The maize transcription factor myb-related protein-1 is a key regulator of the differentiation of transfer cells

Temporal Gene Expression Profiles From Pollination to Seed Maturity in Sorghum Provide Core Candidates for Engineering Seed Traits

Sorghum (Sorghum bicolor (L.) Moench) is a highly nutritional multipurpose millet crop. However, the genetic and molecular regulatory mechanisms governing sorghum grain development and the associated agronomic traits remain unexplored. In this study, we performed a comprehensive transcriptomic analysis of pistils collected 1-2 days before pollination, and developing seeds collected -2, 10, 20 and 30 days after pollination of S. bicolor variety M35-1. Out of 31 337 genes expressed in these stages, 12 804 were differentially expressed in the consecutive stages of seed development. These exhibited 10 dominant expression patterns correlated with the distinct pathways and gene functions. Functional analysis, based on the pathway mapping, transcription factor enrichment and orthology, delineated the key patterns associated with pollination, fertilization, early seed development, grain filling and seed maturation. Furthermore, colocalization with previously reported quantitative trait loci (QTLs) for grain weight/size revealed 48 differentially expressed genes mapping to these QTL regions. Comprehensive literature mining integrated with QTL mapping and expression data shortlisted 25, 17 and 8 core candidates for engineering grain size, starch and protein content, respectively.

Maize transcription factor ZmEREB167 negatively regulates starch accumulation and kernel size

Transcription factors play critical roles in the regulation of gene expression during maize kernel development. The maize endosperm, a large storage organ, accounting for nearly 90% of the dry weight of mature kernels, serves as the primary site for starch storage. In this study, we identify an endosperm-specific EREB gene, ZmEREB167, which encodes a nucleus-localized EREB protein. Knockout of ZmEREB167 significantly increases kernel size and weight, as well as starch and protein content, compared with the wild type. In situ hybridization experiments show that ZmEREB167 is highly expressed in the BETL as well as PED regions of maize kernels. Dual-luciferase assays show that ZmEREB167 exhibits transcriptionally repressor activity in maize protoplasts. Transcriptome analysis reveals that a large number of genes are up-regulated in the Zmereb167-C1 mutant compared with the wild type, including key genetic factors such as ZmMRP-1 and ZmMN1, as well as multiple transporters involved in maize endosperm development. Integration of RNA-seq and ChIP-seq results identify 68 target genes modulated by ZmEREB167. We find that ZmEREB167 directly targets OPAQUE2, ZmNRT1.1, ZmIAA12, ZmIAA19, and ZmbZIP20, repressing their expressions. Our study demonstrates that ZmEREB167 functions as a negative regulator in maize endosperm development and affects starch accumulation and kernel size.

Toward unveiling transcriptome dynamics and regulatory modules at the maternal/filial interface of developing maize kernel

The basal region of maize (Zea mays) kernels, which includes the pedicel, placenta-chalazal, and basal endosperm transfer layers, serves as the maternal/filial interface for nutrient transfer from the mother plant to the developing seed. However, transcriptome dynamics of this maternal/filial interface remain largely unexplored. To address this gap, we conducted high-temporal-resolution RNA sequencing of the basal and upper kernel regions between 4 and 32 days after pollination and deeply analyzed transcriptome dynamics of the maternal/filial interface. Utilizing 790 specifically and highly expressed genes in the basal region, we performed the gene ontology (GO) term and weighted gene co-expression network analyses. In the early-stage basal region, we identified five MADS-box transcription factors (TFs) as hubs. Their homologs have been demonstrated as pivotal regulators at the maternal/filial interface of rice or Arabidopsis, suggesting their potential roles in maize kernel development. In the filling-stage basal region, numerous GO terms associated with transcriptional regulation and transporters are significantly enriched. Furthermore, we investigated the molecular function of three hub TFs. Through genome-wide DNA affinity purification sequencing combined with promoter transactivation assays, we suggested that these three TFs act as regulators of 10 basal-specific transporter genes involved in the transfer of sugars, amino acids, and ions. This study provides insights into transcriptomic dynamic and regulatory modules of the maternal/filial interface. In the future, genetic investigation of these hub regulators must advance our understanding of maternal/filial interface development and function.

A MYB-related transcription factor ZmMYBR29 is involved in grain filling

BACKGROUND

The endosperm serves as the primary source of nutrients for maize (Zea mays L.) kernel embryo development and germination. Positioned at the base of the endosperm, the transfer cells (TCs) of the basal endosperm transfer layer (BETL) generate cell wall ingrowths, which enhance the connectivity between the maternal plant and the developing kernels. These TCs play a crucial role in nutrient transport and defense against pathogens. The molecular mechanism underlying BETL development in maize remains unraveled.

RESULTS

This study demonstrated that the MYB-related transcription factor ZmMYBR29, exhibited specific expression in the basal cellularized endosperm, as evidenced by in situ hybridization analysis. Utilizing the CRISPR/Cas9 system, we successfully generated a loss-of-function homozygous zmmybr29 mutant, which presented with smaller kernel size. Observation of histological sections revealed abnormal development and disrupted morphology of the cell wall ingrowths in the BETL. The average grain filling rate decreased significantly by 26.7% in zmmybr29 mutant in comparison to the wild type, which impacted the dry matter accumulation within the kernels and ultimately led to a decrease in grain weight. Analysis of RNA-seq data revealed downregulated expression of genes associated with starch synthesis and carbohydrate metabolism in the mutant. Furthermore, transcriptomic profiling identified 23 genes that expressed specifically in BETL, and the majority of these genes exhibited altered expression patterns in zmmybr29 mutant.

CONCLUSIONS

In summary, ZmMYBR29 encodes a MYB-related transcription factor that is expressed specifically in BETL, resulting in the downregulation of genes associated with kernel development. Furthermore, ZmMYBR29 influences kernels weight by affecting the grain filling rate, providing a new perspective for the complementation of the molecular regulatory network in maize endosperm development.

Decoding the gene regulatory network of endosperm differentiation in maize

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

The transcription factors ZmNAC128 and ZmNAC130 coordinate with Opaque2 to promote endosperm filling in maize

Endosperm filling in maize (Zea mays), which involves nutrient uptake and biosynthesis of storage reserves, largely determines grain yield and quality. However, much remains unclear about the synchronization of these processes. Here, we comprehensively investigated the functions of duplicate NAM, ATAF1/2, and CUC2 (NAC)-type transcription factors, namely, ZmNAC128 and ZmNAC130, in endosperm filling. The gene-edited double mutant zmnac128 zmnac130 exhibits a poorly filled kernel phenotype such that the kernels have an inner cavity. RNA sequencing and protein abundance analysis revealed that the expression of many genes involved in the biosynthesis of zein and starch is reduced in the filling endosperm of zmnac128 zmnac130. Further, DNA affinity purification and sequencing combined with chromatin-immunoprecipitation quantitative PCR and promoter transactivation assays demonstrated that ZmNAC128 and ZmNAC130 are direct regulators of 3 (16-, 27-, and 50-kD) γ-zein genes and 6 important starch metabolism genes (Brittle2 [Bt2], pullulanase-type starch debranching enzyme [Zpu1], granule-bound starch synthase 1 [GBSS1], starch synthase 1 [SS1], starch synthase IIa [SSIIa], and sucrose synthase 1 [Sus1]). ZmNAC128 and ZmNAC130 recognize an additional cis-element in the Opaque2 (O2) promoter to regulate its expression. The triple mutant zmnac128 zmnac130 o2 exhibits extremely poor endosperm filling, which results in more than 70% of kernel weight loss. ZmNAC128 and ZmNAC130 regulate the expression of the transporter genes sugars that will eventually be exported transporter 4c (ZmSWEET4c), sucrose and glucose carrier 1 (ZmSUGCAR1), and yellow stripe-like2 (ZmYSL2) and in turn facilitate nutrient uptake, while O2 plays a supporting role. In conclusion, ZmNAC128 and ZmNAC130 cooperate with O2 to facilitate endosperm filling, which involves nutrient uptake in the basal endosperm transfer layer (BETL) and the synthesis of zeins and starch in the starchy endosperm (SE).

ZmDRR206 Regulates Nutrient Accumulation in Endosperm through Its Role in Cell Wall Biogenesis during Maize Kernel Development

Dirigent proteins (DIRs) contribute to plant fitness by dynamically reorganizing the cell wall and/or by generating defense compounds during plant growth, development, and interactions with environmental stresses. ZmDRR206 is a maize DIR, it plays a role in maintaining cell wall integrity during seedling growth and defense response in maize, but its role in regulating maize kernel development is unclear. Association analysis of candidate genes indicated that the natural variations of ZmDRR206 were significantly associated with maize hundred-kernel weight (HKW). ZmDRR206 plays a dominant role in storage nutrient accumulation in endosperm during maize kernel development, ZmDRR206 overexpression resulted in small and shrunken maize kernel with significantly reduced starch content and significantly decreased HKW. Cytological characterization of the developing maize kernels revealed that ZmDRR206 overexpression induced dysfunctional basal endosperm transfer layer (BETL) cells, which were shorter with less wall ingrowth, and defense response was constitutively activated in developing maize kernel at 15 and 18 DAP by ZmDRR206 overexpression. The BETL-development-related genes and auxin signal-related genes were down-regulated, while cell wall biogenesis-related genes were up-regulated in developing BETL of the ZmDRR206-overexpressing kernel. Moreover, the developing ZmDRR206-overexpressing kernel had significantly reduced contents of the cell wall components such as cellulose and acid soluble lignin. These results suggest that ZmDRR206 may play a regulatory role in coordinating cell development, storage nutrient metabolism, and stress responses during maize kernel development through its role in cell wall biogenesis and defense response, and provides new insights into understanding the mechanisms of kernel development in maize.

Revisiting maize Brittle endosperm-2 reveals new insights in BETL development and starchy endosperm filling

Rerouting the starch biosynthesis pathway in maize can generate specialty types, like sweet corn and waxy corn, with a drastically increasing global demand. Hence, a fine-tuning of starch metabolism is relevant to create diverse maize cultivars for end-use applications. Here, we characterized a new maize brittle endosperm mutant, referred to as bt1774, which exhibited decreased starch content but a dramatic increase of soluble sugars at maturity. Both endosperm and embryo development was impaired in bt1774 relative to the wild-type (WT), with a prominently arrested basal endosperm transfer layer (BETL). Map-based cloning revealed that BRITTLE ENDOSPERM2 (Bt2), which encodes a small subunit of ADP-glucose pyrophosphorylase (AGPase), is the causal gene for bt1774. A MuA2 element was found to be inserted into intron 2 of Bt2, leading to a severe decrease of its expression, in bt1774. This is in line with the irregular and loosely packed starch granules in the mutant. Transcriptome of endosperm at grain filling stage identified 1, 013 differentially expressed genes in bt1774, which were notably enriched in the BETL compartment, including ZmMRP1, Miniature1, MEG1, and BETLs. Gene expression of the canonical starch biosynthesis pathway was marginally disturbed in Bt1774. Combined with the residual 60% of starch in this nearly null mutant of Bt2, this data strongly suggests that an AGPase-independent pathway compensates for starch synthesis in the endosperm. Consistent with the BETL defects, zein accumulation was impaired in bt1774. Co-expression network analysis revealed that Bt2 probably has a role in intracellular signal transduction, besides starch synthesis. Altogether, we propose that Bt2 is likely involved in carbohydrate flux and balance, thus regulating both the BETL development and the starchy endosperm filling.

A translatome-transcriptome multi-omics gene regulatory network reveals the complicated functional landscape of maize

BACKGROUND

Maize (Zea mays L.) is one of the most important crops worldwide. Although sophisticated maize gene regulatory networks (GRNs) have been constructed for functional genomics and phenotypic dissection, a multi-omics GRN connecting the translatome and transcriptome is lacking, hampering our understanding and exploration of the maize regulatome.

RESULTS

We collect spatio-temporal translatome and transcriptome data and systematically explore the landscape of gene transcription and translation across 33 tissues or developmental stages of maize. Using this comprehensive transcriptome and translatome atlas, we construct a multi-omics GRN integrating mRNAs and translated mRNAs, demonstrating that translatome-related GRNs outperform GRNs solely using transcriptomic data and inter-omics GRNs outperform intra-omics GRNs in most cases. With the aid of the multi-omics GRN, we reconcile some known regulatory networks. We identify a novel transcription factor, ZmGRF6, which is associated with growth. Furthermore, we characterize a function related to drought response for the classic transcription factor ZmMYB31.

CONCLUSIONS

Our findings provide insights into spatio-temporal changes across maize development at both the transcriptome and translatome levels. Multi-omics GRNs represent a useful resource for dissection of the regulatory mechanisms underlying phenotypic variation.

High temporal-resolution transcriptome landscapes of maize embryo sac and ovule during early seed development

Here we provided a high temporal-resolution transcriptome atlas of maize embryo sac and ovule to reveal the gene activity dynamic during early seed development. The early maize (Zea mays) seed development is initiated from double fertilization in the embryo sac and needs to undergo a highly dynamic and complex development process to form the differentiated embryo and endosperm. Despite the importance of maize seed for food, feed, and biofuel, many regulators responsible for controlling its early development are not known yet. Here, we reported a high temporal-resolution transcriptome atlas of embryo sac and ovule based on 44 time point samples collected within the first four days of seed development. A total of 25,187 genes including 1598 transcription factors (TFs) involved in early seed development were detected. Global comparisons of the expressions of these genes revealed five distinct development stages of early seed, which are mainly related to double fertilization, asymmetric cell division of the zygote, as well as coenocyte formation, cellularization and differentiation in endosperm. We identified 3327 seed-specific genes, which more than one thousand seed-specific genes with main expressions during early seed development were newly identified here, including 859 and 186 genes predominantly expressed in the embryo sac and ovule, respectively. Combined with the published transcriptome data of seed, we uncovered the dominant auxin biosynthesis, transport and signaling related genes at different development stages and subregions of seed. These results are helpful for understanding the genetic control of early seed development.

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.

Distinct Hormone Signalling-Modulation Activities Characterize Two Maize Endosperm-Specific Type-A Response Regulators

ZmTCRR1 and 2 are type-A response regulators expressed in the maize endosperm transfer cells (TC). While type-B response regulators transcriptionally control canonical type-A response regulators, as part of the cytokinin signal transduction mechanism, the ZmTCRRs are regulated by ZmMRP1, a master regulator of TC identity. In addition, the corresponding proteins are not detected in the TC, accumulating in the inner endosperm cells instead. These features suggest these molecules are not involved in classical, cell-autonomous, cytokinin signalling pathways. Using transgenic Arabidopsis plants ectopically expressing these genes, we have shown that ZmTCRR1 and 2 can modulate auxin and cytokinin signalling, respectively. In Arabidopsis, the ectopic expression of ZmTCRR2 blocked, almost completely, cytokinin perception. Given the conservation of these signalling pathways at the molecular level, our results suggest that the ZmTCRRs modulate cytokinin and auxin perception in the inner endosperm cells.

Identification of a candidate gene underlying qHKW3, a QTL for hundred-kernel weight in maize

qHKW3, a quantitative trait locus for hundred-kernel weight, harbors the proposed causal gene Zm00001d044081, encoding a homeobox-leucine zipper protein (ATHB-4) that might affect kernel size and weight. Kernel size and weight are key traits that contribute greatly to grain yield per year in maize (Zea mays). Here, we developed the chromosome segment substitution line (CSSL), H15-6-2, with smaller kernel size and lower kernel weight across environments compared to the background line Ye478. Histological analysis suggested that a slower kernel filling rate of H15-6-2 contributes to its small-kernel size and reduced hundred-kernel weight. We identified a quantitative trait locus (QTL) explaining 23% of the phenotypic variation in hundred-kernel weight. This QTL, qHKW3, was fine mapped to an interval of approximately 40.66-kb harboring the gene Zm00001d044081. The upstream sequence and its expression level of Zm00001d044081 in kernels at 6 days after pollination (DAP) showed obvious differences between the near-isogenic lines HKW3^Ye478 and HKW3^H15-6-2. We further confirmed the effects of the Zm00001d044081 promoter on maize kernel size and weight in an independent association mapping panel with 513 lines by candidate regional association analysis. We propose that Zm00001d044081, which encodes the homeobox-leucine zipper protein ATHB-4, is the causal gene of qHKW3, representing an attractive target for the genetic improvement of maize yield.

Molecular mechanisms of maize endosperm transfer cell development

Endosperm transfer cells function as the nutrient transporter, antimicrobic barrier, and signal mediator between filial and maternal tissues. Sugar supply of maternal tissues, sugar demand of filial tissues, and requirement for defence against pathogens are three elemental factors inducing differentiation of endosperm transfer cells. Epigenetic factors, especially MEG1, moderate the key genetic factor ZmMRP-1 to activate endosperm transfer cell-specific genes that control the flange wall ingrowth formation and defensin-like protein secretion in maize. Auxin and cytokinin are primary hormones involved in development of maize endosperm transfer cells. Crosstalk between glucose and hormone signaling regulates endosperm transfer cell development via modifying ZmMRP-1 expression. This review summarizes the current knowledge on maize endosperm transfer cell development, and discusses its potential molecular mechanisms. It is expected to strengthen the theoretical basis for structural and functional optimization of endosperm transfer cells, and yield improvement of kernels in maize.

Maize GSK3-like kinase ZmSK2 is involved in embryonic development

Grain size and weight are closely related to the yield of cereal crops. Abnormal development of the embryo, an important part of the grain, not only affects crop yield but also impacts next-generation survival. Here, we found that maize GSK3-like kinase ZmSK2, a homolog of BIN2 in Arabidopsis, is involved in embryonic development. ZmSK2 overexpression resulted in severe BR defective phenotypes and arrested embryonic development at the transition stage, while the zmsk2 knockout lines showed enlarged embryos. ZmSK2 interacts with Aux/IAA-transcription factor 28 (ZmIAA28), a negative regulator of auxin signaling, and the interaction region is the auxin degron "GWPPV" motif of ZmIAA28 domain II. Coexpression of ZmSK2 with ZmIAA28 increased the accumulation of ZmIAA28 in maize protoplasts, which may have been due to phosphorylation by ZmSK2. In conclusion, this study reveals the function of ZmSK2 in maize embryonic development and proposes that ZmSK2-ZmIAA28 may be another link in the signaling pathway that integrates BR and auxin.

ENB1 encodes a cellulose synthase 5 that directs synthesis of cell wall ingrowths in maize basal endosperm transfer cells

Development of the endosperm is strikingly different in monocots and dicots: it often manifests as a persistent tissue in the former and transient tissue in the latter. Little is known about the controlling mechanisms responsible for these different outcomes. Here we characterized a maize (Zea mays) mutant, endosperm breakdown1 (enb1), in which the typically persistent endosperm (PE) was drastically degraded during kernel development. ENB1 encodes a cellulose synthase 5 that is predominantly expressed in the basal endosperm transfer layer (BETL) of endosperm cells. Loss of ENB1 function caused a drastic reduction in formation of flange cell wall ingrowths (ingrowths) in BETL cells. Defective ingrowths impair nutrient uptake, leading to premature utilization of endosperm starch to nourish the embryo. Similarly, developing wild-type kernels cultured in vitro with a low level of sucrose manifested early endosperm breakdown. ENB1 expression is induced by sucrose via the BETL-specific Myb-Related Protein1 transcription factor. Overexpression of ENB1 enhanced development of flange ingrowths, facilitating sucrose transport into BETL cells and increasing kernel weight. The results demonstrated that ENB1 enhances sucrose supply to the endosperm and contributes to a PE in the kernel.

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.

Rapidly evolving genetic features for desert adaptations in Stipagrostis pennata

BACKGROUND

Stipagrostis pennata is distributed in the mobile and semi-mobile sand dunes which can adapt well to extreme environments such as drought and high temperature. It is a pioneer plant species with potential for stabilizing sand dunes and ecological restoration. It can settle on moving sand dunes earlier than other desert plants. It can effectively improve the stability of sand dunes and help more plants settle down and increase plant diversity. However, despite its important ecological value, the genetic resources available for this species are limited.

RESULTS

We used single-molecule real-time sequencing technology to obtain the complete full-length transcriptome of Stipagrostis pennata, including 90,204 unigenes with an average length of 2624 bp. In addition, the 5436 transcription factors identified in these unigenes are rich in stress resistance genes, such as MYB-related, C3H, bHLH, GRAS and HSF, etc., which may play a role in adapting to desert drought and strong wind stress. Intron retention events are abundant alternative splicing events. Stipagrostis pennata has experienced stronger positive selection, accelerating the fixation of advantageous variants. Thirty-eight genes, such as CPP/TSO1-like gene, have evolved rapidly and may play a role in material transportation, flowering and seed formation.

CONCLUSIONS

The present study captures the complete full-length transcriptome of Stipagrostis pennata and reveals its rapid evolution. The desert adaptation in Stipagrostis pennata is reflected in the regulation of gene expression and the adaptability of gene function. Our findings provide a wealth of knowledge for the evolutionary adaptability of desert grass species.

Maize unstable factor for orange1 is essential for endosperm development and carbohydrate accumulation

Maize (Zea mays L.) Ufo1-1 is a spontaneous dominant mutation of the unstable factor for orange1 (ufo1). We recently cloned ufo1, which is a Poaceae-specific gene highly expressed during seed development in maize. Here, we have characterized Ufo1-1 and a loss-of-function Ds insertion allele (ufo1-Dsg) to decipher the role of ufo1 in maize. We found that both ufo1 mutant alleles impact sugars and hormones, and have defects in the basal endosperm transfer layer (BETL) and adjacent cell types. The Ufo1-1 BETL had reduced cell elongation and cell wall ingrowth, resulting in cuboidal shaped transfer cells. In contrast, the ufo1-Dsg BETL cells showed a reduced overall size with abnormal wall ingrowth. Expression analysis identified the impact of ufo1 on several genes essential for BETL development. The overexpression of Ufo1-1 in various tissues leads to ectopic phenotypes, including abnormal cell organization and stomata subsidiary cell defects. Interestingly, pericarp and leaf transcriptomes also showed that as compared with wild type, Ufo1-1 had ectopic expression of endosperm development-specific genes. This study shows that Ufo1-1 impacts the expression patterns of a wide range of genes involved in various developmental processes.

Localised expression of OsIAA29 suggests a key role for auxin in regulating development of the dorsal aleurone of early rice grains

Non-canonical AUX/IAA protein, OsIAA29, and ZmMPR-1 homologues, OsMRPLs, are part of an auxin-related signalling cascade operating in the dorsal aleurone during early rice grain development. Endosperm of rice and other cereals accumulates high concentrations of the predominant in planta auxin, indole-3-acetic acid (IAA) during early grain development. However, IAA signalling and function during endosperm development are poorly understood. Here, we report that OsYUC12 (an auxin biosynthesis gene) and OsIAA29 (encoding a non-canonical AUX/IAA) are both expressed exclusively in grains, reaching a maximum 5-6 days after pollination. OsYUC12 expression is localised in the aleurone, sub-aleurone and embryo, whereas OsIAA29 expression is restricted to a narrow strip in the dorsal aleurone, directly under the vascular bundle. Although rice has been reported to lack endosperm transfer cells (ETCs), this region of the aleurone is enriched with sugar transporters and is likely to play a key role in apoplastic nutrient transfer, analogous to ETCs in other cereals. OsIAA29 has orthologues only in grass species; expression of which is also specific to early grain development. OsYUC12 and OsIAA29 are temporally co-expressed with two genes (AL1 and OsPR602) previously linked to the development of dorsal aleurone or ETCs. Also up-regulated at the same time is a cluster of MYB-related genes (designated OsMRPLs) homologous to ZmMRP-1, which regulates maize ETC development. Wheat homologues of ZmMRP-1 are similarly expressed in ETCs. Although previous work has suggested that other cereals do not have orthologues of ZmMRP-1, our work suggests OsIAA29 and OsMRPLs and their homologues in other grasses are part of an auxin-regulated, conserved signalling network involved in the differentiation of cells with ETC-like function in developing cereal grains.

A C-terminal encoded peptide, ZmCEP1, is essential for kernel development in maize

C-terminal encoded peptides (CEPs) are peptide hormones which act as mobile signals coordinating important developmental programs. Previous studies have unraveled that CEPs are able to regulate plant growth and abiotic stress via cell-to-cell communication in Arabidopsis and rice; however, little is known about their roles in maize. Here, we examined the spatiotemporal expression pattern of ZmCEP1 and showed that ZmCEP1 is highly expressed in young ears and tassels of maize, particularly in the vascular bundles of ears. Heterologous expression of ZmCEP1 in Arabidopsis results in smaller plants and seed size. Similarly, overexpression of ZmCEP1 in maize decreased the plant and ear height, ear length, kernel size, and 100-kernel weight. Consistently, exogenous application of the synthesized ZmCEP1 peptide to the roots of Arabidopsis and maize inhibited root elongation. Knock-out of ZmCEP1 through CRISPR/Cas9 significantly increased plant and ear height, kernel size and 100-kernel weight. Transcriptome analysis revealed that knock-out of ZmCEP1 up-regulated a subset of genes involved in nitrogen metabolism, nitrate transport, sugar transport and auxin response. Thus, these results provide new insights into the genetic and molecular function of ZmCEP1 in regulating kernel development and plant growth, providing novel opportunities for maize breeding.

A chlorophyll a oxygenase 1 gene ZmCAO1 contributes to grain yield and waterlogging tolerance in maize

Chlorophylls function in photosynthesis, and are critical to plant developmental processes and responses to environmental stimuli. Chlorophyll b is synthesized from chlorophyll a by chlorophyll a oxygenase (CAO). Here, we characterize a yellow-green leaf (ygl) mutant and identify the causal gene which encodes a chlorophyll a oxygenase in maize (ZmCAO1). A 51 bp Popin transposon insertion in ZmCAO1 strongly disrupts its transcription. Low enzyme activity of ZmCAO1 leads to reduced concentrations of chlorophyll a and chlorophyll b, resulting in the yellow-green leaf phenotype of the ygl mutant. The net photosynthetic rate, stomatal conductance, and transpiration rate are decreased in the ygl mutant, while concentrations of δ-aminolevulinic acid (ALA), porphobilinogen (PBG) and protochlorophyllide (Pchlide) are increased. In addition, a ZmCAO1 mutation results in down-regulation of key photosynthetic genes, limits photosynthetic assimilation, and reduces plant height, ear size, kernel weight, and grain yield. Furthermore, the zmcao1 mutant shows enhanced reactive oxygen species production leading to sensitivity to waterlogging. These results demonstrate the pleiotropy of ZmCAO1 function in photosynthesis, grain yield, and waterlogging tolerance in maize.

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.

Identification of virulence associated milRNAs and their bidirectional targets in Rhizoctonia solani and maize during infection

BACKGROUND

Anastomosis group 1 IA (AG1-IA) of Rhizoctonia solani is the major agent of banded leaf and sheath blight (BLSB) disease that causes severe yield loss in many worldwide crops. MicroRNAs (miRNAs) are ~ 22 nt non-coding RNAs that negatively regulate gene expression levels by mRNA degradation or translation inhibition. A better understanding of miRNA function during AG1-IA infection can expedite to elucidate the molecular mechanisms of fungi-host interactions.

RESULTS

In this study, we sequenced three small RNA libraries obtained from the mycelium of AG1-IA isolate, non-infected maize sheath and mixed maize sheath 3 days after inoculation. In total, 137 conserved and 34 novel microRNA-like small RNAs (milRNAs) were identified from the pathogen. Among these, one novel and 17 conserved milRNAs were identified as potential virulence-associated (VA) milRNAs. Subsequently, the prediction of target genes for these milRNAs was performed in both AG1-IA and maize, while functional annotation of these targets suggested a link to pathogenesis-related biological processes. Further, expression patterns of these virulence-associated milRNAs demonstrated that theyparticipate in the virulence of AG1-IA. Finally, regulation of one maize targeting gene, GRMZM2G412674 for Rhi-milRNA-9829-5p, was validated by dual-luciferase assay and identified to play a positive role in BLSB resistance in two maize mutants. These results suggest the global differentially expressed milRNAs of R. solani AG1-IA that participate in the regulation of target genes in both AG1-IA and maize to reinforce its pathogenicity.

CONCLUSIONS

Our data have provided a comprehensive overview of the VA-milRNAs of R. solani and identified that they are probably the virulence factors by directly interfered in host targeting genes. These results offer new insights on the molecular mechanisms of R.solani-maize interactions during the process of infection.

Maize kernel development

Maize (Zea mays) is a leading cereal crop in the world. The maize kernel is the storage organ and the harvest portion of this crop and is closely related to its yield and quality. The development of maize kernel is initiated by the double fertilization event, leading to the formation of a diploid embryo and a triploid endosperm. The embryo and endosperm are then undergone independent developmental programs, resulting in a mature maize kernel which is comprised of a persistent endosperm, a large embryo, and a maternal pericarp. Due to the well-characterized morphogenesis and powerful genetics, maize kernel has long been an excellent model for the study of cereal kernel development. In recent years, with the release of the maize reference genome and the development of new genomic technologies, there has been an explosive expansion of new knowledge for maize kernel development. In this review, we overviewed recent progress in the study of maize kernel development, with an emphasis on genetic mapping of kernel traits, transcriptome analysis during kernel development, functional gene cloning of kernel mutants, and genetic engineering of kernel traits.

Transfer cells: what regulates the development of their intricate wall labyrinths?

Transfer cells (TCs) support high nutrient rates into, or at symplasmic discontinuities within, the plant body. Their transport capacity is conferred by an amplified plasma membrane surface area, enriched in nutrient transporters, supported on an intricately invaginated wall labyrinth (WL). Thus, development of the WL is at the heart of TC function. Enquiry has shifted from describing WL architecture and formation to discovering mechanisms regulating WL assembly. Experimental systems used to examine these phenomena are critiqued. Considerable progress has been made in identifying master regulators that commit stem cells to a TC fate (e.g. the maize Myeloblastosis (MYB)-related R1-type transcription factor) and signals that induce differentiated cells to undergo trans-differentiation to a TC phenotype (e.g. sugar, auxin and ethylene). In addition, signals that provide positional information for assembly of the WL include apoplasmic hydrogen peroxide and cytosolic Ca²⁺ plumes. The former switches on, and specifies the intracellular site for WL construction, while the latter creates subdomains to direct assembly of WL invaginations. Less is known about macromolecule species and their spatial organization essential for WL assembly. Emerging evidence points to a dependency on methyl-esterified homogalacturonan accumulation, unique patterns of cellulose and callose deposition and spatial positioning of arabinogalactan proteins.

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.

Transcriptomics at Maize Embryo/Endosperm Interfaces Identifies a Transcriptionally Distinct Endosperm Subdomain Adjacent to the Embryo Scutellum

Seeds are complex biological systems comprising three genetically distinct tissues nested one inside another (embryo, endosperm, and maternal tissues). However, the complexity of the kernel makes it difficult to understand intercompartment interactions without access to spatially accurate information. Here, we took advantage of the large size of the maize (Zea mays) kernel to characterize genome-wide expression profiles of tissues at different embryo/endosperm interfaces. Our analysis identifies specific transcriptomic signatures in two interface tissues compared with whole seed compartments: the scutellar aleurone layer and the newly named endosperm adjacent to scutellum (EAS). The EAS, which appears around 9 d after pollination and persists for around 11 d, is confined to one to three endosperm cell layers adjacent to the embryonic scutellum. Its transcriptome is enriched in genes encoding transporters. The absence of the embryo in an embryo specific mutant can alter the expression pattern of EAS marker genes. The detection of cell death in some EAS cells together with an accumulation of crushed cell walls suggests that the EAS is a dynamic zone from which cell layers in contact with the embryo are regularly eliminated and to which additional endosperm cells are recruited as the embryo grows.

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.

Dye- and fluorescence-based assay to characterize symplastic and apoplastic trafficking in soybean (Glycime max L.) endosperm

BACKGROUND

Endosperm is a triploid tissue in seed resulting from a sperm nucleus fused with the binucleate central cell after double fertilization. Endosperm may be involved in metabolite production, solute transport, nutrient storage, and germination. In the legume family (Fabaceae), with the greatest number of domesticated crops, approximately 60% of genera contain well-differentiated endosperm in mature seeds. Soybean seeds, the most important legume crop in the worlds, have endosperm surrounding embryos during all stages of seed development. However, the function of soybean endosperm is still unknown.

RESULTS

Flow cytometry assay confirmed that soybean endosperm was triploid. Cytobiological observation showed that soybean endosperm cells were alive with zigzag-shape cell wall. Soybean endosperm cells allowed fusion proteins (42 kDa) to move from bombarded cells to adjacent unbombarded-cells. Such movement is not simple diffusion because the fusion proteins failed to move into dead cells. We used symplastic tracers to test the transport potential of soybean endosperm. Small organic dye and low-molecular-weight symplastic tracers revealed fast symplastic transport. After a treatment of an inhibitor of ATPase, N,N'-dicyclohexylcarbodiimide (DCCD), symplastic transport was blocked, but all tracers still showed fast apolopastic transport. The transport speed of 8-hydroxypyrene-1,3,6-trisulfonic acid in endosperm was 1.5 to 3 times faster than in cotyledon cells or Arabidopsis embryos.

CONCLUSIONS

Soybean endosperm is a membrane-like, semi-transparent, and fully active tissue located between the seed coat and cotyledon. Soybean endosperm cells allowed macromolecules to move fast via plasmodesmata transport. The size exclusion limit is larger for soybean endosperm cells than its cotyledon or even Arabidopsis embryo cells. Soybean endosperm may be involved in fast and horizontal transport during the mid-developmental stage of seeds.

High Temporal-Resolution Transcriptome Landscape of Early Maize Seed Development

The early maize (Zea mays) seed undergoes several developmental stages after double fertilization to become fully differentiated within a short period of time, but the genetic control of this highly dynamic and complex developmental process remains largely unknown. Here, we report a high temporal-resolution investigation of transcriptomes using 31 samples collected at an interval of 4 or 6 h within the first six days of seed development. These time-course transcriptomes were clearly separated into four distinct groups corresponding to the stages of double fertilization, coenocyte formation, cellularization, and differentiation. A total of 22,790 expressed genes including 1415 transcription factors (TFs) were detected in early stages of maize seed development. In particular, 1093 genes including 110 TFs were specifically expressed in the seed and displayed high temporal specificity by expressing only in particular period of early seed development. There were 160, 22, 112, and 569 seed-specific genes predominantly expressed in the first 16 h after pollination, coenocyte formation, cellularization, and differentiation stage, respectively. In addition, network analysis predicted 31,256 interactions among 1317 TFs and 14,540 genes. The high temporal-resolution transcriptome atlas reported here provides an important resource for future functional study to unravel the genetic control of seed development.

Transcriptomic and metabolomic analysis of ZmYUC1 mutant reveals the role of auxin during early endosperm formation in maize

Kernel size in cereal is an important agronomic trait controlled by the interaction of genetic and environmental factors. The endosperm occupies most of the kernel area; for this reason, the endosperm cells dimension, number and metabolic content strongly influence kernel properties. This paper presents the transcriptomic and metabolomic analysis of the maize defective endosperm 18 (de18) mutant, where auxin accumulation in the endosperm is impaired. This mutation, involving the ZmYuc1 gene, leads to a reduced kernel size compared to the wild-type line B37. Our results mainly indicate that IAA concentration controls sugar and protein metabolism during kernel differentiation and it is necessary for BETL formation. Furthermore, a fine tuning of different auxin conjugates is reported as the main mechanism to counteract the auxin deficit. Some candidates as master regulators of endosperm transcriptional regulation mediated by auxin are found between MYB and MADS-box gene families. A link between auxin and storage protein accumulation is highlighted, suggesting that IAA directly or indirectly, through CK or ABA, regulates the transcription of zein coding genes. This study represents a move forward with respect to the current knowledge about the role of auxin during maize endosperm differentiation thus revealing the genes that are modulated by auxin and that control agronomic traits as kernel size and metabolic composition.

Maize Dek15 Encodes the Cohesin-Loading Complex Subunit SCC4 and Is Essential for Chromosome Segregation and Kernel Development

Cohesin complexes maintain sister chromatid cohesion to ensure proper chromosome segregation during mitosis and meiosis. In plants, the exact components and functions of the cohesin complex remain poorly understood. Here, we positionally cloned the classic maize (Zea mays) mutant defective kernel 15 (dek15), revealing that it encodes a homolog of SISTER CHROMATID COHESION PROTEIN 4 (SCC4), a loader subunit of the cohesin ring. Developing dek15 kernels contained fewer cells than the wild type, but had a highly variable cell size. The dek15 mutation was found to disrupt the mitotic cell cycle and endoreduplication, resulting in a reduced endosperm and embryo lethality. The cells in the dek15 endosperm and embryo exhibited precocious sister chromatid separation and other chromosome segregation errors, including misaligned chromosomes, lagging chromosomes, and micronuclei, resulting in a high percentage of aneuploid cells. The loss of Dek15/Scc4 function upregulated the expression of genes involved in cell cycle progression and stress responses, and downregulated key genes involved in organic synthesis during maize endosperm development. Our yeast two-hybrid screen identified the chromatin remodeling proteins chromatin remodeling factor 4, chromatin remodeling complex subunit B (CHB)102, CHB105, and CHB106 as SCC4-interacting proteins, suggesting a possible mechanism by which the cohesin ring is loaded onto chromatin in plant cells. This study revealed biological functions for DEK15/SCC4 in mitotic chromosome segregation and kernel development in maize.

Cell wall invertase activity regulates the expression of the transfer cell-specific transcription factor ZmMRP-1

Studies in cell wall bound invertase mutants indicate that the promoter of the transfer cell-specific transcription factor, ZmMRP - 1 , is modulated by the carbohydrate balance. Transfer cells are highly specialized plant cells located at the surfaces that need to support an intensive exchange of nutrients, such as the entrance of fruits, seeds and nodules or the young branching points along the stem. ZmMRP-1 is a one-domain MYB-related transcription factor specifically expressed at the transfer cell layer of the maize endosperm. Previous studies demonstrated that this factor regulates the expression of a large number of transfer cell-specific genes, and suggested that ZmMRP-1 is a key regulator of the differentiation of this tissue. The expression of this gene is largely dominated by positional cues, but within the ZmMRP-1 expressing cells the promoter appears to be modulated by sugars. Here we have investigated in vivo this modulation. Using maize and Arabidopsis mutants for cell wall invertase genes, we found that the absence of cell wall invertase activity is a major inductive signal of the ZmMRP-1 expression.

Highly interwoven communities of a gene regulatory network unveil topologically important genes for maize seed development

The complex interactions between transcription factors (TFs) and their target genes in a spatially and temporally specific manner are crucial to all cellular processes. Reconstruction of gene regulatory networks (GRNs) from gene expression profiles can help to decipher TF-gene regulations in a variety of contexts; however, the inevitable prediction errors of GRNs hinder optimal data mining of RNA-Seq transcriptome profiles. Here we perform an integrative study of Zea mays (maize) seed development in order to identify key genes in a complex developmental process. First, we reverse engineered a GRN from 78 maize seed transcriptome profiles. Then, we studied collective gene interaction patterns and uncovered highly interwoven network communities as the building blocks of the GRN. One community, composed of mostly unknown genes interacting with opaque2, brittle endosperm1 and shrunken2, contributes to seed phenotypes. Another community, composed mostly of genes expressed in the basal endosperm transfer layer, is responsible for nutrient transport. We further integrated our inferred GRN with gene expression patterns in different seed compartments and at various developmental stages and pathways. The integration facilitated a biological interpretation of the GRN. Our yeast one-hybrid assays verified six out of eight TF-promoter bindings in the reconstructed GRN. This study identified topologically important genes in interwoven network communities that may be crucial to maize seed development.

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.

Signaling in Early Maize Kernel Development

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

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.

Parent-of-Origin-Effect rough endosperm Mutants in Maize

Parent-of-origin-effect loci have non-Mendelian inheritance in which phenotypes are determined by either the maternal or paternal allele alone. In angiosperms, parent-of-origin effects can be caused by loci required for gametophyte development or by imprinted genes needed for seed development. Few parent-of-origin-effect loci have been identified in maize (Zea mays) even though there are a large number of imprinted genes known from transcriptomics. We screened rough endosperm (rgh) mutants for parent-of-origin effects using reciprocal crosses with inbred parents. Six maternal rough endosperm (mre) and three paternal rough endosperm (pre) mutants were identified with three mre loci mapped. When inherited from the female parent, mre/+ seeds reduce grain fill with a rough, etched, or pitted endosperm surface. Pollen transmission of pre mutants results in rgh endosperm as well as embryo lethality. Eight of the mutants had significant distortion from the expected one-to-one ratio for parent-of-origin effects. Linked markers for mre1, mre2, and mre3 indicated that the mutant alleles have no bias in transmission. Histological analysis of mre1, mre2, mre3, and pre*-949 showed altered timing of starch grain accumulation and basal endosperm transfer cell layer (BETL) development. The mre1 locus delays BETL and starchy endosperm development, while mre2 and pre*-949 cause ectopic starchy endosperm differentiation. We conclude that many parent-of-origin effects in maize have incomplete penetrance of kernel phenotypes and that there is a large diversity of endosperm developmental roles for parent-of-origin-effect loci.

Identification of Candidate Transcriptional Regulators of Epidermal Transfer Cell Development in Vicia faba Cotyledons

Transfer cells (TCs) are anatomically-specialized cells formed at apoplasmic-symplasmic bottlenecks in nutrient transport pathways in plants. TCs form invaginated wall ingrowths which provide a scaffold to amplify plasma membrane surface area and thus increase the density of nutrient transporters required to achieve enhanced nutrient flow across these bottlenecks. Despite their importance to nutrient transport in plants, little is known of the transcriptional regulation of wall ingrowth formation. Here, we used RNA-Seq to identify transcription factors putatively involved in regulating epidermal TC development in cotyledons of Vicia faba. Comparing cotyledons cultured for 0, 3, 9, and 24 h to induce trans-differentiation of epidermal TCs identified 43 transcription factors that showed either epidermal-specific or epidermal-enhanced expression, and 10 that showed epidermal-specific down regulation. Members of the WRKY and ethylene-responsive families were prominent in the cohort of transcription factors showing epidermal-specific or epidermal-enhanced expression, consistent with the initiation of TC development often representing a response to stress. Members of the MYB family were also prominent in these categories, including orthologs of MYB genes involved in localized secondary wall deposition in Arabidopsis thaliana. Among the group of transcription factors showing down regulation were various homeobox genes and members of the MADs-box and zinc-finger families of poorly defined functions. Collectively, this study identified several transcription factors showing expression characteristics and orthologous functions that indicate likely participation in transcriptional regulation of epidermal TC development in V. faba cotyledons.

Evidence for maternal control of seed size in maize from phenotypic and transcriptional analysis

Seed size is an important component of grain yield and a key determinant trait for crop domestication. The Krug Yellow Dent long-term selection experiment for large and small seed provides a valuable resource to dissect genetic and phenotypic changes affecting seed size within a common genetic background. In this study, inbred lines derived from Krug Large Seed (KLS) and Krug Small Seed (KSS) populations and reciprocal F1 crosses were used to investigate developmental and molecular mechanisms governing seed size. Seed morphological characteristics showed striking differences between KLS and KSS inbred lines, and the reciprocal cross experiment revealed a strong maternal influence on both seed weight and seed size. Quantification of endosperm area, starchy endosperm cell size, and kernel dry mass accumulation indicated a positive correlation between seed size, endosperm cell number, and grain filling rate, and patterns of grain filling in reciprocal crosses mirrored that of the maternal parent. Consistent with the maternal contribution to seed weight, transcriptome profiling of reciprocal F1 hybrids showed substantial similarities to the maternal parent. A set of differentially expressed genes between KLS and KSS inbreds were found, which fell into a broad number of functional categories including DNA methylation, nucleosome assembly, and heat stress response. In addition, gene co-expression network analysis of parental inbreds and reciprocal F1 hybrids identified co-expression modules enriched in ovule development and DNA methylation, implicating these two processes in seed size determination. These results expand our understanding of seed size regulation and help to uncover the developmental and molecular basis underlying maternal control of seed size in maize.

Genome-Wide Mapping of Targets of Maize Histone Deacetylase HDA101 Reveals Its Function and Regulatory Mechanism during Seed Development

Histone deacetylases (HDACs) regulate histone acetylation levels by removing the acetyl group from lysine residues. The maize (Zea mays) HDACHDA101 influences several aspects of development, including kernel size; however, the molecular mechanism by which HDA101 affects kernel development remains unknown. In this study, we find that HDA101 regulates the expression of transfer cell-specific genes, suggesting that their misregulation may be associated with the defects in differentiation of endosperm transfer cells and smaller kernels observed in hda101 mutants. To investigate HDA101 function during the early stages of seed development, we performed genome-wide mapping of HDA101 binding sites. We observed that, like mammalian HDACs, HDA101 mainly targets highly and intermediately expressed genes. Although loss of HDA101 can induce histone hyperacetylation of its direct targets, this often does not involve variation in transcript levels. A small subset of inactive genes that must be negatively regulated during kernel development is also targeted by HDA101 and its loss leads to hyperacetylation and increased expression of these inactive genes. Finally, we report that HDA101 interacts with members of different chromatin remodeling complexes, such as NFC103/MSI1 and SNL1/SIN3-like protein corepressors. Taken together, our results reveal a complex genetic network regulated by HDA101 during seed development and provide insight into the different mechanisms of HDA101-mediated regulation of transcriptionally active and inactive genes.

Seed filling in domesticated maize and rice depends on SWEET-mediated hexose transport

Carbohydrate import into seeds directly determines seed size and must have been increased through domestication. However, evidence of the domestication of sugar translocation and the identities of seed-filling transporters have been elusive. Maize ZmSWEET4c, as opposed to its sucrose-transporting homologs, mediates transepithelial hexose transport across the basal endosperm transfer layer (BETL), the entry point of nutrients into the seed, and shows signatures indicative of selection during domestication. Mutants of both maize ZmSWEET4c and its rice ortholog OsSWEET4 are defective in seed filling, indicating that a lack of hexose transport at the BETL impairs further transfer of sugars imported from the maternal phloem. In both maize and rice, SWEET4 was likely recruited during domestication to enhance sugar import into the endosperm.

Peptide signalling during angiosperm seed development

Cell-cell communication is pivotal for the coordination of various features of plant development. Recent studies in plants have revealed that, as in animals, secreted signal peptides play critical roles during reproduction. However, the precise signalling mechanisms in plants are not well understood. In this review, we discuss the known and putative roles of secreted peptides present in the seeds of angiosperms as key signalling factors involved in coordinating different aspects of seed development.

A putative plant organelle RNA recognition protein gene is essential for maize kernel development

Basal endosperm transfer layer (BETL) cells are responsible for transferring apoplastic solutes from the maternal pedicel into the endosperm, supplying the grain with compounds required for embryo development and storage reserve accumulation. Here, we analyze the maize (Zea mays L.) empty pericarp6 (emp6) mutant, which causes early arrest in grain development. The Emp6+gene function is required independently in both the embryo and endosperm. The emp6 mutant causes a notable effect on the differentiation of BETL cells; the extensive cell wall ingrowths that distinguish BETL cells are diminished and BETL marker gene expression is compromised in mutant kernels. Transposon tagging identified the emp6 locus as encoding a putative plant organelle RNA recognition (PORR) protein, 1 of 15 PORR family members in maize. The emp6 transcript is widely detected in plant tissues with highest levels in embryos and developing kernels. EMP6-green fluorescent protein (GFP) fusion proteins transiently expressed in Nicotiana benthamiana leaves were targeted specifically to mitochondria. These results suggest that BETL cell differentiation might be particularly energy intensive, or alternatively, that mitochondria might confer a developmental function.

RNA sequencing of laser-capture microdissected compartments of the maize kernel identifies regulatory modules associated with endosperm cell differentiation

Endosperm is an absorptive structure that supports embryo development or seedling germination in angiosperms. The endosperm of cereals is a main source of food, feed, and industrial raw materials worldwide. However, the genetic networks that regulate endosperm cell differentiation remain largely unclear. As a first step toward characterizing these networks, we profiled the mRNAs in five major cell types of the differentiating endosperm and in the embryo and four maternal compartments of the maize (Zea mays) kernel. Comparisons of these mRNA populations revealed the diverged gene expression programs between filial and maternal compartments and an unexpected close correlation between embryo and the aleurone layer of endosperm. Gene coexpression network analysis identified coexpression modules associated with single or multiple kernel compartments including modules for the endosperm cell types, some of which showed enrichment of previously identified temporally activated and/or imprinted genes. Detailed analyses of a coexpression module highly correlated with the basal endosperm transfer layer (BETL) identified a regulatory module activated by MRP-1, a regulator of BETL differentiation and function. These results provide a high-resolution atlas of gene activity in the compartments of the maize kernel and help to uncover the regulatory modules associated with the differentiation of the major endosperm cell types.

Observation and investigation of three endosperm transport tissues in sorghum caryopses

Endosperm transport tissues in sorghum caryopses include endosperm transfer cells, endosperm conducting cells, and the embryo surrounding region. To elucidate the structural changes of these tissues and their relationship with the caryopsis development, sorghum caryopses were analyzed at different days after pollination using light, fluorescence, and electron microscopy. The following results were obtained: post-phloem maternal tissues included the placentochalaza and the nucellar projection-like nucellus. Well-developed endosperm transfer cells exhibited very evident flange-type wall ingrowths. Very few wall ingrowths were present in the initially developed endosperm transfer cells when the level of sucrose from the initially developed vascular system was low. At the middle stage of caryopsis development, the level of sucrose from the well-developed vascular system was high. Endosperm transfer cells increased in both area and layer amount, and their wall ingrowths increased in both length and density. Later in caryopsis development, the level of sucrose from the degenerated vascular system was low and wall ingrowths distorted in the degenerated endosperm transfer cells. Endosperm conducting cells primarily occupied the most part of endosperm, but decreased gradually because the upper part transformed into the starchy endosperm and the lower part degenerated to give space to the embryo growth. Although the embryo surrounding region initially enveloped the small embryo, it rapidly degenerated and finally disappeared. Our data showed that (1) the caryopsis vascular system influenced the differentiation of endosperm transfer cells by controlling the sugar levels (2) and configuration of endosperm transport tissues were probably altered to favor the growth of filial tissues.

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.

The cereal starch endosperm development and its relationship with other endosperm tissues and embryo

The cereal starch endosperm is the central part of endosperm, and it is rich in starch and protein which are the important resources for human food. The starch and protein are separately accumulated in starch granules and protein bodies. Content and configuration of starch granules and protein bodies affect the quality of the starch endosperm. The development of starch endosperm is mediated by genes, enzymes, and hormones, and it also has a close relationship with other endosperm tissues and embryo. This paper reviews the latest investigations on the starch endosperm and will provide some useful information for the future researches on the development of cereal endosperm.