The thick aleurone1 mutant defines a negative regulation of maize aleurone cell fate that functions downstream of defective kernel1

Membrane-anchored calpains - hidden regulators of growth and development beyond plants?

Calpains are modulatory proteases that modify diverse cellular substrates and play essential roles in eukaryots. The best studied are animal cytosolic calpains. Here, we focus on enigmatic membrane-anchored calpains, their structural and functional features as well as phylogenetic distribution. Based on domain composition, we identified four types of membrane-anchored calpains. Type 1 and 2 show broad phylogenetic distribution among unicellular protists and streptophytes suggesting their ancient evolutionary origin. Type 3 and 4 diversified early and are present in brown algae and oomycetes. The plant DEK1 protein is the only representative of membrane-anchored calpains that has been functionally studied. Here, we present up to date knowledge about its structural features, putative regulation, posttranslational modifications, and biological role. Finally, we discuss potential model organisms and available tools for functional studies of membrane-anchored calpains with yet unknown biological role. Mechanistic understanding of membrane-anchored calpains may provide important insights into fundamental principles of cell polarization, cell fate control, and morphogenesis beyond plants.

Dissection of Developmental Programs and Regulatory Modules Directing Endosperm Transfer Cell and Aleurone Identity in the Syncytial Endosperm of Barley

Endosperm development in barley starts with the formation of a multinucleate syncytium, followed by cellularization in the ventral part of the syncytium generating endosperm transfer cells (ETCs) as first differentiating subdomain, whereas aleurone (AL) cells will originate from the periphery of the enclosing syncytium. Positional signaling in the syncytial stage determines cell identity in the cereal endosperm. Here, we performed a morphological analysis and employed laser capture microdissection (LCM)-based RNA-seq of the ETC region and the peripheral syncytium at the onset of cellularization to dissect developmental and regulatory programs directing cell specification in the early endosperm. Transcriptome data revealed domain-specific characteristics and identified two-component signaling (TCS) and hormone activities (auxin, ABA, ethylene) with associated transcription factors (TFs) as the main regulatory links for ETC specification. On the contrary, differential hormone signaling (canonical auxin, gibberellins, cytokinin) and interacting TFs control the duration of the syncytial phase and timing of cellularization of AL initials. Domain-specific expression of candidate genes was validated by in situ hybridization and putative protein-protein interactions were confirmed by split-YFP assays. This is the first transcriptome analysis dissecting syncytial subdomains of cereal seeds and provides an essential framework for initial endosperm differentiation in barley, which is likely also valuable for comparative studies with other cereal crops.

Multi-omic characterization of the maize GPI synthesis mutant gwt1 with defects in kernel development

BACKGROUND

Glycosylphosphatidylinositol (GPI) and GPI-anchored proteins (GAPs) are important for cell wall formation and reproductive development in Arabidopsis. However, monocot counterparts that function in kernel endosperm development have yet to be discovered. Here, we performed a multi-omic analysis to explore the function of GPI related genes on kernel development in maize.

RESULTS

In maize, 48 counterparts of human GPI synthesis and lipid remodeling genes were identified, in which null mutation of the glucosaminyl-phosphatidylinositol O-acyltransferase1 gene, ZmGWT1, caused a kernel mutant (named gwt1) with defects in the basal endosperm transport layer (BETL). We performed plasma membrane (PM) proteomics to characterize the potential GAPs involved in kernel development. In total, 4,981 proteins were successfully identified in 10-DAP gwt1 kernels of mutant and wild-type (WT), including 1,638 membrane-anchored proteins with different posttranslational modifications. Forty-seven of the 256 predicted GAPs were differentially accumulated between gwt1 and WT. Two predicted BETL-specific GAPs (Zm00001d018837 and Zm00001d049834), which kept similar abundance at general proteome but with significantly decreased abundance at membrane proteome in gwt1 were highlighted.

CONCLUSIONS

Our results show the importance of GPI and GAPs for endosperm development and provide candidate genes for further investigation of the regulatory network in which ZmGWT1 participates.

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.

Cereal Endosperms: Development and Storage Product Accumulation

The persistent triploid endosperms of cereal crops are the most important source of human food and animal feed. The development of cereal endosperms progresses through coenocytic nuclear division, cellularization, aleurone and starchy endosperm differentiation, and storage product accumulation. In the past few decades, the cell biological processes involved in endosperm formation in most cereals have been described. Molecular genetic studies performed in recent years led to the identification of the genes underlying endosperm differentiation, regulatory network governing storage product accumulation, and epigenetic mechanism underlying imprinted gene expression. In this article, we outline recent progress in this area and propose hypothetical models to illustrate machineries that control aleurone and starchy endosperm differentiation, sugar loading, and storage product accumulations. A future challenge in this area is to decipher the molecular mechanisms underlying coenocytic nuclear division, endosperm cellularization, and programmed cell death.

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.

CRISPR-Cas9 mediated mutation in GRAIN WIDTH and WEIGHT2 (GW2) locus improves aleurone layer and grain nutritional quality in rice

Enhancing crop productivity and their nutritional quality are the key components and primary focus of crop improvement strategy for fulfilling future food demand and improving human health. Grain filling and endosperm development are the key determinants of grain yield and nutritional quality. GRAIN WIDTH and WEIGHT2 (GW2) gene encodes a RING-type E3 ubiquitin ligase and determines the grain weight in cereal crops. Here we report GW2 knockout (KO) mutants in Indica (var. MTU1010) through CRISPR/Cas9 genome editing. The endosperm of GW2-KO mutant seed displays a thick aleurone layer with enhanced grain protein content. Further the loss of function of OsGW2 results in improved accumulation of essential dietary minerals (Fe, Zn, K, P, Ca) in the endosperm of rice grain. Additionally, the mutants displayed an early growth vigour phenotype with an improved root and shoot architecture. The hull morphology of GW2-KO lines also showed improved, grain filling thereby promoting larger grain architecture. Together, our findings indicate that GW2 may serve as a key regulator of improved grain architecture, grain nutritional quality and an important modulator of plant morphology. The study offers a strategy for the development of improved rice cultivars with enriched nutritional quality and its possible implementation in other cereals as well.

Comparative transcriptomics and network analysis define gene coexpression modules that control maize aleurone development and auxin signaling

The naked endosperm1 (nkd1), naked endosperm2 (nkd2), and thick aleurone1 (thk1) genes are important regulators of maize (Zea mays L.) endosperm development. Double mutants of nkd1 and nkd2 (nkd1,2) show multiple aleurone (AL) cell layers with disrupted AL cell differentiation, whereas mutants of thk1 cause multiple cell layers of fully differentiated AL cells. Here, we conducted a comparative analysis of nkd1,2 and thk1 mutant endosperm transcriptomes to study how these factors regulate gene networks to control AL layer specification and cell differentiation. Weighted gene coexpression network analysis was incorporated with published laser capture microdissected transcriptome datasets to identify a coexpression module associated with AL development. In this module, both Nkd1,2+ and Thk1+ appear to regulate cell cycle and division, whereas Nkd1,2+, but not Thk1+, regulate auxin signaling. Further investigation of nkd1,2 differentially expressed genes combined with published putative targets of auxin response factors (ARFs) identified 61 AL-preferential genes that may be directly activated by NKD-modulated ARFs. All 61 genes were upregulated in nkd1,2 mutant and the enriched Gene Ontology terms suggested that they are associated with hormone crosstalk, lipid metabolism, and developmental growth. Expression of a transgenic DR5-red fluorescent protein auxin reporter was significantly higher in nkd1,2 mutant endosperm than in wild type, supporting the prediction that Nkd1,2+ negatively regulate auxin signaling in developing AL. Overall, these results suggest that Nkd1,2+ and Thk1+ may normally restrict AL development to a single cell layer by limiting cell division, and that Nkd1,2+ restrict auxin signaling in the AL to maintain normal cell patterning and differentiation processes.

High-resolution spatiotemporal transcriptome analyses during cellularization of rice endosperm unveil the earliest gene regulation critical for aleurone and starchy endosperm cell fate specification

The major tissues of the cereal endosperm are the starchy endosperm (SE) in the inner and the aleurone layer (AL) at the outer periphery. The fates of the cells that comprise these tissues are determined according to positional information; however, our understanding of the underlying molecular mechanisms remains limited. Here, we conducted a high-resolution spatiotemporal analysis of the rice endosperm transcriptome during early cellularization. In rice, endosperm cellularization proceeds in a concentric pattern from a primary alveolus cell layer, such that developmental progression can be defined by the number of cell layers. Using laser-capture microdissection to obtain precise tissue sections, transcriptomic changes were followed through five histologically defined stages of cellularization from the syncytial to 3-cell layer (3 L) stage. In addition, transcriptomes were compared between the inner and the outermost peripheral cell layers. Large differences in the transcriptomes between stages and between the inner and the peripheral cells were found. SE attributes were expressed at the alveolus-cell-layer stage but were preferentially activated in the inner cell layers that resulted from periclinal division of the alveolus cell layer. Similarly, AL attributes started to be expressed only after the 2 L stage and were localized to the outermost peripheral cell layer. These results indicate that the first periclinal division of the alveolus cell layer is asymmetric at the transcriptome level, and that the cell-fate-specifying positional cues and their perception system are already operating before the first periclinal division. Several genes related to epidermal identity (i.e., type IV homeodomain-leucine zipper genes and wax biosynthetic genes) were also found to be expressed at the syncytial stage, but their expression was localized to the outermost peripheral cell layer from the 2 L stage onward. We believe that our findings significantly enhance our knowledge of the mechanisms underlying cell fate specification in rice endosperm.

Defective mitochondrial function by mutation in THICK ALEURONE 1 encoding a mitochondrion-targeted single-stranded DNA-binding protein leads to increased aleurone cell layers and improved nutrition in rice

Cereal endosperm comprises an outer aleurone and an inner starchy endosperm. Although these two tissues have the same developmental origin, they differ in morphology, cell fate, and storage product accumulation, with the mechanism largely unknown. Here, we report the identification and characterization of rice thick aleurone 1 (ta1) mutant that shows an increased number of aleurone cell layers and increased contents of nutritional factors including proteins, lipids, vitamins, dietary fibers, and micronutrients. We identified that the TA1 gene, which is expressed in embryo, aleurone, and subaleurone in caryopses, encodes a mitochondrion-targeted protein with single-stranded DNA-binding activity named OsmtSSB1. Cytological analyses revealed that the increased aleurone cell layers in ta1 originate from a developmental switch of subaleurone toward aleurone instead of starchy endosperm in the wild type. We found that TA1/OsmtSSB1 interacts with mitochondrial DNA recombinase RECA3 and DNA helicase TWINKLE, and downregulation of RECA3 or TWINKLE also leads to ta1-like phenotypes. We further showed that mutation in TA1/OsmtSSB1 causes elevated illegitimate recombinations in the mitochondrial genome, altered mitochondrial morphology, and compromised energy supply, suggesting that the OsmtSSB1-mediated mitochondrial function plays a critical role in subaleurone cell-fate determination in rice.

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.

The thick aleurone1 Gene Encodes a NOT1 Subunit of the CCR4-NOT Complex and Regulates Cell Patterning in Endosperm

Maize (Zea mays) thick aleurone1 (thk1-R) mutants form multiple aleurone layers in the endosperm and have arrested embryogenesis. Prior studies suggest that thk1 functions downstream of defective kernel1 (dek1) in a regulatory pathway that controls aleurone cell fate and other endosperm traits. The original thk1-R mutant contained an ∼2-Mb multigene deletion, which precluded identification of the causal gene. Here, ethyl methanesulfonate mutagenesis produced additional alleles, and RNA sequencing from developing endosperm was used to identify a candidate gene based on differential expression compared with the wild-type progenitor. Gene editing confirmed the gene identity by producing mutant alleles that failed to complement existing thk1 mutants and that produced multiple-aleurone homozygous phenotypes. Thk1 encodes a homolog of NEGATIVE ON TATA-LESS1, a protein that acts as a scaffold for the CARBON CATABOLITE REPRESSION4-NEGATIVE ON TATA-LESS complex. This complex is highly conserved and essential in all eukaryotes for regulating a wide array of gene expression and cellular activities. Maize also harbors a duplicate locus, thick aleurone-like1, which likely accounts for the ability of thk1 mutants to form viable cells. Transcriptomic analysis indicated that THK1 regulates activities involving cell division, signaling, differentiation, and metabolism. Identification of thk1 provides an important new component of the DEK1 regulatory system that patterns cell fate in endosperm.

The Modular Control of Cereal Endosperm Development

Expansion of the human population demands a significant increase in cereal production. The main component of cereal grains is endosperm, a body of starchy endosperm (SE) cells surrounded by aleurone (AL) cells with transfer cells (TC) at the base and embryo surrounding (ESR) cells adjacent to the embryo. The data reviewed here emphasize the modular nature of endosperm by first suggesting that sucrose promotes development of the fertilized triploid endosperm cell. Next, that the basal syncytial endosperm responds to glucose by turning on TC development. The default endosperm cell fate is SE and ESR differentiation is likely activated by signaling from the embryo. Cells on the exterior surface of the endosperm are specified as AL cells.

Epigenetic responses to abiotic stresses during reproductive development in cereals

KEY MESSAGE

Overview of current understanding of epigenetic alterations after abiotic stresses during reproductive development in cereals. Abiotic stresses, including heat, drought, cold, flooding, and salinity, negatively impact crop productivity. Various stages during reproductive development are especially sensitive to environmental stresses, which may lead to complete sterility and severe yield losses. Plants exhibit diverse responses to ameliorate stress damage. Changes in DNA methylation, histone modification as well as regulation of small RNA and long noncoding RNA pathways have been shown to represent key modulators in plant stress responses. During reproductive development in cereals, various protein complexes controlling histone and DNA methylation have been identified, revealing conserved and novel mechanisms regulating abiotic stress responses in cereals and other plant species. New findings highlight the role of transposable elements during stress periods. Here, we review our current understanding of epigenetic stress responses during male and female gametophyte formation (germline development), fertilization, early seed devolvement, and seed maturation in cereals. An integrative model of epigenetic responses during reproductive development in cereals is proposed, emphasizing the role of DNA methylation and histone modifications during abiotic stresses.

Instability of endosperm development in amphiploids and their parental species in the genus Avena L

The development of oat endosperm is modified by chromatin and nuclei elimination, intrusive growth of cell walls, and polyploidisation of cell clones. The last event is correlated with somatic crossing-over. Grass endosperm is a variable tissue in terms of its cytogenetics and development. Free-nuclear syncytium and starchy and aleurone endosperm were the main focus of the research. These were studied in oat amphiploids (4x, 6x, and 8x) and parental species (2x, 4x, and 6x). What the levels of cytogenetic disorders and developmental anomalies in species versus hybrids are, and, what the factors are determining phenotypes of both tissue components, are open questions for oats. Chromosome bridges and micronuclei are the main cytogenetic disorders showing the elimination of parts of genomes. Bridges are formed by the AT-heterochromatin-rich and -free ends of chromosomes. In the starchy tissue, various sectors are separated structurally due to the elongation or intrusive growth of aleurone cells. The development of the aleurone layer is highly disturbed locally due to the amplification of aleurone cell divisions. Changes related to their structure and metabolism occur in the aleurone cells, for example, clones of small versus large aleurone cells. Somatic crossing-over (SCO) is expressed in clones of large polyploidised cells (r = 0.80***), giving rise to new aleurone phenotypes. The multivariate description of the endosperm instability showed that endospermal disorders were more frequent in amphiploids than in the oat species. Avena strigosa and the amphiploid A. fatua × A. sterilis appeared to be extreme units in an ordination space. Nuclear DNA elimination, periclinal and multidirectional cytokineses, polyploidisation, intrusive growth, and SCO appeared to be important factors determining oat endospermal variations.

The U6 Biogenesis-Like 1 Plays an Important Role in Maize Kernel and Seedling Development by Affecting the 3' End Processing of U6 snRNA

Regulation of gene expression at the post-transcriptional level is of crucial importance in the development of an organism. Here we present the characterization of a maize gene, U6 biogenesis-like 1 (UBL1), which plays an important role in kernel and seedling development by influencing pre-mRNA splicing. The ubl1 mutant, exhibiting small kernel and weak seedling, was isolated from a Mutator-tagged population. Transgenic complementation and three independent mutant alleles confirmed that UBL1, which encodes a putative RNA exonuclease belonging to the 2H phosphodiesterase superfamily, is responsible for the phenotype of ubl1. We demonstrated that UBL1 possess the RNA exonuclease activity in vitro and found that loss of UBL1 function in ubl1 causes decreased level and abnormal 3' end constitution of snRNA U6, resulting in splicing defect of mRNAs. Through the in vitro and in vivo studies replacing two histidines with alanines in the H-X-T/S-X (X is a hydrophobic residue) motifs we demonstrated that these two motifs are essential for the normal function of UBL1. We further showed that the function of UBL1 may be conserved across a wide phylogenetic distance as the heterologous expression of maize UBL1 could complement the Arabidopsis ubl1 mutant.

zmsbt1 and zmsbt2, two new subtilisin-like serine proteases genes expressed in early maize kernel development

Two subtilisin-like proteases show highly specific and complementary expression patterns in developing grains. These genes label the complete surface of the filial-maternal interface, suggesting a role in filial epithelial differentiation. The cereal endosperm is the most important source of nutrition and raw materials for mankind, as well as the storage compartment enabling initial growth of the germinating plantlets. The development of the different cell types in this tissue is regulated environmentally, genetically and epigenetically, resulting in the formation of top-bottom, adaxial-abaxial and surface-central axes. However, the mechanisms governing the interactions among the different inputs are mostly unknown. We have screened a kernel cDNA library for tissue-specific transcripts as initial step to identify genes relevant in cell differentiation. We report here on the isolation of two maize subtilisin-related genes that show grain-specific, surficial expression. zmsbt1 (Zea mays Subtilisin1) is expressed at the developing aleurone in a time-regulated manner, while zmsbt2 concentrates at the pedicel in front of the endosperm basal transfer layer. We have shown that their presence, early in the maize caryopsis development, is dependent on proper initial tissue determination, and have isolated their promoters to produce transgenic reporter lines that assist in the study of their regulation.

ZmDof3, a maize endosperm-specific Dof protein gene, regulates starch accumulation and aleurone development in maize endosperm

To explore the function of Dof transcription factors during kernel development in maize, we first identified Dof genes in the maize genome. We found that ZmDof3 was exclusively expressed in the endosperm of maize kernel and had the features of a Dof transcription factor. Suppression of ZmDof3 resulted in a defective kernel phenotype with reduced starch content and a partially patchy aleurone layer. The expression levels of starch synthesis-related genes and aleurone differentiation-associated genes were down-regulated in ZmDof3 knockdown kernels, indicating that ZmDof3 plays an important role in maize endosperm development. The maize endosperm, occupying a large proportion of the kernel, plays an important role in seed development and germination. Current knowledge regarding the regulation of endosperm development is limited. Dof proteins, a family of plant-specific transcription factors, play critical roles in diverse biological processes. In this study, an endosperm-specific Dof protein gene, ZmDof3, was identified in maize through genome-wide screening. Suppression of ZmDof3 resulted in a defective kernel phenotype. The endosperm of ZmDof3 knockdown kernels was loosely packed with irregular starch granules observed by electronic microscope. Through genome-wide expression profiling, we found that down-regulated genes were enriched in GO terms related to carbohydrate metabolism. Moreover, ZmDof3 could bind to the Dof core element in the promoter of starch biosynthesis genes Du1 and Su2 in vitro and in vivo. In addition, the aleurone at local position in mature ZmDof3 knockdown kernels varied from one to three layers, which consisted of smaller and irregular cells. Further analyses showed that knockdown of ZmDof3 reduced the expression of Nkd1, which is involved in aleurone cell differentiation, and that ZmDof3 could bind to the Dof core element in the Nkd1 promoter. Our study reveals that ZmDof3 functions in maize endosperm development as a positive regulator in the signaling system controlling starch accumulation and aleurone development.

Extreme-phenotype genome-wide association study (XP-GWAS): a method for identifying trait-associated variants by sequencing pools of individuals selected from a diversity panel

Although approaches for performing genome-wide association studies (GWAS) are well developed, conventional GWAS requires high-density genotyping of large numbers of individuals from a diversity panel. Here we report a method for performing GWAS that does not require genotyping of large numbers of individuals. Instead XP-GWAS (extreme-phenotype GWAS) relies on genotyping pools of individuals from a diversity panel that have extreme phenotypes. This analysis measures allele frequencies in the extreme pools, enabling discovery of associations between genetic variants and traits of interest. This method was evaluated in maize (Zea mays) using the well-characterized kernel row number trait, which was selected to enable comparisons between the results of XP-GWAS and conventional GWAS. An exome-sequencing strategy was used to focus sequencing resources on genes and their flanking regions. A total of 0.94 million variants were identified and served as evaluation markers; comparisons among pools showed that 145 of these variants were statistically associated with the kernel row number phenotype. These trait-associated variants were significantly enriched in regions identified by conventional GWAS. XP-GWAS was able to resolve several linked QTL and detect trait-associated variants within a single gene under a QTL peak. XP-GWAS is expected to be particularly valuable for detecting genes or alleles responsible for quantitative variation in species for which extensive genotyping resources are not available, such as wild progenitors of crops, orphan crops, and other poorly characterized species such as those of ecological interest.

Calpain-Mediated Positional Information Directs Cell Wall Orientation to Sustain Plant Stem Cell Activity, Growth and Development

Eukaryotic development and stem cell control depend on the integration of cell positional sensing with cell cycle control and cell wall positioning, yet few factors that directly link these events are known. The DEFECTIVE KERNEL1 (DEK1) gene encoding the unique plant calpain protein is fundamental for development and growth, being essential to confer and maintain epidermal cell identity that allows development beyond the globular embryo stage. We show that DEK1 expression is highest in the actively dividing cells of seeds, meristems and vasculature. We further show that eliminating Arabidopsis DEK1 function leads to changes in developmental cues from the first zygotic division onward, altered microtubule patterns and misshapen cells, resulting in early embryo abortion. Expression of the embryonic marker genes WOX2, ATML1, PIN4, WUS and STM, related to axis organization, cell identity and meristem functions, is also altered in dek1 embryos. By monitoring cell layer-specific DEK1 down-regulation, we show that L1- and 35S-induced down-regulation mainly affects stem cell functions, causing severe shoot apical meristem phenotypes. These results are consistent with a requirement for DEK1 to direct layer-specific cellular activities and set downstream developmental cues. Our data suggest that DEK1 may anchor cell wall positions and control cell division and differentiation, thereby balancing the plant's requirement to maintain totipotent stem cell reservoirs while simultaneously directing growth and organ formation. A role for DEK1 in regulating microtubule-orchestrated cell wall orientation during cell division can explain its effects on embryonic development, and suggests a more general function for calpains in microtubule organization in eukaryotic cells.

The naked endosperm genes encode duplicate INDETERMINATE domain transcription factors required for maize endosperm cell patterning and differentiation

The aleurone is the outermost layer of cereal endosperm and functions to digest storage products accumulated in starchy endosperm cells as well as to confer important dietary health benefits. Whereas normal maize (Zea mays [Zm]) has a single aleurone layer, naked endosperm (nkd) mutants produce multiple outer cell layers of partially differentiated cells that show sporadic expression of aleurone identity markers such as a viviparous1 promoter-β-glucuronidase transgene. The 15:1 F2 segregation ratio suggested that two recessive genes were involved, and map-based cloning identified two homologous genes in duplicated regions of the genome. The nkd1 and nkd2 genes encode the INDETERMINATE1 domain (IDD) containing transcription factors ZmIDDveg9 and ZmIDD9 on chromosomes 2 and 10, respectively. Independent mutant alleles of nkd1 and nkd2, as well as nkd2-RNA interference lines in which both nkd genes were knocked down, also showed the nkd mutant phenotype, confirming the gene identities. In wild-type kernels, the nkd transcripts were most abundant around 11 to 16 d after pollination. The NKD proteins have putative nuclear localization signals, and green fluorescent protein fusion proteins showed nuclear localization. The mutant phenotype and gene identities suggest that NKD controls a gene regulatory network involved in aleurone cell fate specification and cell differentiation.

Using transposons for genetic mosaic analysis of plant development

Genetic mosaics, or chimeras, are individual organisms composed of cells or tissues of two or more distinct genotypes. They are experimentally useful for addressing several key biological questions. These include fate mapping through analysis of marked clonal lineages, analyzing cell or tissue interactions such as the induction of developmental events, and analyzing whether a gene acts cell autonomously. Genetic mosaics can arise in many ways, including through the action of transposable elements. Naturally occurring transposons can generate genetic mosaics by somatically inserting into a gene to cause a mutant sector, somatically excising from a mutant gene to create a revertant wild-type sector, or causing chromosomal breaks or rearrangements leading to loss of a gene or genes. Transposons have also been cleverly engineered to allow the generation of marked somatic sectors, sometimes in controlled ways. Here we review ways in which transposon-induced genetic mosaics have been used experimentally, the various methods that have been used, and general considerations for designing genetic mosaic studies using transposon methods.

Endosperm: food for humankind and fodder for scientific discoveries

The endosperm is an essential constituent of seeds in flowering plants. It originates from a fertilization event parallel to the fertilization that gives rise to the embryo. The endosperm nurtures embryo development and, in some species including cereals, stores the seed reserves and represents a major source of food for humankind. Endosperm biology is characterized by specific features, including idiosyncratic cellular controls of cell division and epigenetic controls associated with parental genomic imprinting. This review attempts a comprehensive summary of our current knowledge of endosperm development and highlights recent advances in this field.

Endosperm development: dynamic processes and cellular innovations underlying sibling altruism

The endosperm is a product of fertilization that evolved to support and nourish its genetic twin sibling embryo. Cereal endosperm accumulates starch and protein stores, which later support the germinating seedling. These nutritional stores prompted the domestication of cereals and are the focus of ongoing efforts for crop improvement and biotechnological innovations. Endosperm development entails several novel modifications to basic cellular and developmental processes. Cereals display nuclear endosperm development, which begins with a period of free nuclear division to generate a coenocyte. Cytoskeletal arrays distribute nuclei around the periphery of the cytoplasm and direct the subsequent deposition of cell wall material during cellularization. Positional cues and signaling systems function dynamically in the specification of the four major cell types: transfer cells, embryo-surrounding cells, starchy endosperm (SE), and aleurone. Genome balance, epigenetic gene regulation, and parent-of-origin effects are essential for directing these processes. Transfer cells transport solutes, including sugars and amino acids, from the maternal plant tissues into the developing grain where they are partitioned between embryo and SE cells. Cells of the embryo-surrounding region appear to coordinate development of the embryo and endosperm. As the seed matures, SE cells assimilate starch and protein stores, undergo DNA endoreduplication, and finally undergo programmed cell death. In contrast, aleurone cells follow a maturation program similar to the embryo, allowing them to survive desiccation. At germination, the aleurone cells secrete amylases and proteases that hydrolyze the storage products of the SE to nourish the germinating seedling.