Atypical response regulators expressed in the maize endosperm transfer cells link canonical two component systems and seed biology

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.

Using NGS Technology and Association Mapping to Identify Candidate Genes Associated with Fusarium Stalk Rot Resistance

Stalk rot caused by Fusarium fungi is one of the most widespread and devastating diseases of maize, and the introduction of resistant genotypes is one of the most effective strategies for controlling the disease. Breeding genotypes with genetically determined resistance will also allow less use of crop protection products. The aim of the research was to identify molecular markers and associated candidate genes determining maize plant resistance to Fusarium stalk rot. The plant material for this study consisted of 122 maize hybrids. The experiment was conducted in two localities: Smolice and Kobierzyce. The Fusarium stalk rot values ranged from 1.65% (for genotype G01.10) to 31.18% (for genotype G03.07) in Kobierzyce and from 0.00% (for 58 genotypes) to 6.36% (G05.03) in Smolice. The analyzed genotypes were simultaneously subjected to next-generation sequencing using the Illumina platform. Illumina sequencing identified 60,436 SilicoDArT markers and 32,178 SNP markers (92,614 in total). For association mapping, 32,900 markers (26,234 SilicoDArT and 6666 SNP) meeting the criteria (MAF > 0.25 and the number of missing observations <10%) were used. The results of the observation of the degree of infection and sequencing were used for association mapping, which ultimately resulted in the selection of ten molecular markers important at both places. Among the identified markers, two SNP markers that are located inside candidate genes play an important role. Marker 4772836 is located inside the serine/threonine-protein kinase bsk3 gene, while marker 4765764 is located inside the histidine kinase 1 gene. Both genes can be associated with plant resistance to Fusarium stalk rot, and these genes can also be used in breeding programs to select resistant varieties.

Genetic Dissection of Spike Productivity Traits in the Siberian Collection of Spring Barley

Barley (Hordeum vulgare L.) is one of the most commonly cultivated cereals worldwide. Its local varieties can represent a valuable source of unique genetic variants useful for crop improvement. The aim of this study was to reveal loci contributing to spike productivity traits in Siberian spring barley and to develop diagnostic DNA markers for marker-assisted breeding programs. For this purpose we conducted a genome-wide association study using a panel of 94 barley varieties. In total, 64 SNPs significantly associated with productivity traits were revealed. Twenty-three SNP markers were validated by genotyping in an independent sample set using competitive allele-specific PCR (KASP). Finally, fourteen markers associated with spike productivity traits on chromosomes 2H, 4H and 5H can be suggested for use in breeding programs.

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.

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

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

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.

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 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.

Phosphorylation-mediated signalling in flowering: prospects and retrospects of phosphoproteomics in crops

Protein phosphorylation is a major post-translational modification, regulating protein function, stability, and subcellular localization. To date, annotated phosphorylation data are available mainly for model organisms and humans, despite the economic importance of crop species and their large kinomes. Our understanding of the phospho-regulation of flowering in relation to the biology and interaction between the pollen and pistil is still significantly lagging, limiting our knowledge on kinase signalling and its potential applications to crop production. To address this gap, we bring together relevant literature that were previously disconnected to present an overview of the roles of phosphoproteomic signalling pathways in modulating molecular and cellular regulation within specific tissues at different morphological stages of flowering. This review is intended to stimulate research, with the potential to increase crop productivity by providing a platform for novel molecular tools.

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.

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.

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.

Hormonal regulation of cereal endosperm development with a focus on rice (Oryza sativa)

The endosperm of cereal grain forms the staple diet for most of the world's population, and feeds much of their stock. Grain size and quality are determined largely by events taking place during coenocytic nuclear division, endosperm cellularisation and cell differentiation, and the production of storage molecules. Thus, understanding the complex signalling processes occurring at each of these steps is essential for maintaining and improving our food supply. Here, we critically review evidence for the effects of phytohormones on grain size, as well as hormone homeostasis, signalling and crosstalk. We focus on rice endosperm due to the importance of rice as a food crop and a model grass, as well as its relative neglect in recent reviews; however, data from other cereals are also discussed due to strong evidence for conserved signalling networks operating during grain development. Discussion is restricted to auxin, cytokinin, ethylene, abscisic acid and gibberellin. Our review highlights the need for accurate hormone determinations combined with information on gene expression. We present evidence for separate, localised signalling roles for auxin at different stages of grain development and highlight key research questions for other hormones where much less data are available.

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.

Change in Auxin and Cytokinin Levels Coincides with Altered Expression of Branching Genes during Axillary Bud Outgrowth in Chrysanthemum

In the production and breeding of Chrysanthemum sp., shoot branching is an important quality aspect as the outgrowth of axillary buds determines the final plant shape. Bud outgrowth is mainly controlled by apical dominance and the crosstalk between the plant hormones auxin, cytokinin and strigolactone. In this work the hormonal and genetic regulation of axillary bud outgrowth was studied in two differently branching cut flower Chrysanthemum morifolium (Ramat) genotypes. C17 is a split-type which forms an inflorescence meristem after a certain vegetative period, while C18 remains vegetative under long day conditions. Plant growth of both genotypes was monitored during 5 subsequent weeks starting one week before flower initiation occurred in C17. Axillary bud outgrowth was measured weekly and samples of shoot apex, stem and axillary buds were taken during the first two weeks. We combined auxin and cytokinin measurements by UPLC-MS/MS with RT-qPCR expression analysis of genes involved in shoot branching regulation pathways in chrysanthemum. These included bud development genes (CmBRC1, CmDRM1, CmSTM, CmLsL), auxin pathway genes (CmPIN1, CmTIR3, CmTIR1, CmAXR1, CmAXR6, CmAXR2, CmIAA16, CmIAA12), cytokinin pathway genes (CmIPT3, CmHK3, CmRR1) and strigolactone genes (CmMAX1 and CmMAX2). Genotype C17 showed a release from apical dominance after floral transition coinciding with decreased auxin and increased cytokinin levels in the subapical axillary buds. As opposed to C17, C18 maintained strong apical dominance with vegetative growth throughout the experiment. Here high auxin levels and decreasing cytokinin levels in axillary buds and stem were measured. A differential expression of several branching genes accompanied the different hormonal change and bud outgrowth in C17 and C18. This was clear for the strigolactone biosynthesis gene CmMAX1, the transcription factor CmBRC1 and the dormancy associated gene CmDRM1, that all showed a decreased expression in C17 at floral transition and an increased expression in C18 with continuous vegetative growth. These results offer a case study for Chrysanthemum, showing an altered cytokinin to auxin balance and differential gene expression between vegetative growth with apical dominance and transition to generative growth with loss of apical dominance and axillary bud outgrowth. This suggests a conservation of several aspects of the hormonal and genetical regulation of bud outgrowth in Chrysanthemum. Furthermore, 15 previously uncharacterised genes in chrysanthemum, were described in this study. Of those genes involved in axillary bud outgrowth we identified CmDRM1, CmBRC1 and CmMAX1 as having an altered expression preceding axillary bud outgrowth, which could be useful as markers for bud activity.

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.

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.

Development of endosperm transfer cells in barley

Endosperm transfer cells (ETCs) are positioned at the intersection of maternal and filial tissues in seeds of cereals and represent a bottleneck for apoplasmic transport of assimilates into the endosperm. Endosperm cellularization starts at the maternal-filial boundary and generates the highly specialized ETCs. During differentiation barley ETCs develop characteristic flange-like wall ingrowths to facilitate effective nutrient transfer. A comprehensive morphological analysis depicted distinct developmental time points in establishment of transfer cell (TC) morphology and revealed intracellular changes possibly associated with cell wall metabolism. Embedded inside the grain, ETCs are barely accessible by manual preparation. To get tissue-specific information about ETC specification and differentiation, laser microdissection (LM)-based methods were used for transcript and metabolite profiling. Transcriptome analysis of ETCs at different developmental stages by microarrays indicated activated gene expression programs related to control of cell proliferation and cell shape, cell wall and carbohydrate metabolism reflecting the morphological changes during early ETC development. Transporter genes reveal distinct expression patterns suggesting a switch from active to passive modes of nutrient uptake with the onset of grain filling. Tissue-specific RNA-seq of the differentiating ETC region from the syncytial stage until functionality in nutrient transfer identified a high number of novel transcripts putatively involved in ETC differentiation. An essential role for two-component signaling (TCS) pathways in ETC development of barley emerged from this analysis. Correlative data provide evidence for abscisic acid and ethylene influences on ETC differentiation and hint at a crosstalk between hormone signal transduction and TCS phosphorelays. Collectively, the data expose a comprehensive view on ETC development, associated pathways and identified candidate genes for ETC specification.

Expression and functional analysis of genes encoding cytokinin receptor-like histidine kinase in maize (Zea mays L.)

Cytokinin signaling is vital for plant growth and development which function via the two-component system (TCS). As one of the key component of TCS, transmembrane histidine kinases (HK) are encoded by a small gene family in plants. In this study, we focused on expression and functional analysis of cytokinin receptor-like HK genes (ZmHK) in maize. Firstly, bioinformatics analysis revealed that seven cloned ZmHK genes have different expression patterns during maize development. Secondly, ectopic expression by CaMV35S promoter in Arabidopsis further revealed that functional differentiation exists among these seven members. Among them, the ZmHK1a2-OX transgenic line has the lowest germination rate in the dark, ZmHK1-OX and ZmHK2a2-OX can delay leaf senescence, and seed size of ZmHK1-OX, ZmHK1a2-OX, ZmHK2-OX, ZmHK3b-OX and ZmHK2a2-OX was obviously reduced as compared to wild type. Additionally, ZmHK genes play opposite roles in shoot and root development; all ZmHK-OX transgenic lines display obvious shorter root length and reduced number of lateral roots, but enhanced shoot development compared with the wild type. Most notably, Arabidopsis response regulator ARR5 gene was up-regulated in ZmHK1-OX, ZmHK1a2-OX, ZmHK2-OX, ZmHK3b-OX and ZmHK2a2-OX as compared to wild type. Although the causal link between ZmHK genes and cytokinin signaling pathway is still an area to be further elucidated, these findings reflected that the diversification of ZmHK genes expression patterns and functions occurred in the course of maize evolution, indicating that some ZmHK genes might play different roles during maize development.

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.

Two maize END-1 orthologs, BETL9 and BETL9like, are transcribed in a non-overlapping spatial pattern on the outer surface of the developing endosperm

In the course of a project aimed to isolate transfer cells-specific genes in maize endosperm we have identified the BETL9 gene. BETL9 encodes for a small protein very similar in sequence to the product of the barley transfer cell-specific gene END-1. Both BETL9 and END-1 proteins are lipid transfer proteins, but their function is currently unknown. In situ hybridization analysis confirms that the BETL9 gene is exclusively transcribed in the basal endosperm transfer cell layer during seed development since 10 days after pollination. However, immunolocalization data indicates that the BETL9 protein accumulates in the maternal placento-chalaza cells located just beside the transfer cell layer. This suggests that the BETL9 protein should be transported to the maternal side to exert its, still unknown, function. In addition, we have identified a second maize gene very similar in sequence to BETL9 and we have named it BETL9like. In situ hybridization shows that BETL9like is also specifically transcribed in the developing maize endosperm within the same time frame that BETL9, but in this case it is exclusively expressed in the aleurone cell layer. Consequently, the BETL9 and BETL9like genes are transcribed in a non-overlapping pattern on the outer surface of the maize endosperm. The BETL9 and BETL9like promoter sequences, fused to the GUS reporter gene, accurately reflected the expression pattern observed for the genes in maize. Finally, we have identified in the Arabidopsis genome a set of four genes orthologous to BETL9 and BETL9like and analyzed the activity of their promoters in Arabidopsis transgenic plants carrying fusions of their promoter sequences to the GUS reporter. As in the case of the maize genes, the Arabidopsis orthologs showed highly complementary expression patterns.

Histidine kinases in plants: cross talk between hormone and stress responses

Two-component signaling pathways involve sensory histidine kinases (HK), histidine phosphotransfer proteins (HpT) and response regulators (RR). Recent advancements in genome sequencing projects for a number of plant species have established the TCS family to be multigenic one. In plants, HKs operate through the His-Asp phosphorelay and control many physiological and developmental processes throughout the lifecycle of plants. Despite the huge diversity reported for the structural features of the HKs, their functional redundancy has also been reported via mutant approach. Several sensory HKs having a CHASE domain, transmembrane domain(s), transmitter domain and receiver domain have been reported to be involved in cytokinin and ethylene signaling. On the other hand, there are also increasing evidences for some of the sensory HKs to be performing their role as osmosensor, clearly indicating toward a possible cross-talk between hormone and stress responsive cascades. In this review, we bring out the latest knowledge about the structure and functions of histidine kinases in cytokinin and ethylene signaling and their role(s) in development and the regulation of environmental stress responses.

454 Transcriptome sequencing suggests a role for two-component signalling in cellularization and differentiation of barley endosperm transfer cells

Background

Cell specification and differentiation in the endosperm of cereals starts at the maternal-filial boundary and generates the endosperm transfer cells (ETCs). Besides the importance in assimilate transfer, ETCs are proposed to play an essential role in the regulation of endosperm differentiation by affecting development of proximate endosperm tissues. We attempted to identify signalling elements involved in early endosperm differentiation by using a combination of laser-assisted microdissection and 454 transcriptome sequencing.

Principal Findings

454 sequencing of the differentiating ETC region from the syncytial state until functionality in transfer processes captured a high proportion of novel transcripts which are not available in existing barley EST databases. Intriguingly, the ETC-transcriptome showed a high abundance of elements of the two-component signalling (TCS) system suggesting an outstanding role in ETC differentiation. All components and subfamilies of the TCS, including distinct kinds of membrane-bound receptors, have been identified to be expressed in ETCs. The TCS system represents an ancient signal transduction system firstly discovered in bacteria and has previously been shown to be co-opted by eukaryotes, like fungi and plants, whereas in animals and humans this signalling route does not exist. Transcript profiling of TCS elements by qRT-PCR suggested pivotal roles for specific phosphorelays activated in a coordinated time flow during ETC cellularization and differentiation. ETC-specificity of transcriptionally activated TCS phosphorelays was assessed for early differentiation and cellularization contrasting to an extension of expression to other grain tissues at the beginning of ETC maturation. Features of candidate genes of distinct phosphorelays and transcriptional activation of genes putatively implicated in hormone signalling pathways hint at a crosstalk of hormonal influences, putatively ABA and ethylene, and TCS signalling.

Significance

Our findings suggest an integral function for the TCS in ETC differentiation possibly coupled to sequent hormonal regulation by ABA and ethylene.

A novel histidine kinase gene, ZmHK9, mediate drought tolerance through the regulation of stomatal development in Arabidopsis

Plants have developed complex signaling networks to regulate biochemical and physiological acclimation, environmental signals were perceived and transmitted to cellular machinery to activate adaptive responses. Here, a novel drought responsive histidine kinase gene was identified and designated as ZmHK9. Under normal conditions, ZmHK9 was predominantly expressed in roots, and the roots of ZmHK9-OX transgenic lines are markedly hypersensitive to ABA and ethylene, as compare to wild type. Consistent with its expression induced by PEG and exogenous ABA treatment, promoter sequence of this gene possessed drought and ABA responsive element. Moreover, the transgenic plants were much less affected by drought stress and recovered quickly after rewatering, stomatal complex size and stomatal density in the transgenic plants are significantly smaller and lower than those of the wild-type plants. In addition, ABA induced stomatal closure and the stomatal aperture of ZmHK9-OX lines was smaller than that of wild type. Collectively, it can be concluded that ZmHK9 regulates root elongation, stomatal development and drought tolerance through ABA dependent signaling pathway in Arabidopsis.

The more, the merrier: cytokinin signaling beyond Arabidopsis

The phytohormone cytokinin is a key player in many developmental processes and in the response of plants to biotic and abiotic stress. The cytokinin signal is perceived and transduced via a multistep variant of the bacterial two-component signaling system. Most of the research on cytokinin signaling has been done in the model plant Arabidopsis thaliana. Research on cytokinin signaling has expanded to a much broader range of plants species in recent years. This is due to the natural limitation of Arabidopsis as a model species for the investigation of processes like nodulation or wood formation. The rapidly increasing number of sequenced plant genomes also facilitates the use of other species in this line of research. This review summarizes what is known about the cytokinin signaling in the different organisms and highlights differences to Arabidopsis.