs. We found that in maize (Zea mays), an invertase inhibitor homolog (ZM-INVINH1) is expressed early in kernel development, between 4 and 7 d after pollination. Invertase activity is reduced in vitro in the presence of recombinant ZM-INVINH1, and inhibition is attenuated by pre-incubation with sucrose. The presence of a putative signal peptide, fractionation experiments, and ZM-INVINH1::green fluorescent protein fusion experiments indicate that the protein is exported to the apoplast. Moreover, association of ZM-INVINH1 with the glycoprotein fraction by concanavalin A chromatogaphy suggests that ZM-INVINH1 interacts with an apoplastic invertase during early kernel development. ZM-INVINH1 was localized to the embryo surrounding region by in situ analysis, suggesting that this region forms a boundary, compartmentalizing apoplast invertase activity to allow different embryo and endosperm developmental rates.

Receptor kinase signaling in plant development

The Arabidopsis genome sequence has revealed that plants contain a much larger complement of receptor kinase genes than other organisms. Early analysis of these genes revealed involvement in a diverse array of developmental and defense functions that included gametophyte development, pollen-pistil interactions, shoot apical meristem equilibrium, hormone perception, and cell morphogenesis. Amino acid sequence motifs and binding studies indicate that the ectodomains are capable of binding, either directly or indirectly, various classes of molecules including proteins, carbohydrates, and steroids. Genetic and biochemical approaches have begun to identify other components of several signal transduction pathways. Some receptor-like kinases (RLKs) appear to function with coreceptors lacking kinase domains, and genome analysis suggests this might be true for many RLKs. The KAPP protein phosphatase functions as a negative regulator of at least two RLK systems, and in vitro studies suggest it could be a common component of more. Whether plant signaling systems display a modularity similar to animal systems remains to be determined. Future efforts will reveal unknown functions of other RLKs and elucidate the relationships among their signaling networks.

Endosperm: the crossroad of seed development

The endosperm is an essential part of the seed that sustains embryo development and reserve storage. Several genes that are involved in endosperm differentiation and that have domains of expression arranged along a conserved antero-posterior axis have been isolated in Arabidopsis and in cereals. Endosperm polarity is controlled maternally by chromatin-remodeling complexes. Endosperm development appears to be predominantly under epigenetic controls that might be linked with its evolutionary origin.

Shoot and floral meristem maintenance in arabidopsis

The shoot apical meristem (SAM) of higher plants functions as a site of continuous organogenesis within which a small pool of pluripotent stem cells replenishes the cells incorporated into lateral organs. This article summarizes recent results demonstrating that the fate of stem cells in Arabidopsis shoot and floral meristems is controlled by overlapping spatial and temporal signaling systems. Stem cell maintenance is an active process requiring constant communication between neighboring groups of SAM cells. Information flows via a ligand-receptor signal transduction pathway, resulting in the formation of a spatial feedback loop that stabilizes the size of the stem cell population. Termination of stem cell activity during flower development is achieved by a temporal feedback loop involving both stem cell maintenance genes and flower patterning genes. These investigations are providing exciting insights into the components and activities of the stem cell regulatory pathway and into the interaction of this pathway with molecular mechanisms that control floral patterning.

Cell fate specification in the cereal endosperm

While superficially simple, endosperm development is a complex, dynamic process. Cereal endosperms contain three major cell types: starchy endosperm, transfer cells and aleurone. The localized accumulation of the END1 transcript in the syncitial endosperm suggests that signals from the maternal placental tissue specify transfer cell type early. Aleurone fate is plastic and requires the continual input of positional cues to maintain cell identity. Starchy endosperm appears to be the default cell type. Mutant patterns suggest that a regulatory hierarchy integrates endosperm development. Requirements for gametic imprinting, maternal : paternal genome ratios and putative chromatin modeling factors indicate the importance of genomic control.

Gene expression patterns in the maize caryopsis: clues to decisions in embryo and endosperm development

We will describe gene expression patterns in the maize caryopsis, which provide clues to developmental decisions and questions in the embryo and endosperm. The emphasis will be on the development of the root/shoot axis, which is the main achievement of plant embryogenesis. Data obtained in the vegetative seedling are included as far as they may be relevant to the elaboration of the shoot/root axis. Development of the embryo will be briefly compared to endosperm as both seed compartment exhibit pronounced differences.

ENDOSPERM DEVELOPMENT: Cellularization and Cell Fate Specification

The endosperm develops from the central cell of the megagametophyte after introduction of the second male gamete into the diploid central cell. Of the three forms of endosperm in angiosperms, the nuclear type is prevalent in economically important species, including the cereals. Landmarks in nuclear endosperm development are the coenocytic, cellularization, differentiation, and maturation stages. The differentiated endosperm contains four major cell types: starchy endosperm, aleurone, transfer cells, and the cells of the embryo surrounding region. Recent research has demonstrated that the first two phases of endosperm occur via mechanisms that are conserved among all groups of angiosperms, involving directed nuclear migration during the coenocytic stage and anticlinal cell wall deposition by cytoplasmic phragmoplasts formed in interzones between radial microtubular systems emanating from nuclear membranes. Complete cellularization of the endosperm coenocyte is achieved through centripetal growth of cell files, extending to the center of the endosperm cavity. Key points in cell cycle control and control of the MT (microtubular) cytoskeletal apparatus central to endosperm development are discussed. Specification of cell fates in the cereal endosperm appears to occur via positional signaling; cells in peripheral positions, except over the main vascular tissues, assume aleurone cell fate. Cells over the main vascular tissue become transfer cells and all interior cells become starchy endosperm cells. Studies in maize have implicated Crinkly4, a protein receptor kinase-like molecule, in aleurone cell fate specification.