The globby1-1 (glo1-1) mutation disrupts nuclear and cell division in the developing maize seed causing alterations in endosperm cell fate and tissue differentiation

The transcriptome landscape of developing barley seeds

Cereal grains are an important source of food and feed. To provide comprehensive spatiotemporal information about biological processes in developing seeds of cultivated barley (Hordeum vulgare L. subsp. vulgare), we performed a transcriptomic study of the embryo, endosperm, and seed maternal tissues collected from grains 4-32 days after pollination. Weighted gene co-expression network and motif enrichment analyses identified specific groups of genes and transcription factors (TFs) potentially regulating barley seed tissue development. We defined a set of tissue-specific marker genes and families of TFs for functional studies of the pathways controlling barley grain development. Assessing selected groups of chromatin regulators revealed that epigenetic processes are highly dynamic and likely play a major role during barley endosperm development. The repressive H3K27me3 modification is globally reduced in endosperm tissues and at specific genes related to development and storage compounds. Altogether, this atlas uncovers the complexity of developmentally regulated gene expression in developing barley grains.

Coordinated Transcriptome and Metabolome Analyses of a Barley hvhggt Mutant Reveal a Critical Role of Tocotrienols in Endosperm Starch Accumulation

Tocotrienols and tocopherols (vitamin E) are potent antioxidants that are synthesized in green plants. Unlike ubiquitous tocopherols, tocotrienols predominantly accumulate in the endosperm of monocot grains, catalyzed by homogentiate geranylgeranyl transferase (HGGT). Previously, we generated a tocotrienol-deficient hvhggt mutant with shrunken barley grains. However, the relationship between tocotrienols and grain development remains unclear. Here, we found that the hvhggt lines displayed hollow endosperms with defective transfer cells and reduced aleurone layers. The carbohydrate and starch contents of the hvhggt endosperm decreased by approximately 20 and 23%, respectively. Weighted gene coexpression network analyses identified a critical gene module containing HvHGGT, which was strongly associated with the hvhggt mutation and enriched with gene functions in starch and sucrose metabolism. Metabolome measurements revealed an elevated soluble sugar content in the hvhggt endosperm, which was significantly associated with the identified gene modules. The hvhggt endosperm had significantly higher NAD(H) and NADP(H) contents and lower levels of ADPGlc (regulated by redox balance) than the wild-type, consistent with the absence of tocotrienols. Interestingly, exogenous α-tocotrienol spraying on developing hvhggt spikes partially rescued starch accumulation and endosperm defects. Our study supports a potential novel function of tocotrienols in grain starch accumulation and endosperm development in monocot crops.

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.

Barley HISTIDINE KINASE 1 (HvHK1) coordinates transfer cell specification in the young endosperm

Cereal endosperm represents the most important source of the world's food; nevertheless, the molecular mechanisms underlying cell and tissue differentiation in cereal grains remain poorly understood. Endosperm cellularization commences at the maternal-filial intersection of grains and generates endosperm transfer cells (ETCs), a cell type with a prominent anatomy optimized for efficient nutrient transport. Barley HISTIDINE KINASE1 (HvHK1) was identified as a receptor component with spatially restricted expression in the syncytial endosperm where ETCs emerge. Here, we demonstrate its function in ETC fate acquisition using RNA interference-mediated downregulation of HvHK1. Repression of HvHK1 impairs cell specification in the central ETC region and the development of transfer cell morphology, and consecutively defects differentiation of adjacent endosperm tissues. Coinciding with reduced expression of HvHK1, disturbed cell plate formation and fusion were observed at the initiation of endosperm cellularization, revealing that HvHK1 triggers initial cytokinesis of ETCs. Cell-type-specific RNA sequencing confirmed loss of transfer cell identity, compromised cell wall biogenesis and reduced transport capacities in aberrant cells and elucidated two-component signaling and hormone pathways that are mediated by HvHK1. Gene regulatory network modeling was used to specify the direct targets of HvHK1; this predicted non-canonical auxin signaling elements as the main regulatory links governing cellularization of ETCs, potentially through interaction with type-B response regulators. This work provides clues to previously unknown molecular mechanisms directing ETC specification, a process with fundamental impact on grain yield in cereals.

Comparative transcriptomics reveals the difference in early endosperm development between maize with different amylose contents

In seeds, the endosperm is a crucial organ that plays vital roles in supporting embryo development and determining seed weight and quality. Starch is the predominant storage carbohydrate of the endosperm and accounts for ∼70% of the mature maize kernel weight. Nonetheless, because starch biosynthesis is a complex process that is orchestrated by multiple enzymes, the gene regulatory networks of starch biosynthesis, particularly amylose and amylopectin biosynthesis, have not been fully elucidated. Here, through high-throughput RNA sequencing, we developed a temporal transcriptome atlas of the endosperms of high-amylose maize and common maize at 5-, 10-, 15- and 20-day after pollination and found that 21,986 genes are involved in the programming of the high-amylose and common maize endosperm. A coexpression analysis identified multiple sequentially expressed gene sets that are closely correlated with cellular and metabolic programmes and provided valuable insight into the dynamic reprogramming of the transcriptome in common and high-amylose maize. In addition, a number of genes and transcription factors were found to be strongly linked to starch synthesis, which might help elucidate the key mechanisms and regulatory networks underlying amylose and amylopectin biosynthesis. This study will aid the understanding of the spatiotemporal patterns and genetic regulation of endosperm development in different types of maize and provide valuable genetic information for the breeding of starch varieties with different contents.

Comparative transcriptome profiling of multi-ovary wheat under heterogeneous cytoplasm suppression

DUOII is a multi-ovary wheat line with two or three pistils and three stamens in each floret. The multi-ovary trait of DUOII is controlled by a dominant gene, whose expression can be suppressed by the heterogeneous cytoplasm of TeZhiI (TZI), a line with the nucleus of common wheat and the cytoplasm of Aegilops. DUOII (♀) × TZI (♂) shows multi-ovary trait, while TZI (♀) × DUOII (♂) shows mono-ovary. Observing the developmental process, we found that the critical stage of additional pistil primordium development was when the young spikes were 2–6 mm long. To elucidate the molecular mechanisms that are responsible for the heterogeneous cytoplasmic suppression of the multi-ovary gene, we RNA-sequenced the entire transcriptome of 2–6 mm long young spikes obtained from the reciprocal crosses between DUOII and TZI. A total of 600 differentially expressed genes (DEGs) was identified. Functional annotation of these DEGs showed that the heterogeneous cytoplasmic suppression of additional pistil development mainly involved four pathways, i.e., chloroplast metabolism, DNA replication and repair, hormone signal transduction, and trehalose-6-phosphate in the primordium development stage, which cooperated to modulate the multi-ovary gene expression under heterogeneous cytoplasmic suppression.

Comparative proteomic analysis of multi-ovary wheat under heterogeneous cytoplasm suppression

BACKGROUND

DUOII is a multi-ovary wheat (Triticum aestivum L.) line with two or three pistils and three stamens in each floret. The multi-ovary trait of DUOII is controlled by a dominant gene, whose expression can be suppressed by the heterogeneous cytoplasm of TeZhiI (TZI), a line with the nucleus of common wheat and the cytoplasm of Aegilops. Crosses between female DUOII plants and male TZI plants resulted in multi-ovary F₁s; whereas, the reciprocal crosses resulted in mono-ovary F₁s. Although the multi-ovary trait is inherited as single trait controlled by a dominant allele in lines with a Triticum cytoplasm, the mechanism by which the special heterogeneous cytoplasm suppresses the expression of multi-ovary is not well understood.

RESULTS

Observing the developmental process, we found that the critical stage of additional pistil primordium development was when the young spikes were 2-6 mm long. Then, we compared the quantitative proteomic profiles of 2-6 mm long young spikes obtained from the reciprocal crosses between DUOII and TZI. A total of 90 differentially expressed proteins were identified and analyzed based on their biological functions. These proteins had obvious functional pathways mainly implicated in chloroplast metabolism, nuclear and cell division, plant respiration, protein metabolism, and flower development. Importantly, we identified two key proteins, Flowering Locus K Homology Domain and PEPPER, which are known to play an essential role in the specification of pistil organ identity. By drawing relationships between the 90 differentially expressed proteins, we found that these proteins revealed a complex network which is associated with multi-ovary gene expression under heterogeneous cytoplasmic suppression.

CONCLUSIONS

Our proteomic analysis has identified certain differentially expressed proteins in 2-6 mm long young spikes, which was the critical stage of additional primordium development. This paper provided a universal proteomic profiling involved in the cytoplasmic suppression of wheat floral meristems; and our findings have laid a solid foundation for further mechanistic studies on the underlying mechanisms that control the heterogeneous cytoplasm-induced suppression of the nuclear multi-ovary gene in wheat.

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.

Genetic and epigenetic control of transfer cell development in plants

The inter-cellular translocation of nutrients in plant is mediated by highly specialized transfer cells (TCs). TCs share similar functional and structural features across a wide range of plant species, including location at plant exchange surfaces, rich in secondary wall ingrowths, facilitation of nutrient flow, and passage of select molecules. The fate of endosperm TCs is determined in the TC fate acquisition stage (TCF), before the structure features are formed in the TC differentiation stage (TCD). At present, the molecular basis of TC development in plants remains largely unknown. In this review, we summarize the important roles of the signaling molecules in different development phases, such as sugars in TCF and phytohormones in TCD, and discuss the genetic and epigenetic factors, including TC-specific genes and endogenous plant peptides, and their crosstalk with these signaling molecules as a complex regulatory network in regulation of TC development in plants.

Cell layer-specific distribution of transiently expressed barley ESCRT-III component HvVPS60 in developing barley endosperm

The significance of the endosomal sorting complexes required for transport (ESCRT)-III in cereal endosperm has been shown by the identification of the recessive mutant supernumerary aleurone layer1 (SAL1) in maize. ESCRT-III is indispensable in the final membrane fission step during biogenesis of multivesicular bodies (MVBs), responsible for protein sorting to vacuoles and to the cell surface. Here, we annotated barley ESCRT-III members in the (model) crop Hordeum vulgare and show that all identified members are expressed in developing barley endosperm. We used fluorescently tagged core ESCRT-III members HvSNF7a/CHMP4 and HvVPS24/CHMP3 and the associated ESCRT-III component HvVPS60a/CHMP5 for transient localization studies in barley endosperm. In vivo confocal microscopic analyses show that the localization of recombinantly expressed HvSNF7a, HvVPS24 and HvVPS60a differs within barley endosperm. Whereas HvSNF7a induces large agglomerations, HvVPS24 shows mainly cytosolic localization in aleurone and subaleurone. In contrast, HvVPS60a localizes strongly at the plasma membrane in aleurone. In subaleurone, HvVPS60a was found to a lesser extent at the plasma membrane and at vacuolar membranes. These results indicate that the steady-state association of ESCRT-III may be influenced by cell layer-specific protein deposition or trafficking and remodelling of the endomembrane system in endosperm. We show that sorting of an artificially mono-ubiquitinated Arabidopsis plasma membrane protein is inhibited by HvVPS60a in aleurone. The involvement of HvVPS60a in different cell layer-specific trafficking pathways, reflected by localization of HvVPS60a at the plasma membrane in aleurone and at the PSV membrane in subaleurone, is discussed.

Embryo defective 14 encodes a plastid-targeted cGTPase essential for embryogenesis in maize

The embryo defective (emb) mutants in maize genetically define a unique class of loci that is required for embryogenesis but not endosperm development, allowing dissection of two developmental processes of seed formation. Through characterization of the emb14 mutant, we report here that Emb14 gene encodes a circular permuted, YqeH class GTPase protein that likely functions in 30S ribosome formation in plastids. Loss of Emb14 function in the null mutant arrests embryogenesis at the early transition stage. Emb14 was cloned by transposon tagging and was confirmed by analysis of four alleles. Subcellular localization indicated that the EMB14 is targeted to chloroplasts. Recombinant EMB14 is shown to hydrolyze GTP in vitro (Km  = 2.42 ± 0.3 μm). Emb14 was constitutively expressed in all tissues examined and high level of expression was found in transition stage embryos. Comparison of emb14 and WT indicated that loss of EMB14 function severely impairs accumulation of 16S rRNA and several plastid encoded ribosomal genes. We show that an EMB14 transgene complements the pale green, slow growth phenotype conditioned by mutations in AtNOA1, a closely related YqeH GTPase of Arabidopsis. Taken together, we propose that the EMB14/AtNOA1/YqeH class GTPases function in assembly of the 30S subunit of the chloroplast ribosome, and that this function is essential to embryogenesis in plants.

Physical, metabolic and developmental functions of the seed coat

The conventional understanding of the role of the seed coat is that it provides a protective layer for the developing zygote. Recent data show that the picture is more nuanced. The seed coat certainly represents a first line of defense against adverse external factors, but it also acts as channel for transmitting environmental cues to the interior of the seed. The latter function primes the seed to adjust its metabolism in response to changes in its external environment. The purpose of this review is to provide the reader with a comprehensive view of the structure and functionality of the seed coat, and to expose its hidden interaction with both the endosperm and embryo. Any breeding and/or biotechnology intervention seeking to increase seed size or modify seed features will have to consider the implications on this tripartite interaction.

Maize early endosperm growth and development: from fertilization through cell type differentiation

•

PREMISE OF THE STUDY

Given the worldwide economic importance of maize endosperm, it is surprising that its development is not the most comprehensively studied of the cereals. We present detailed morphometric and cytological descriptions of endosperm development in the maize inbred line B73, for which the genome has been sequenced, and compare its growth with four diverse Nested Association Mapping (NAM) founder lines.•

METHODS

The first 12 d of B73 endosperm development were described using semithin sections of plastic-embedded kernels and confocal microscopy. Longitudinal sections were used to compare endosperm length, thickness, and area.•

KEY RESULTS

Morphometric comparison between Arizona- and Michigan-grown B73 showed a common pattern. Early endosperm development was divided into four stages: coenocytic, cellularization through alveolation, cellularization through partitioning, and differentiation. We observed tightly synchronous nuclear divisions in the coenocyte, elucidated that the onset of cellularization was coincident with endosperm size, and identified a previously undefined cell type (basal intermediate zone, BIZ). NAM founders with small mature kernels had larger endosperms (0-6 d after pollination) than lines with large mature kernels.•

CONCLUSIONS

Our B73-specific model of early endosperm growth links developmental events to relative endosperm size, while accounting for diverse growing conditions. Maize endosperm cellularizes through alveolation, then random partitioning of the central vacuole. This unique cellularization feature of maize contrasts with the smaller endosperms of Arabidopsis, barley, and rice that strictly cellularize through repeated alveolation. NAM analysis revealed differences in endosperm size during early development, which potentially relates to differences in timing of cellularization across diverse lines of maize.

Endosperm transfer cell-specific genes and proteins: structure, function and applications in biotechnology

Endosperm transfer cells (ETC) are one of four main types of cells in endosperm. A characteristic feature of ETC is the presence of cell wall in-growths that create an enlarged plasma membrane surface area. This specialized cell structure is important for the specific function of ETC, which is to transfer nutrients from maternal vascular tissue to endosperm. ETC-specific genes are of particular interest to plant biotechnologists, who use genetic engineering to improve grain quality and yield characteristics of important field crops. The success of molecular biology-based approaches to manipulating ETC function is dependent on a thorough understanding of the functions of ETC-specific genes and ETC-specific promoters. The aim of this review is to summarize the existing data on structure and function of ETC-specific genes and their products. Potential applications of ETC-specific genes, and in particular their promoters for biotechnology will be discussed.

Intersection of transfer cells with phloem biology-broad evolutionary trends, function, and induction

Transfer cells (TCs) are ubiquitous throughout the plant kingdom. Their unique ingrowth wall labyrinths, supporting a plasma membrane enriched in transporter proteins, provides these cells with an enhanced membrane transport capacity for resources. In certain plant species, TCs have been shown to function to facilitate phloem loading and/or unloading at cellular sites of intense resource exchange between symplasmic/apoplasmic compartments. Within the phloem, the key cellular locations of TCs are leaf minor veins of collection phloem and stem nodes of transport phloem. In these locations, companion and phloem parenchyma cells trans-differentiate to a TC morphology consistent with facilitating loading and re-distribution of resources, respectively. At a species level, occurrence of TCs is significantly higher in transport than in collection phloem. TCs are absent from release phloem, but occur within post-sieve element unloading pathways and particularly at interfaces between generations of developing Angiosperm seeds. Experimental accessibility of seed TCs has provided opportunities to investigate their inductive signaling, regulation of ingrowth wall formation and membrane transport function. This review uses this information base to explore current knowledge of phloem transport function and inductive signaling for phloem-associated TCs. The functional role of collection phloem and seed TCs is supported by definitive evidence, but no such information is available for stem node TCs that present an almost intractable experimental challenge. There is an emerging understanding of inductive signals and signaling pathways responsible for initiating trans-differentiation to a TC morphology in developing seeds. However, scant information is available to comment on a potential role for inductive signals (auxin, ethylene and reactive oxygen species) that induce seed TCs, in regulating induction of phloem-associated TCs. Biotic phloem invaders have been used as a model to speculate on involvement of these signals.

Embryo defective12 encodes the plastid initiation factor 3 and is essential for embryogenesis in maize

Embryo-specific mutants in maize define a unique class of genetic loci that affect embryogenesis without a significant deleterious impact on endosperm development. Here we report the characterization of an embryo specific12 (emb12) mutant in maize. Embryogenesis in the emb12 mutants is arrested at or before transition stage. The mutant embryo at an early stage exhibits abnormal cell structure with increased vacuoles and dramatically reduced internal membrane organelles. In contrast, the mutant endosperm appears normal in morphology, cell structure, starch, lipid and protein accumulation. The Emb12 locus was cloned by transposon tagging and predicts a protein with a high similarity to prokaryotic translation initiation factor 3 (IF3). EMB12-GFP fusion analysis indicates that EMB12 is localized in plastids. The RNA in situ hybridization and protein immunohistochemical analyses indicate that a high level of Emb12 expression localizes in the embryo proper at early developmental stages and in the embryo axis at later stages. Western analysis indicates that plastid protein synthesis is impaired. These results indicate that Emb12 encodes the plastid IF3 which is essential for embryogenesis but not for endosperm development in maize.

Kin conflict in seed development: an interdependent but fractious collective

Seeds are complex structures that unite diploid maternal tissues with filial tissues that may be haploid (gametophyte), diploid (embryo), or triploid (endosperm). Maternal tissues are predicted to favor smaller seeds than are favored by filial tissues, and filial genes of maternal origin are predicted to favor smaller seeds than are favored by filial genes of paternal origin. Consistent with these predictions, seed size is determined by an interplay between growth of maternal integuments, which limits seed size, and of filial endosperm, which promotes larger seeds. Within endosperm, genes of paternal origin favor delayed cellularization of endosperm and larger seeds, whereas genes of maternal origin favor early cellularization and smaller seeds. The ratio of maternal and paternal gene products in endosperm contributes to the failure of crosses between different ploidy levels of the same species and crosses between species. Maternally expressed small-interfering RNAs (siRNAs) are predicted to associate with growth-enhancing genes.

Scarecrow plays a role in establishing Kranz anatomy in maize leaves

More than a quarter of the primary productivity on land, and a large fraction of the food that humans consume, is contributed by plants that fix atmospheric CO(2) by C(4) photosynthesis. It has been estimated that transferring the C(4) pathway to C(3) crops could boost yield by 50% and also increase water use efficiency and reduce the need for fertilizer, particularly in dry, hot environments. The high productivity of maize (Zea mays), sugarcane (Saccharum spp.) and several emerging bioenergy grasses is due largely to C(4) photosynthesis, which is enabled by the orderly arrangement, in concentric rings, of specialized bundle sheath and mesophyll cells in leaves in a pattern known as Kranz anatomy. Here we show that PIN, the auxin efflux protein, is present in the end walls of maize bundle sheath cells, as it is in the endodermis of the root. Since this marker suggests the expression of endodermal genetic programs in bundle sheath cells, we determined whether the transcription factor SCARECROW, which regulates structural differentiation of the root endodermis, also plays a role in the development of Kranz anatomy in maize. Mutations in the Scarecrow gene result in proliferation of bundle sheath cells, abnormal differentiation of bundle sheath chloroplasts, vein disorientation, loss of minor veins and reduction of vein density. Further characterization of this signal transduction pathway should facilitate the transfer of the C(4) trait into C(3) crop species, including rice.

Maternal to offspring resource allocation in plants and mammals

Appropriate allocation of resources to the offspring is critical for successful reproduction, particularly in species that reproduce on more than one occasion. The offspring must be provisioned adequately to ensure its vigour, whereas the parent must not become so depleted such that its survival is endangered. In both flowering plants and mammals specialised structures have evolved to support the offspring during its development. In this review we consider common themes that may indicate conservation of nutrient transfer function and regulation by genomic imprinting across the two kingdoms.

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.

Maternal control of nutrient allocation in plant seeds by genomic imprinting

Imprinted genes are commonly expressed in mammalian placentas and in plant seed endosperms, where they exhibit preferential uniparental allelic expression. In mammals, imprinted genes directly regulate placental function and nutrient distribution from mother to fetus; however, none of the >60 imprinted genes thus far reported in plants have been demonstrated to play an equivalent role in regulating the flow of resources to the embryo. Here we show that imprinted Maternally expressed gene1 (Meg1) in maize is both necessary and sufficient for the establishment and differentiation of the endosperm nutrient transfer cells located at the mother:seed interface. Consistent with these findings, Meg1 also regulates maternal nutrient uptake, sucrose partitioning, and seed biomass yield. In addition, we generated an imprinted and nonimprinted synthetic Meg1 ((syn)Meg1) dosage series whereby increased dosage and absence of imprinting both resulted in an unequal investment of maternal resources into the endosperm. These findings highlight dosage regulation by genomic imprinting as being critical for maintaining a balanced distribution of maternal nutrients to filial tissues in plants, as in mammals. However, unlike in mammals, Meg1 is a maternally expressed imprinted gene that surprisingly acts to promote rather than restrict nutrient allocation to the offspring.

Maize rough endosperm3 encodes an RNA splicing factor required for endosperm cell differentiation and has a nonautonomous effect on embryo development

Endosperm and embryo development are coordinated via epigenetic regulation and signaling between these tissues. In maize (Zea mays), the endosperm-embryo signals are not known, but endosperm cellularization is a key event for embryos to form shoots and roots. We screened seed mutants for nonautonomous functions in endosperm and embryo development with genetically nonconcordant seeds and identified the recessive mutant rough endosperm3 (rgh3). The wild-type Rgh3 allele is required in the endosperm for embryos to develop and has an autonomous role in embryo and seedling development. Endosperm cell differentiation is defective in rgh3. Results from endosperm cell culture indicate that rgh3 mutants remain in a proliferative state through mid-seed development. Rgh3 encodes the maize U2AF(35) Related Protein (URP), an RNA splicing factor involved in both U2 and U12 splicing. The Rgh3 allele produces at least 19 alternative splice variants with only one isoform encoding a full-length ortholog to URP. The full-length RGH3α isoform localizes to the nucleolus and displays a speckled pattern within the nucleoplasm, and RGH3α colocalizes with U2AF(65). A survey of alternatively spliced transcripts found that, in the rgh3 mutant, a fraction of noncanonical splicing events are altered. Our findings suggest that differentiation of maize endosperm cell types is necessary for embryos to develop. The molecular cloning of Rgh3 suggests that alternative RNA splicing is needed for cell differentiation, development, and plant viability.

Regulation of aleurone development in cereal grains

The aleurone layer of cereal grains is important biologically as well as nutritionally and economically. Here, current knowledge on the regulation of aleurone development is reviewed. Recent reports suggest that the control of aleurone development is more complex than earlier models portrayed. Multiple levels of genetic regulation control aleurone cell fate, differentiation, and organization. The hormones auxin and cytokinin can also influence aleurone development. New technical advances promise to facilitate future progress.

Analysis of stunter1, a maize mutant with reduced gametophyte size and maternal effects on seed development

Many higher eukaryotes have evolved strategies for the maternal control of growth and development of their offspring. In higher plants this is achieved in part by postmeiotic gene activity controlling the development of the haploid female gametophyte. stunter1 (stt1) is a novel, recessive, maternal effect mutant in maize that displays viable, miniature kernels. Maternal inheritance of stt1 results in seeds with reduced but otherwise normal endosperms and embryos. The stt1 mutation displays reduced transmission through the male and female parents and causes significant changes in the sizes of both male and female gametophytes. stt1 pollen grains are smaller than wild type, have reduced germination efficiency, and reduced pollen tube growth. stt1 embryo sacs have smaller central cells and abnormal antipodal cells that are larger, more vacuolated, and fewer in number than wild type. Embryos and endosperms produced by fertilization of stt1 embryo sacs develop and grow more slowly than wild type. The data suggest that the morphology of mutant embryo sacs influences endosperm development, leading to the production of miniature kernels in stt1. Analysis of seeds carrying a mutant maternal allele of stt1 over a deletion of the paternal allele demonstrates that both parental alleles are active after fertilization in both the endosperm and embryo. This analysis also indicates that embryo development until the globular stage in maize can proceed without endosperm development and is likely supported directly by the diploid mother plant.

Endosperm development in Brachypodium distachyon

Grain development and its evolution in grasses remains poorly understood, despite cereals being our most important source of food. The grain, for which many grass species have been domesticated, is a single-seeded fruit with prominent and persistent endosperm. Brachypodium distachyon, a small wild grass, is being posited as a new model system for the temperate small grain cereals, but little is known about its endosperm development and how this compares with that of the domesticated cereals. A cellular and molecular map of domains within the developing Brachypodium endosperm is constructed. This provides the first detailed description of grain development in Brachypodium for the reference strain, Bd21, that will be useful for future genetic and comparative studies. Development of Brachypodium grains is compared with that of wheat. Notably, the aleurone is not regionally differentiated as in wheat, suggesting that the modified aleurone region may be a feature of only a subset of cereals. Also, the central endosperm and the nucellar epidermis contain unusually prominent cell walls that may act as a storage material. The composition of these cell walls is more closely related to those of barley and oats than to those of wheat. Therefore, although endosperm development is broadly similar to that of temperate small grain cereals, there are significant differences that may reflect its phylogenetic position between the Triticeae and rice.

Current opinions on endosperm transfer cells in maize

Endosperm transfer cells (ETC) mainly occur in the endosperm epithelial layer near the pedicel. They transport the nutrient unloaded by the maternal vascular tissue to filial tissues. Wall ingrowths of ETC can facilitate solute transportation. Sugar, especially glucose, is found to modulate the promoter activity of ZmMRP-1, a determinant of transfer cell-specific expression. The ZmMRP-1-encoded protein can transactivate the promoters of transfer cell-specific genes. Signalling and early events leading to wall ingrowth formation depend upon gene expression. Sucrose synthase and the cytoskeleton probably play a primary role in the wall ingrowth formation. The major solutes transferred by ETC are amino acids, sucrose, and monosaccharides, which is consistent with the expression of their transporters and transport-associated genes. In this paper, we review current opinions on the differentiation, wall ingrowth formation, and function of ETC in maize. According to the experimental materials provided by predecessors, we also give some speculations about the differentiation mechanisms of ETC and process of wall ingrowth formation.

Barley grain development toward an integrative view

Seeds are complex structures composed of several maternal and filial tissues which undergo rapid changes during development. In this review, the barley grain is taken as a cereal seed model. Following a brief description of the developing grain, recent progress in grain development modeling is described. 3-D/4-D models based on histological sections or nondestructive NMR measurements can be used to integrate a variety of datasets. Extensive transcriptome data are taken as a frame to augment our understanding of various molecular-physiological processes. Discussed are maternal influences on grain development and the role of different tissues (pericarp, nucellus, nucellar projection, endosperm, endosperm transfer cells). Programmed cell death (PCD) is taken to pinpoint tissue specificities and the importance of remobilization processes for grain development. Transcriptome data have also been used to derive transcriptional networks underlying differentiation and maturation in endosperm and embryo. They suggest that the "maturation hormone" ABA is important also in early grain development. Massive storage product synthesis during maturation is dependent on sufficient energy, which can only be provided by specific metabolic adaptations due to severe oxygen deficiencies within the seed. To integrate the great variety of data from different research areas in complex, predictive computational modeling as part of a systems biology approach is an important challenge of the future. First attempts of modeling barley grain metabolism are summarized.

Miniature1-encoded cell wall invertase is essential for assembly and function of wall-in-growth in the maize endosperm transfer cell

The miniature1 (mn1) seed phenotype in maize (Zea mays) is due to a loss-of-function mutation at the Mn1 locus that encodes a cell wall invertase (INCW2) that localizes exclusively to the basal endosperm transfer cells (BETCs) of developing seeds. A common feature of all transfer cells is the labyrinth-like wall-in-growth (WIG) that increases the plasma membrane area, thereby enhancing transport capacity in these cells. To better understand WIG formation and roles of INCW2 in the BETC development, we examined wild-type and mn1 mutant developing kernels by cryofixation and electron microscopy. In Mn1 seeds, WIGs developed uniformly in the BETC layer during 7 to 17 d after pollination, and the secretory/endocytic organelles proliferated in the BETCs. Mitochondria accumulated in the vicinity of WIGs, suggesting a functional link between them. In the mn1 BETCs, WIGs were stunted and their endoplasmic reticulum was swollen; Golgi density in the mutant BETCs was 51% of the Mn1 Golgi density. However, the polarized distribution of mitochondria was not affected. INCW2-specific immunogold particles were detected in WIGs, the endoplasmic reticulum, Golgi stacks, and the trans-Golgi network in the Mn1 BETCs, while immunogold particles were extremely rare in the mutant BETCs. Levels of WIG development in the empty pericarp4 mutant was heterogeneous among BETCs, and INCW2 immunogold particles were approximately four times more abundant in the larger WIGs than in the stunted WIGs. These results indicate that polarized secretion is activated during WIG formation and that INCW2 is required for normal development of WIGs to which INCW2 is localized.

A cellular study of teosinte Zea mays subsp. parviglumis (Poaceae) caryopsis development showing several processes conserved in maize

The evolutionary history of maize (Zea mays subsp. mays) is of general interest because of its economic and scientific importance. Here we show that many cellular traits described previously in developing caryopses of maize are also seen in its wild progenitor teosinte (Zea mays subsp. parviglumis). These features, each with a possible role in development, include (1) an early programmed cell death in the maternal placento-chalazal (P-C) layer that may lead to increased hydrolytic conductance to the developing seed; (2) accumulation of phenolics and flavonoids in the P-C layer that may be related to antimicrobial activity; (3) formation of wall ingrowths in the basal endosperm transfer layer (BETL); (4) localization of cell wall invertase in the BETL, which is attributed to the increased transport capacity of photosynthates to the sink; and (5) endoreduplication in endosperm nuclei suggested to contribute to increased gene expression and greater sink capacity of the developing seed. In maize caryopsis, these cellular traits have been previously attributed to domestication and selection for larger seed size and vigor. Given the conservation of the entire cellular program in developing teosinte caryopses described here, we suggest that these traits evolved independently of domestication and predate human selection pressure.

The maize Dof protein PBF activates transcription of gamma-zein during maize seed development

Maize PBF (prolamin-box binding factor) belongs to the Dof class of plant specific transcription factors containing one highly conserved zinc finger DNA-binding domain, called Dof (DNA binding with one finger) domain. Maize PBF trans-activates the gamma-zein gene (gammaZ) promoter in developing maize seeds as shown by transient expression in maize endosperms. Co-transfection of a gammaZ:GUS construct with 35S:PBF resulted in a sevenfold increase in GUS expression, however, PBF mutation in Cys residues within the Dof domain abolishes both, binding to DNA and the capacity to activate gammaZ promoter. We present two pieces of evidence that PBF transactivates gammaZ promoter by binding to the Pb3 motif (TGTAAAG). First, recombinant Dof domain of PBF (bdPBF) specifically recognized Pb3 site as shown by gel mobility shift assays and second, co-expression of PBF with gammaZ promoter mutated in Pb3 motif suppressed PBF trans-activation capacity. Immunocytochemical analysis on developing endosperm sections shows that PBF is localized in the nuclei of the peripheral layer cells of starchy endosperm, the tissue in which the initial accumulation of gamma-zein protein occurs. By contrast, PBF is detected in the cytosol of the starchy endosperm cells newly differentiated from aleurone daughter cells, where gamma-zein was absent. Taken together these data indicate that maize PBF plays an essential role in the regulation of the temporal and spatial expression of gammaZ gene.

Abscisic acid and stress signals induce Viviparous1 expression in seed and vegetative tissues of maize

Viviparous1 (Vp1) encodes a B3 domain-containing transcription factor that is a key regulator of seed maturation in maize (Zea mays). However, the mechanisms of Vp1 regulation are not well understood. To examine physiological factors that may regulate Vp1 expression, transcript levels were monitored in maturing embryos placed in culture under different conditions. Expression of Vp1 decreased after culture in hormone-free medium, but was induced by salinity or osmotic stress. Application of exogenous abscisic acid (ABA) also induced transcript levels within 1 h in a dose-dependent manner. The Vp1 promoter fused to beta-glucuronidase or green fluorescent protein reproduced the endogenous Vp1 expression patterns in transgenic maize plants and also revealed previously unknown expression domains of Vp1. The Vp1 promoter is active in the embryo and aleurone cells of developing seeds and, upon drought stress, was also found in phloem cells of vegetative tissues, including cobs, leaves, and stems. Sequence analysis of the Vp1 promoter identified a potential ABA-responsive complex, consisting of an ACGT-containing ABA response element (ABRE) and a coupling element 1-like motif. Electrophoretic mobility shift assay confirmed that the ABRE and putative coupling element 1 components specifically bound proteins in embryo nuclear protein extracts. Treatment of embryos in hormone-free Murashige and Skoog medium blocked the ABRE-protein interaction, whereas exogenous ABA or mannitol treatment restored this interaction. Our data support a model for a VP1-dependent positive feedback mechanism regulating Vp1 expression during seed maturation.

Signalling during epidermal development

The process of L1 specification early in plant embryogenesis, and subsequent maintenance and elaboration of epidermal organization, are fundamental to plant growth and fitness. To occur in a co-ordinated fashion, these processes require considerable cell–cell cross-talk. It is perhaps then unsurprising that several classes of plant RLKs (receptor-like kinases), as well as other membrane-localized signalling components, have been implicated both in epidermal specification and in patterning events governing the distribution of epidermal cell types. However, despite our growing knowledge of the roles of these signalling molecules, remarkably little is understood regarding their function at the cellular level. In particular the potential role of regulated proteolytic cleavage in controlling the activity of signalling molecules at the plant plasma membrane has remained largely unaddressed despite its massive importance in signalling in animal systems. Because of the relative physical accessibility of their expression domains, molecules involved in epidermal development present opportunities for investigating mechanisms of cell–cell signalling in planta. Advances in understanding the potential regulatory processing of membrane-localized signalling molecules during epidermal development will be examined using parallels with animal systems to highlight potential future directions for this field of research.

empty pericarp4 encodes a mitochondrion-targeted pentatricopeptide repeat protein necessary for seed development and plant growth in maize

The pentatricopeptide repeat (PPR) family represents one of the largest gene families in plants, with >440 members annotated in Arabidopsis thaliana. PPR proteins are thought to have a major role in the regulation of posttranscriptional processes in organelles. Recent studies have shown that Arabidopsis PPR proteins play an essential, nonredundant role during embryogenesis. Here, we demonstrate that mutations in empty pericarp4 (emp4), a maize (Zea mays) PPR-encoding gene, confer a seed-lethal phenotype. Mutant endosperms are severely impaired, with highly irregular differentiation of transfer cells in the nutrient-importing basal endosperm. Analysis of homozygous mutant plants generated from embryo-rescue experiments indicated that emp4 also affects general plant growth. The emp4-1 mutation was identified in an active Mutator (Mu) population, and cosegregation analysis revealed that it arose from a Mu3 element insertion. Evidence of emp4 molecular cloning was provided by the isolation of four additional emp4 alleles obtained by a reverse genetics approach. emp4 encodes a novel type of PPR protein of 614 amino acids. EMP4 contains nine 35-amino acid PPR motifs and an N-terminal mitochondrion-targeted sequence peptide, which was confirmed by a translational EMP4-green fluorescent protein fusion that localized to mitochondria. Molecular analyses further suggest that EMP4 is necessary to regulate the correct expression of a small subset of mitochondrial transcripts in the endosperm.

A mutational approach to the study of seed development in maize

The maize seed comprises two major compartments, the embryo and the endosperm, both originating from the double fertilization event. The embryogenetic process allows the formation of a well-differentiated embryonic axis, surrounded by a single massive cotyledon, the scutellum. The mature endosperm constitutes the bulk of the seed and comprises specific regions containing reserve proteins, complex carbohydrates, and oils. To gain more insight into molecular events that underlie seed development, three monogenic mutants were characterized, referred to as emp (empty pericarp) on the basis of their extreme endosperm reduction, first recognizable at about 12 d after pollination. Their histological analysis reveals a partial development of the endosperm domains as well as loss of adhesion between pedicel tissues and the basal transfer layer. In the endosperm, programmed cell death (PCD) is delayed. The embryo appears retarded in its growth, but not impaired in its morphogenesis. The mutants can be rescued by culturing immature embryos, even though the seedlings appear retarded in their growth. The analysis of seeds with discordant embryo-endosperm phenotype (mutant embryo, normal endosperm and vice-versa), obtained using B-A translocations, suggests that emp expression in the embryo is necessary, but not sufficient, for proper seed development. In all three mutants the picture emerging is one of a general delay in processes related to growth, as a result of a mutation affecting endosperm development as a primary event.

Maternal gametophytic baseless1 is required for development of the central cell and early endosperm patterning in maize (Zea mays)

In angiosperms, double fertilization of an egg cell and a central cell with two sperm cells results in the formation of a seed containing a diploid embryo and a triploid endosperm. The extent to which the embryo sac controls postfertilization events in the seed is unknown. The novel gametophytic maternal-effect maize mutation, baseless1 (bsl1) affects central cell development within the embryo sac, frequently by altering the position of the two polar nuclei. Despite this irregularity, fertilization is as efficient as in wild type. The spatial expression of basal endosperm-specific transcripts is altered in free-nuclear and cellular mutant endosperms. At later stages of seed development, bsl1 predominantly affects development of the basal endosperm transfer layer (BETL). When bsl1/+ diploid plants were pollinated by wild-type tetraploid plants, the BETL abnormalities observed in bsl1/bsl1/+/+ tetraploid endosperms were diverse and of variable severity. Moreover, the frequency of kernels with severely perturbed BETL development correlated with the percentage of severely affected bsl1 central cells. Therefore, BSL1 is likely required in the central cell before fertilization for correct BETL patterning to occur. These findings provide new genetic evidence that a maternal gametophytic component is necessary for correct endosperm patterning.

Surface position, not signaling from surrounding maternal tissues, specifies aleurone epidermal cell fate in maize

Maize (Zea mays) endosperm consists of an epidermal-like surface layer of aleurone cells, an underlying body of starchy endosperm cells, and a basal layer of transfer cells. To determine whether surrounding maternal tissues perform a role in specifying endosperm cell fates, a maize endosperm organ culture technique was established whereby the developing endosperm is completely removed from surrounding maternal tissues. Using cell type-specific fluorescence markers, we show that aleurone cell fate specification occurs exclusively in response to surface position and does not require specific, continued maternal signal input. The starchy endosperm and aleurone cell fates are freely interchangeable throughout the lifespan of the endosperm, with internalized aleurone cells converting to starchy endosperm cells and with starchy endosperm cells that become positioned at the surface converting to aleurone cells. In contrast to aleurone and starchy endosperm cells, transfer cells fail to develop in in vitro-grown endosperm, supporting earlier indications that maternal tissue interaction is required to fully differentiate this cell type. Several parameters confirm that the maize endosperm organ cultures described herein retain the main developmental features of in planta endosperm, including fidelity of aleurone mutant phenotypes, temporal and spatial control of cell type-specific fluorescent markers, specificity of cell type transcripts, and control of mitotic cell divisions.

Construction and screening of subtracted cDNA libraries from limited populations of plant cells: a comparative analysis of gene expression between maize egg cells and central cells

The analysis of cell type-specific gene expression is an essential step in understanding certain biological processes during plant development, such as differentiation. Although methods for isolating specific cell types have been established, the application of cDNA subtraction to small populations of isolated cell types for direct identification of specific or differentially expressed transcripts has not yet been reported. As a first step in the identification of genes expressed differentially between maize egg cells and central cells, we have manually isolated these types of cell, and applied a suppression-subtractive hybridization (SSH) strategy. After microarray screening of 1030 cDNAs obtained from the subtracted libraries, we identified 340 differentially expressed clones. Of these, 142 were sequenced, which resulted in the identification of 62 individual cDNAs. The expression patterns of 20 cDNAs were validated by quantitative RT-PCR, through which we identified five transcripts with cell type-specific expression. The specific localization of some of these transcripts was also confirmed by in situ hybridization on embryo sac sections. Taken together, our data demonstrate the effectiveness of our approach in identifying differentially expressed and cell type-specific transcripts of relatively low abundance. This was also confirmed by the identification of previously reported egg cell- and central cell-specific genes in our screen. Importantly, from our analysis we identified a significant number of novel sequences not present in other embryo sac or, indeed, in other plant expressed sequence tag (EST) databases. Thus, in combination with standard EST sequencing and microarray hybridization strategies, our approach of differentially screening subtracted cDNAs will add substantially to the expression information in spatially highly resolved transcriptome analyses.

Aleurone cell identity is suppressed following connation in maize kernels

Expression of the cytokinin-synthesizing isopentenyl transferase enzyme under the control of the Arabidopsis (Arabidopsis thaliana) SAG12 senescence-inducible promoter reverses the normal abortion of the lower floret from a maize (Zea mays) spikelet. Following pollination, the upper and lower floret pistils fuse, producing a connated kernel with two genetically distinct embryos and the endosperms fused along their abgerminal face. Therefore, ectopic synthesis of cytokinin was used to position two independent endosperms within a connated kernel to determine how the fused endosperm would affect the development of the two aleurone layers along the fusion plane. Examination of the connated kernel revealed that aleurone cells were present for only a short distance along the fusion plane whereas starchy endosperm cells were present along most of the remainder of the fusion plane, suggesting that aleurone development is suppressed when positioned between independent starchy endosperms. Sporadic aleurone cells along the fusion plane were observed and may have arisen from late or imperfect fusion of the endosperms of the connated kernel, supporting the observation that a peripheral position at the surface of the endosperm and not proximity to maternal tissues such as the testa and pericarp are important for aleurone development. Aleurone mosaicism was observed in the crown region of nonconnated SAG12-isopentenyl transferase kernels, suggesting that cytokinin can also affect aleurone development.

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.

Vacuolar H+-translocating inorganic pyrophosphatase (Vpp1) marks partial aleurone cell fate in cereal endosperm development

Cereal endosperm is a model system for cell fate determination in plants. In wild-type plants the outermost endosperm cells adopt aleurone cell fate, while all underlying cells display starchy endosperm cell fate. Mutant analysis showed that cell fate is determined by position rather than lineage. To further characterise the precise cell fate of the outermost cells, we performed a differential screen and isolated the novel marker gene Vpp1 . It encodes a vacuolar H+-translocating inorganic pyrophosphatase (V-PPase) and is mainly expressed in kernels, leaves and tassels. In kernels, its expression is restricted to the aleurone layer with the maximum of expression shifting from the adaxial to the abaxial side during early stages. Together with three other marker genes Vpp1 was then used to analyse the cell fate of the outermost cells in Dap3 , Dap7 , cr4 and dek1 mutants, all of which have aberrant aleurone layers. In the Dap3 and Dap7 mutants the Vpp1 and Ltp2 markers but not the A1 and Zein markers were expressed in patches without aleurone indicating that the outermost cells had some but not all features of aleurone cells and did not simply adopt starchy endosperm cell fate. A similar result was obtained in the cr4 mutant, although Ltp2 expression was less generalised. In other Dap7 patches characterised by multiple aleurone-like cell layers the expression of Vpp1 and Ltp2 confirmed the aleurone cell fate of the cells in the additional cell layers. The analysis of dek1 mutants confirmed the starchy endosperm cell fate of the majority but not all outermost cells. Based on these data we propose a model suggesting a stepwise commitment to aleurone cell fate. Sequential steps are marked by the expression of Vpp1 , the expression of Ltp2 , the acquisition of a regular shape and thick walls and finally pigmentation coupled with A1 expression.