Molecular mapping of the shrunken endosperm genes seg8 and sex1 in barley (Hordeum vulgare L.)

A guide to barley mutants

BACKGROUND

Mutants have had a fundamental impact upon scientific and applied genetics. They have paved the way for the molecular and genomic era, and most of today's crop plants are derived from breeding programs involving mutagenic treatments.

RESULTS

Barley (Hordeum vulgare L.) is one of the most widely grown cereals in the world and has a long history as a crop plant. Barley breeding started more than 100 years ago and large breeding programs have collected and generated a wide range of natural and induced mutants, which often were deposited in genebanks around the world. In recent years, an increased interest in genetic diversity has brought many historic mutants into focus because the collections are regarded as valuable resources for understanding the genetic control of barley biology and barley breeding. The increased interest has been fueled also by recent advances in genomic research, which provided new tools and possibilities to analyze and reveal the genetic diversity of mutant collections.

CONCLUSION

Since detailed knowledge about phenotypic characters of the mutants is the key to success of genetic and genomic studies, we here provide a comprehensive description of mostly morphological barley mutants. The review is closely linked to the International Database for Barley Genes and Barley Genetic Stocks ( bgs.nordgen.org ) where further details and additional images of each mutant described in this review can be found.

Loss of ADP-glucose transporter in barley sex1 mutant caused shrunken endosperm but with elevated protein and β-glucan content in whole meal

Grain shape and plumpness affect barley yield. Despite numerous studies on shrunken endosperm mutants in barley, their molecular mechanism and application potential in the food industry are largely unknown. Here, map-based cloning, co-segregation analyses, and allelic variant validation revealed that the loss of HORVU6Hr1G037950 encoding an ADP-glucose transporter caused the shrunken endosperm in sex1. Haplotype analysis suggested that hap4 in the promoter sequence was positively related to the hundred-grain weight showing a breeding potential. A pair of near-isogenic lines targeting HORVU6Hr1G037950 was produced and characterized to investigate molecular mechanisms that SEX1 regulates endosperm development. Results presented that the absence of the SEX1 gene led to the decrease of starch content and A-type granules size, the increase of β-glucan, protein, gelatinization temperature, soluble sugar content, amylopectin A chains, and B1 chains. Enzymatic activity, transcriptome and metabolome analyses revealed the loss of SEX1 results in an impaired ADP-glucose-to-starch conversion process, consequently leading to higher soluble sugar contents and lower starch accumulation, thereby inducing a shrunken-endosperm phenotype in sex1. Taken together, this study provides new insights into barley grain development, and the elevated protein and β-glucan contents of the whole meal in sex1 imply its promising application in the food industry.

Identification and characterization of mRNAs and lncRNAs of a barley shrunken endosperm mutant using RNA-seq

Barley shrunken endosperm mutants have been extensively reported. However, knowledge of the underlying molecular mechanisms of these mutants remains limited. Here, a pair of near isogenic lines (normal endosperm: Bowman and shrunken endosperm: sex1) was subjected to transcriptome analysis to identify mRNAs and lncRNAs related to endosperm development to further dissect its mechanism of molecular regulation. A total of 2123 (1140 up- and 983 down-regulated) unique differentially expressed genes (DEGs) were detected. Functional analyses showed that these DEGs were mainly involved in starch and sucrose metabolism, biosynthesis of secondary metabolites, and plant hormone signal transduction. A total of 343 unique target genes were identified for 57 differentially expressed lncRNAs (DE lncRNAs). These DE lncRNAs were mainly involved in glycerophospholipid metabolism, starch and sucrose metabolism, hormone signal transduction, and stress response. In addition, key lncRNAs were identified by constructing a co-expression network of the target genes of DE lncRNAs. Transcriptome results suggested that mRNA and lncRNA played a critical role in endosperm development. The shrunken endosperm in barley seems to be closely related to plant hormone signal transduction, starch and sucrose metabolism, and cell apoptosis. This study provides a foundation for fine mapping, elucidates the molecular mechanism of shrunken endosperm mutants, and also provides a reference for further studies of lncRNAs during the grain development of plants.

Spatiotemporal Dynamics of Oligofructan Metabolism and Suggested Functions in Developing Cereal Grains

Oligofructans represent one of the most important groups of sucrose-derived water-soluble carbohydrates in the plant kingdom. In cereals, oligofructans accumulate in above ground parts of the plants (stems, leaves, seeds) and their biosynthesis leads to the formation of both types of glycosidic linkages [β(2,1); β(2,6)-fructans] or mixed patterns. In recent studies, tissue- and development- specific distribution patterns of the various oligofructan types in cereal grains have been shown, which are possibly related to the different phases of grain development, such as cellular differentiation of grain tissues and storage product accumulation. Here, we summarize the current knowledge about oligofructan biosynthesis and accumulation kinetics in cereal grains. We focus on the spatiotemporal dynamics and regulation of oligofructan biosynthesis and accumulation in developing barley grains (deduced from a combination of metabolite, transcript and proteome analyses). Finally, putative physiological functions of oligofructans in developing grains are discussed.

Gibberellin-to-abscisic acid balances govern development and differentiation of the nucellar projection of barley grains

In cereal grains, the maternal nucellar projection (NP) constitutes the link to the filial organs, forming a transfer path for assimilates and signals towards the endosperm. At transition to the storage phase, the NP of barley (Hordeum vulgare) undergoes dynamic and regulated differentiation forming a characteristic pattern of proliferating, elongating, and disintegrating cells. Immunolocalization revealed that abscisic acid (ABA) is abundant in early non-elongated but not in differentiated NP cells. In the maternally affected shrunken-endosperm mutant seg8, NP cells did not elongate and ABA remained abundant. The amounts of the bioactive forms of gibberellins (GAs) as well as their biosynthetic precursors were strongly and transiently increased in wild-type caryopses during the transition and early storage phases. In seg8, this increase was delayed and less pronounced together with deregulated gene expression of specific ABA and GA biosynthetic genes. We concluded that differentiation of the barley NP is driven by a distinct and specific shift from lower to higher GA:ABA ratios and that the spatial-temporal change of GA:ABA balances is required to form the differentiation gradient, which is a prerequisite for ordered transfer processes through the NP. Deregulated ABA:GA balances in seg8 impair the differentiation of the NP and potentially compromise transfer of signals and assimilates, resulting in aberrant endosperm growth. These results highlight the impact of hormonal balances on the proper release of assimilates from maternal to filial organs and provide new insights into maternal effects on endosperm differentiation and growth of barley grains.

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.

Genetic dissection of barley morphology and development

Since the early 20th century, barley (Hordeum vulgare) has been a model for investigating the effects of physical and chemical mutagens and for exploring the potential of mutation breeding in crop improvement. As a consequence, extensive and well-characterized collections of morphological and developmental mutants have been assembled that represent a valuable resource for exploring a wide range of complex and fundamental biological processes. We constructed a collection of 881 backcrossed lines containing mutant alleles that induce a majority of the morphological and developmental variation described in this species. After genotyping these lines with up to 3,072 single nucleotide polymorphisms, comparison to their recurrent parent defined the genetic location of 426 mutant alleles to chromosomal segments, each representing on average <3% of the barley genetic map. We show how the gene content in these segments can be predicted through conservation of synteny with model cereal genomes, providing a route to rapid gene identification.

De-regulation of abscisic acid contents causes abnormal endosperm development in the barley mutant seg8

Grain development of the maternal effect shrunken endosperm mutant seg8 was analysed by comprehensive molecular, biochemical and histological methods. The most obvious finding was de-regulation of ABA levels, which were lower compared to wild-type during the pre-storage phase but higher during the transition from cell division/differentiation to accumulation of storage products. Ploidy levels and ABA amounts were inversely correlated in the developing endosperms of both mutant and wild-type, suggesting an influence of ABA on cell-cycle regulation. The low ABA levels found in seg8 grains between anthesis and beginning endosperm cellularization may result from a gene dosage effect in the syncytial endosperm that causes impaired transfer of ABA synthesized in vegetative tissues into filial grain parts. Increased ABA levels during the transition phase are accompanied by higher chlorophyll and carotenoid/xanthophyll contents. The data suggest a disturbed ABA-releasing biosynthetic pathway. This is indicated by up-regulation of expression of the geranylgeranyl reductase (GGR) gene, which may be induced by ABA deficiency during the pre-storage phase. Abnormal cellularization/differentiation of the developing seg8 endosperm and reduced accumulation of starch are phenotypic characteristics that reflect these disturbances. The present study did not reveal the primary gene defect causing the seg8 phenotype, but presents new insights into the maternal/filial relationships regulating barley endosperm development.

Barley grain development toward an integrative view

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