Duplicate polyphenol oxidase genes on barley chromosome 2H and their functional differentiation in the phenol reaction of spikes and grains

Crop Evolution of Foxtail Millet

Studies on the domestication, genetic differentiation, and crop evolution of foxtail millet are reviewed in this paper. Several genetic studies were carried out to elucidate the genetic relationships among foxtail millet accessions originating mainly from Eurasia based on intraspecific hybrid pollen semi-sterility, isozymes, DNA markers, and single-nucleotide polymorphisms. Most studies suggest that China is the center of diversity of foxtail millet, and landraces were categorized into geographical groups. These results indicate that this millet was domesticated in China and spread over Eurasia, but independent origin in other regions cannot be ruled out. Furthermore, the evolution of genes was reviewed (i.e., the Waxy gene conferring amylose content in the endosperm, the Si7PPO gene controlling polyphenol oxidase, the HD1 and SiPRR37 genes controlling heading time, the Sh1 and SvLes1 genes involved in grain shattering, and the C gene controlling leaf sheath pigmentation), and the variation and distribution of these genes suggested complex patterns of evolution under human and/or natural selection.

Polyphenol oxidase genes in barley (Hordeum vulgare L.): functional activity with respect to black grain pigmentation

Polyphenol oxidase (PPO) is an oxidoreductase. In damaged plant tissues, it catalyzes enzymatic browning by oxidizing o-diphenols to highly reactive o-quinones, which polymerize producing heterogeneous dark polymer melanin. In intact tissues, functions of PPO are not well understood. The aim of the study was to investigate the barley PPO gene family and to reveal the possible involvement of Ppo genes in melanization of barley grain, which is controlled by the Blp1 gene. Based on known barley Ppo genes on chromosome 2H (Ppo1 and Ppo2), two additional genes-Ppo3 and Ppo4-were found on chromosomes 3H and 4H, respectively. These genes have one and two exons, respectively, contain a conserved tyrosinase domain and are thought to be functional. Comparative transcriptional analyzes of the genes in samples of developing grains (combined hulls and pericarp tissues) were conducted in two barley lines differing by melanin pigmentation. The genes were found to be transcribed with increasing intensity (while grains mature) independently from the grain color, except for Ppo2, which is transcribed only in black-grained line i:BwBlp1 accumulating melanin in grains. Analysis of this gene's expression in detached hulls and pericarps showed its elevated transcription in both tissues in comparison with yellow ones, while it was significantly higher in hulls than in pericarp. Segregation analysis in two F2 populations obtained based on barley genotypes carrying dominant Blp1 and recessive ppo1 (I) and dominant Blp1 and recessive ppo1 and ppo2 (II) was carried out. In population I, only two phenotypic classes corresponding to parental black and white ones were observed; the segregation ratio was 3 black to 1 white, corresponding to monogenic. In population II, aside from descendants with black and white grains, hybrids with a gray phenotype - light hulls and dark pericarp - were observed; the segregation ratio was 9 black to 3 gray to 4 white, corresponding to the epistatic interaction of two genes. Most hybrids with the gray phenotype carry dominant Blp1 and a homozygous recessive allele of Ppo2. Based on transcription and segregation assays one may conclude involvement of Ppo2 but not Ppo1 in melanin formation in barley hulls.

Multi-target genome editing reduces polyphenol oxidase activity in wheat (Triticum aestivum L.) grains

Introduction

Polyphenol oxidases (PPO) are dual activity metalloenzymes that catalyse the production of quinones. In plants, PPO activity may contribute to biotic stress resistance and secondary metabolism but is undesirable for food producers because it causes the discolouration and changes in flavour profiles of products during post-harvest processing. In wheat (Triticum aestivum L.), PPO released from the aleurone layer of the grain during milling results in the discolouration of flour, dough, and end-use products, reducing their value. Loss-of-function mutations in the PPO1 and PPO2 paralogous genes on homoeologous group 2 chromosomes confer reduced PPO activity in the wheat grain. However, limited natural variation and the proximity of these genes complicates the selection of extremely low-PPO wheat varieties by recombination. The goal of the current study was to edit all copies of PPO1 and PPO2 to drive extreme reductions in PPO grain activity in elite wheat varieties.

Results

A CRISPR/Cas9 construct with one single guide RNA (sgRNA) targeting a conserved copper binding domain was used to edit all seven PPO1 and PPO2 genes in the spring wheat cultivar 'Fielder'. Five of the seven edited T1 lines exhibited significant reductions in PPO activity, and T2 lines had PPO activity up to 86.7% lower than wild-type. The same construct was transformed into the elite winter wheat cultivars 'Guardian' and 'Steamboat', which have five PPO1 and PPO2 genes. In these varieties PPO activity was reduced by >90% in both T1 and T2 lines. In all three varieties, dough samples from edited lines exhibited reduced browning.

Discussion

This study demonstrates that multi-target editing at late stages of variety development could complement selection for beneficial alleles in crop breeding programs by inducing novel variation in loci inaccessible to recombination.

Genome-wide analysis of the polyphenol oxidase gene family reveals that MaPPO1 and MaPPO6 are the main contributors to fruit browning in Musa acuminate

INTRODUCTION

Polyphenol oxidases (PPOs), which are widely present in plants, play an important role in the growth, development, and stress responses. They can catalyze the oxidization of polyphenols and result in the browning of damaged or cut fruit, which seriously affects fruit quality and compromises the sale of fruit. In banana (Musa acuminata, AAA group), 10 PPO genes were determined based on the availability of a high-quality genome sequence, but the role of PPO genes in fruit browning remains unclear.

METHODS

In this study, we analyzed the physicochemical properties, gene structure, conserved structural domains, and evolutionary relationship of the PPO gene family of banana. The expression patterns were analyzed based on omics data and verified by qRT-PCR analysis. Transient expression assay in tobacco leaves was used to identify the subcellular localization of selected MaPPOs, and we analyzed the polyphenol oxidase activity using recombinant MaPPOs and transient expression assay.

RESULTS AND DISCUSSION

We found that more than two-thirds of the MaPPO genes had one intron, and all contained three conserved structural domains of PPO, except MaPPO4. Phylogenetic tree analysis revealed that MaPPO genes were categorized into five groups. MaPPOs did not cluster with Rosaceae and Solanaceae, indicating distant affinities, and MaPPO6/7/8/9/10 clustered into an individual group. Transcriptome, proteome, and expression analyses showed that MaPPO1 exhibits preferential expression in fruit tissue and is highly expressed at respiratory climacteric during fruit ripening. Other examined MaPPO genes were detectable in at least five different tissues. In mature green fruit tissue, MaPPO1 and MaPPO6 were the most abundant. Furthermore, MaPPO1 and MaPPO7 localized in chloroplasts, and MaPPO6 was a chloroplast- and Endoplasmic Reticulum (ER)-localized protein, whereas MaPPO10 only localized in the ER. In addition, the enzyme activity in vivo and in vitro of the selected MaPPO protein showed that MaPPO1 had the highest PPO activity, followed by MaPPO6. These results imply that MaPPO1 and MaPPO6 are the main contributors to banana fruit browning and lay the foundation for the development of banana varieties with low fruit browning.

The loss of polyphenol oxidase function is associated with hilum pigmentation and has been selected during pea domestication

Seed coats serve as protective tissue to the enclosed embryo. As well as mechanical there are also chemical defence functions. During domestication, the property of the seed coat was altered including the removal of the seed dormancy. We used a range of genetic, transcriptomic, proteomic and metabolomic approaches to determine the function of the pea seed polyphenol oxidase (PPO) gene. Sequencing analysis revealed one nucleotide insertion or deletion in the PPO gene, with the functional PPO allele found in all wild pea samples, while most cultivated peas have one of the three nonfunctional ppo alleles. PPO functionality cosegregates with hilum pigmentation. PPO gene and protein expression, as well as enzymatic activity, was downregulated in the seed coats of cultivated peas. The functionality of the PPO gene relates to the oxidation and polymerisation of gallocatechin in the seed coat. Additionally, imaging mass spectrometry supports the hypothesis that hilum pigmentation is conditioned by the presence of both phenolic precursors and sufficient PPO activity. Taken together these results indicate that the nonfunctional polyphenol oxidase gene has been selected during pea domestication, possibly due to better seed palatability or seed coat visual appearance.

Features of Activity of the Phenylpropanoid Biosynthesis Pathway in Melanin-Accumulating Barley Grains

Barley (Hordeum vulgare L.) grain pigmentation is caused by two types of phenolic compounds: anthocyanins (which are flavonoids) give a blue or purple color, and melanins (which are products of enzymatic oxidation and polymerization of phenolic compounds) give a black or brown color. Genes Ant1 and Ant2 determine the synthesis of purple anthocyanins in the grain pericarp, whereas melanins are formed under the control of the Blp1 gene in hulls and pericarp tissues. Unlike anthocyanin synthesis, melanin synthesis is poorly understood. The objective of the current work was to reveal features of the phenylpropanoid biosynthesis pathway functioning in melanin-accumulating barley grains. For this purpose, comparative transcriptomic and metabolomic analyses of three barley near-isogenic lines accumulating anthocyanins, melanins, or both in the grain, were performed. A comparative analysis of mRNA libraries constructed for three stages of spike development (booting, late milk, and early dough) showed transcriptional activation of genes encoding enzymes of the general phenylpropanoid pathway in all the lines regardless of pigmentation; however, as the spike matured, unique transcriptomic patterns associated with melanin and anthocyanin synthesis stood out. Secondary activation of transcription of the genes encoding enzymes of the general phenylpropanoid pathway together with genes of monolignol synthesis was revealed in the line accumulating only melanin. This pattern differs from the one observed in the anthocyanin-accumulating lines, where - together with the genes of general phenylpropanoid and monolignol synthesis pathways - flavonoid biosynthesis genes were found to be upregulated, with earlier activation of these genes in the line accumulating both types of pigments. These transcriptomic shifts may underlie the observed differences in concentrations of phenylpropanoid metabolites analyzed in the grain at a late developmental stage by high-performance liquid chromatography. Both melanin-accumulating lines showed an increased total level of benzoic acids. By contrast, anthocyanin-accumulating lines showed higher concentrations of flavonoids and p-coumaric and ferulic acids. A possible negative effect of melanogenesis on the total flavonoid content and a positive influence on the anthocyanin content were noted in the line accumulating both types of pigments. As a conclusion, redirection of metabolic fluxes in the phenylpropanoid biosynthesis pathway occurs when melanin is synthesized.

MC03g0810, an Important Candidate Gene Controlling Black Seed Coat Color in Bitter Gourd (Momordica spp.)

Seed coat color is one of the most intuitive phenotypes in bitter gourd (Momordica spp.). Although the inheritance of the seed coat color has been reported, the gene responsible for it is still unknown. This study used two sets of parents, representing, respectively, the intersubspecific and intraspecific materials of bitter gourd, and their respective F₁ and F₂ progenies for genetic analysis and primary mapping of the seed coat color. A large F_2:3 population comprising 2,975 seedlings from intraspecific hybridization was used to fine-map the seed coat color gene. The results inferred that a single gene, named McSC1, controlled the seed coat color and that the black color was dominant over the yellow color. The McSC1 locus was mapped to a region with a physical length of ∼7.8 Mb and 42.7 kb on pseudochromosome 3 via bulked segregant analysis with whole-genome resequencing (BSA-seq) and linkage analysis, respectively. Subsequently, the McSC1 locus was further fine-mapped to a 13.2-kb region containing only one candidate gene, MC03g0810, encoding a polyphenol oxidase (PPO). Additionally, the variations of MC03g0810 in the 89 bitter gourd germplasms showed a complete correlation with the seed coat color. Expression and PPO activity analyses showed a positive correlation between the expression level of MC03g0810 and its product PPO and the seed coat color. Therefore, MC03g0810 was proposed as the causal gene of McSC1. Our results provide an important reference for molecular marker-assisted breeding based on the seed coat color and uncover molecular mechanisms of the seed coat color formation in bitter gourd.

Chlorophyll deficiency delays but does not prevent melanogenesis in barley seed melanoplasts

Plant melanin is a dark polymerized polyphenolic substance that can by synthesized in seed tissues. Unlike well-defined enzymatic browning reaction leading to melanin synthesis in senescent and damaged plant tissues, melanin formation in intact tissues was not studied properly. Recently, melanin synthesis was demonstrated in chloroplast-derived melanoplasts in pericarp and husk cells of barley seeds. In barley, there are two independent genes, Blp1 and Alm1, affecting respectively the biosynthesis of melanin and chlorophyll in seeds. Even though different genetic systems are responsible for these traits, the localization of these biosynthetic pathways in the same organelle prompted us to conduct an in-depth study of the i:Bwalm1Blp1 line characterized by simultaneous chlorophyll deficiency caused by recessive allele alm1 and melanin accumulation controlled by dominant allele Blp1. This barley line and parental ones-Bowman, i:BwBlp1, and i:Bwalm1, which are characterized by different combinations of pigments chlorophyll and melanin in seeds-were subjected to a comparative cytological analysis. Three markers were analyzed: the presence of visible pigments, chlorophyll, and PsbA protein (a thylakoid membrane marker). Plastids of the barley pericarp and husk showed prominent differences among the lines, with internal structures that are more developed in husk cells. Although chlorophyll deficiency did not prevent melanogenesis in the spike of the hybrid line, a 7-day delay in melanization initiation and a decrease in its magnitude were observed in comparison with the melanin-and-chlorophyll-containing line. Thus, melanin biosynthesis is not related to photosynthetic processes directly but may be dependent on the presence of plastids with well-developed internal membranes.

The R2R3 Transcription Factor CsMYB59 Regulates Polyphenol Oxidase Gene CsPPO1 in Tea Plants (Camellia sinensis)

Polyphenol oxidase (PPO) plays a role in stress response, secondary metabolism, and other physiological processes during plant growth and development, and is also a critical enzyme in black tea production. However, the regulatory mechanisms of PPO genes and their activity in tea plants are still unclear. In this study, we measured PPO activity in two different tea cultivars, Taoyuandaye (TYDY) and Bixiangzao (BXZ), which are commonly used to produce black tea and green tea, respectively. The expression pattern of CsPPO1 was assessed and validated via transcriptomics and quantitative polymerase chain reaction in both tea varieties. In addition, we isolated and identified an R2R3-MYB transcription factor CsMYB59 that may regulate CsPPO1 expression. CsMYB59 was found to be a nuclear protein, and its expression in tea leaves was positively correlated with CsPPO1 expression and PPO activity. Transcriptional activity analysis showed that CsMYB59 was a transcriptional activator, and the dual-luciferase assay indicated that CsMYB59 could activate the expression of CsPPO1 in tobacco leaves. In summary, our study demonstrates that CsMYB59 represents a transcriptional activator in tea plants and may mediate the regulation of PPO activity by activating CsPPO1 expression. These findings provide novel insights into the regulatory mechanism of PPO gene in Camellia sinensis, which might help to breed tea cultivars with high PPO activity.

Recent advances in polyphenol oxidase-mediated plant stress responses

Plant polyphenol oxidases (PPOs) are ubiquitous copper metalloenzymes with a biochemistry that has been known for more than a century. By the 1990s, biologists began to recognize the importance of PPOs in plant response to the infestation of herbivores and pathogens; ideas concerning a defensive role for PPOs arose to address observed evidence, and several testable hypotheses were suggested. Two pivotal discoveries in tomato (Lycopersicon esculentum Miller) plants, an inverse correlation between PPO levels and insect growth and PPO induction by defence signals, have driven many studies of PPO defence functions in the context of abiotic and biotic stresses. During the past three decades, extensive molecular research in transgenic and non-transgenic systems has partly revealed the sophisticated mechanisms underlying PPO defence against herbivores and pathogens. These understandings, rather than theoretical predictions, have driven the development of new hypotheses and advanced PPO-related studies. Here, we review progress in PPO family features, expression regulation and the defensive role of PPOs in plants. We propose assumptions of an extended range of co- and post-transcriptional processes to the regulation of unexplored PPO expression. In addition, the identification of endogenous PPO substrates and downstream targets of PPO action will be useful for elucidating PPO defensive roles. The potential effects of PPO-mediated oxidative defences on herbivore performance ultimately needs to be further investigated. Therefore, expanding multidisciplinary approaches to unexplored dimensions of PPO defence function should be a future priority.

Phylogenetic analysis of the Si7PPO gene in foxtail millet, Setaria italica, provides further evidence for multiple origins of the negative phenol color reaction phenotype

To elucidate the diversity and evolution of the Si7PPO gene that controls phenol color reaction (Phr) in foxtail millet, Setaria italica, we analyzed sequence polymorphisms of the Si7PPO gene in 39 accessions consisting of foxtail millet landraces (32 accessions) and their wild ancestor ssp. viridis (seven accessions) collected from various regions in Europe and Asia. The accessions included wild type (positive Phr) and three different types of loss-of-function phenotype (negative Phr), "stop codon type", "TE1-insertion type" and "6-bp duplication type", found in our previous study. We constructed a phylogenetic tree of the gene and found that accessions with positive Phr showed higher genetic diversity at the nucleotide sequence level. We also found that the three different loss-of-function types formed different clusters, suggesting that landraces with negative Phr have multiple origins from three different lineages including both landrace and ssp. viridis accessions with positive Phr.

Genetic Mapping and Discovery of the Candidate Gene for Black Seed Coat Color in Watermelon (Citrullus lanatus)

Seed coat color is an important trait highly affecting the seed quality and flesh appearance of watermelon (Citrullus lanatus). However, the molecular regulation mechanism of seed coat color in watermelon is still unclear. In the present study, genetic analysis was performed by evaluating F₁, F₂ and BC₁ populations derived from two parental lines (9904 with light yellow seeds and Handel with black seeds), suggesting that a single dominant gene controls the black seed coat. The initial mapping result revealed a region of interest spanning 370 kb on chromosome 3. Genetic mapping with CAPS and SNP markers narrowed down the candidate region to 70.2 kb. Sequence alignment of the three putative genes in the candidate region suggested that there was a single-nucleotide insertion in the coding region of Cla019481 in 9904, resulting in a frameshift mutation and premature stop codon. The results indicated that Cla019481 named ClCS1 was the candidate gene for black seed coat color in watermelon. In addition, gene annotation revealed that Cla019481 encoded a polyphenol oxidase (PPO), which involved in the oxidation step of the melanin biosynthesis. This research finding will facilitate maker-assisted selection in watermelon and provide evidence for the study of black seed coat coloration in plants.

Reduced Enzymatic Browning in Potato Tubers by Specific Editing of a Polyphenol Oxidase Gene via Ribonucleoprotein Complexes Delivery of the CRISPR/Cas9 System

Polyphenol Oxidases (PPOs) catalyze the conversion of phenolic substrates to quinones, leading to the formation of dark-colored precipitates in fruits and vegetables. This process, known as enzymatic browning, is the cause of undesirable changes in organoleptic properties and the loss of nutritional quality in plant-derived products. In potato (Solanum tubersoum L.), PPOs are encoded by a multi-gene family with different expression patterns. Here, we have studied the application of the CRISPR/Cas9 system to induce mutations in the StPPO2 gene in the tetraploid cultivar Desiree. We hypothesized that the specific editing of this target gene would result in a lower PPO activity in the tuber with the consequent reduction of the enzymatic browning. Ribonucleoprotein complexes (RNPs), formed by two sgRNAs and Cas9 nuclease, were transfected to potato protoplasts. Up to 68% of regenerated plants contained mutations in at least one allele of the target gene, while 24% of edited lines carried mutations in all four alleles. No off-target mutations were identified in other analyzed StPPO genes. Mutations induced in the four alleles of StPPO2 gene, led to lines with a reduction of up to 69% in tuber PPO activity and a reduction of 73% in enzymatic browning, compared to the control. Our results demonstrate that the CRISPR/Cas9 system can be applied to develop potato varieties with reduced enzymatic browning in tubers, by the specific editing of a single member of the StPPO gene family.

Genomic Variation Among and Within Six Juglans Species

Genomic analysis in Juglans (walnuts) is expected to transform the breeding and agricultural production of both nuts and lumber. To that end, we report here the determination of reference sequences for six additional relatives of Juglans regia: Juglans sigillata (also from section Dioscaryon), Juglans nigra, Juglans microcarpa, Juglans hindsii (from section Rhysocaryon), Juglans cathayensis (from section Cardiocaryon), and the closely related Pterocarya stenoptera. While these are ‘draft’ genomes, ranging in size between 640Mbp and 990Mbp, their contiguities and accuracies can support powerful annotations of genomic variation that are often the foundation of new avenues of research and breeding. We annotated nucleotide divergence and synteny by creating complete pairwise alignments of each reference genome to the remaining six. In addition, we have re-sequenced a sample of accessions from four Juglans species (including regia). The variation discovered in these surveys comprises a critical resource for experimentation and breeding, as well as a solid complementary annotation. To demonstrate the potential of these resources the structural and sequence variation in and around the polyphenol oxidase loci, PPO1 and PPO2 were investigated. As reported for other seed crops variation in this gene is implicated in the domestication of walnuts. The apparently Juglandaceae specific PPO1 duplicate shows accelerated divergence and an excess of amino acid replacement on the lineage leading to accessions of the domesticated nut crop species, Juglans regia and sigillata.

Molecular cloning and expression analysis of duplicated polyphenol oxidase genes reveal their functional differentiations in sorghum

Polyphenol oxidase (PPO) is believed to play a role in plant growth, reproduction, and resistance to pathogens and pests. PPO causes browning of grains in cereals. In this study, genetic mapping of sorghum grain for phenol color reaction (PHR) was performed using a recombinant inbred line population. Only one locus was detected between SSR markers SM06072 and Xtxp176 on chromosome 6. Two linked orthologous genes (Sb06PPO1 and Sb06PPO2) within the mapped region were discovered and cloned. Transformation experiments using Nipponbare (a PHR negative rice cultivar) showed that Sb06PPO1 from LTR108 and two Sb06PPO2 alleles from both varieties could complement Nipponbare, whereas Sb06PPO1 from 654 could not. Subsequent quantitative real-time PCR (qPCR) experiments showed that Sb06PPO1 and Sb06PPO2 functioned diversely, Sb06PPO1 was mainly expressed in young panicles before flowering. Sb06PPO2 was strongly expressed in flowering panicles, especially in hulls and branches at filling stage. Moreover, the expression of Sb06PPO1 was found to be significantly up-regulated by exogenous ABA and salt, whereas Sb06PPO2 was not changed significantly, further demonstrating functional differentiation between the two genes.

Polyphenol Oxidases in Crops: Biochemical, Physiological and Genetic Aspects

Enzymatic browning is a colour reaction occurring in plants, including cereals, fruit and horticultural crops, due to oxidation during postharvest processing and storage. This has a negative impact on the colour, flavour, nutritional properties and shelf life of food products. Browning is usually caused by polyphenol oxidases (PPOs), following cell damage caused by senescence, wounding and the attack of pests and pathogens. Several studies indicated that PPOs play a role in plant immunity, and emerging evidence suggested that PPOs might also be involved in other physiological processes. Genomic investigations ultimately led to the isolation of PPO homologs in several crops, which will be possibly characterized at the functional level in the near future. Here, focusing on the botanic families of Poaceae and Solanaceae, we provide an overview on available scientific literature on PPOs, resulting in useful information on biochemical, physiological and genetic aspects.

A PPO Promoter from Betalain-Producing Red Swiss Chard, Directs Petiole- and Root-Preferential Expression of Foreign Gene in Anthocyanins-Producing Plants

A 1670 bp 5'-flanking region of the polyphenol oxidase (PPO) gene was isolated from red Swiss chard, a betalain-producing plant. This region, named promoter BvcPPOP, and its 5'-truncated versions were fused with the GUS gene and introduced into Arabidopsis, an anthocyanins-producing plant. GUS histochemical staining and quantitative analysis of transgenic plants at the vegetative and reproductive stages showed that BvcPPOP could direct GUS gene expression in vegetative organs with root- and petiole-preference, but not in reproductive organs including inflorescences shoot, inflorescences leaf, flower, pod and seed. This promoter was regulated by developmental stages in its driving strength, but not in expression pattern. It was also regulated by the abiotic stressors tested, positively by salicylic acid (SA) and methyl jasmonate (MeJA) but negatively by abscisic acid (ABA), gibberellin (GA), NaCl and OH(-). Its four 5'-truncated versions varied in the driving strength, but not obviously in expression pattern, and even the shortest version (-225 to +22) retained the root- and petiole- preference. This promoter is, to our knowledge, the first PPO promoter cloned and functionally elucidated from the betalain-producing plant, and thus provides not only a useful tool for expressing gene(s) of agricultural interest in vegetative organs, but also a clue to clarify the function of metabolism-specific PPO in betalain biosynthesis.

Genetic Diversity of Individual Phenolic Acids in Barley and Their Correlation with Barley Malt Quality

Phenolic acids have been quite extensively studied in food science research because of their antioxidative effect. In this study, the genotypic difference and genetic control of phenolic acids, and their correlation with malt quality, were investigated in barley. Ferulic acid (FA) and p-coumaric acid (p-CA) were identified as two main phenolic acids, showing wide variations among 68 barley genotypes. The mean content of FA and p-CA were 2.15 μg g(-1) and 1.10 μg g(-1) in grains and 4.07 μg g(-1) and 1.44 μg g(-1) in malt, respectively. After malting, FA and p-CA were increased significantly in 55 and 37 genotypes and were reduced in 2 and 14 genotypes, respectively. Both malt FA and p-CA were positively correlated with soluble N content and Kolbach index and negatively correlated with malt extract and viscosity. The results indicated that the effect of malting on the change of an individual phenolic acid is genotype independent. Association mapping identified that 8 markers on Chromosomes 1H, 2H, 4H, and 7H are associated with grain p-CA and 4 markers on Chromosomes 3H and 7H are linked with grain FA. However, only a single marker on Chromosome 3H was found to be associated with malt FA. Moreover, a lack of overlapping markers between grain and malt indicated the genetic diversity of phenolic acids in barley grain and malt. Our results strengthen the understanding of phenolic acids in barley and their responses to the malting process.

Multiple origins of the phenol reaction negative phenotype in foxtail millet, Setaria italica (L.) P. Beauv., were caused by independent loss-of-function mutations of the polyphenol oxidase (Si7PPO) gene during domestication

Foxtail millet shows variation in positive phenol color reaction (Phr) and negative Phr in grains, but predominant accessions of this crop are negative reaction type, and the molecular genetic basis of the Phr reaction remains unresolved. In this article, we isolated polyphenol oxidase (PPO) gene responsible for Phr using genome sequence information and investigated molecular genetic basis of negative Phr and crop evolution of foxtail millet. First of all, we searched for PPO gene homologs in a foxtail millet genome database using a rice PPO gene as a query and successfully found three copies of the PPO gene. One of the PPO gene homologs on chromosome 7 showed the highest similarity with PPO genes expressed in hulls (grains) of other cereal species including rice, wheat, and barley and was designated as Si7PPO. Phr phenotypes and Si7PPO genotypes completely co-segregated in a segregating population. We also analyzed the genetic variation conferring negative Phr reaction. Of 480 accessions of the landraces investigated, 87 (18.1 %) showed positive Phr and 393 (81.9 %) showed negative Phr. In the 393 Phr negative accessions, three types of loss-of-function Si7PPO gene were predominant and independently found in various locations. One of them has an SNP in exon 1 resulting in a premature stop codon and was designated as stop codon type, another has an insertion of a transposon (Si7PPO-TE1) in intron 2 and was designated as TE1-insertion type, and the other has a 6-bp duplication in exon 3 resulting in the duplication of 2 amino acids and was designated as 6-bp duplication type. As a rare variant of the stop codon type, one accession additionally has an insertion of a transposon, Si7PPO-TE2, in intron 2 and was designated as "stop codon +TE2 insertion type". The geographical distribution of accessions with positive Phr and those with three major types of negative Phr was also investigated. Accessions with positive Phr were found in subtropical and tropical regions at frequencies of ca. 25-67 % and those with negative Phr were broadly found in Europe and Asia. The stop codon type was found in 285 accessions and was broadly distributed in Europe and Asia, whereas the TE-1 insertion type was found in 99 accessions from Europe and Asia but was not found in India. The 6-bp duplication type was found in only 8 accessions from Nansei Islands (Okinawa Prefecture) of Japan. We also analyzed Phr in the wild ancestor and concluded that the negative Phr type was likely to have originated after domestication of foxtail millet. It was also implied that negative Phr of foxtail millet arose by multiple independent loss of function of PPO gene through dispersal because of some advantages under some environmental conditions and human selection as in rice and barley.

Marker-trait associations in Virginia Tech winter barley identified using genome-wide mapping

Genome-wide association studies (GWAS) provide an opportunity to examine the genetic architecture of quantitatively inherited traits in breeding populations. The objectives of this study were to use GWAS to identify chromosome regions governing traits of importance in six-rowed winter barley (Hordeum vulgare L.) germplasm and to identify single-nucleotide polymorphisms (SNPs) markers that can be implemented in a marker-assisted breeding program. Advanced hulled and hulless lines (329 total) were screened using 3,072 SNPs as a part of the US. Barley Coordinated Agricultural Project (CAP). Phenotypic data collected over 4 years for agronomic and food quality traits and resistance to leaf rust (caused by Puccinia hordei G. Otth), powdery mildew [caused by Blumeria graminis (DC.) E.O. Speer f. sp. hordei Em. Marchal], net blotch (caused by Pyrenophora teres), and spot blotch [caused by Cochliobolus sativus (Ito and Kuribayashi) Drechsler ex Dastur] were analyzed with SNP genotypic data in a GWAS to determine marker-trait associations. Significant SNPs associated with previously described quantitative trait loci (QTL) or genes were identified for heading date on chromosome 3H, test weight on 2H, yield on 7H, grain protein on 5H, polyphenol oxidase activity on 2H and resistance to leaf rust on 2H and 3H, powdery mildew on 1H, 2H and 4H, net blotch on 5H, and spot blotch on 7H. Novel QTL also were identified for agronomic, quality, and disease resistance traits. These SNP-trait associations provide the opportunity to directly select for QTL contributing to multiple traits in breeding programs.

Hordeum chilense genome, a useful tool to investigate the endosperm yellow pigment content in the Triticeae

BACKGROUND

The wild barley Hordeum chilense fulfills some requirements for being a useful tool to investigate the endosperm yellow pigment content (YPC) in the Triticeae including its diploid constitution, the availability of genetic resources (addition and deletion stocks and a high density genetic map) and, especially, its high seed YPC not silenced in tritordeums (amphiploids derived from H. chilense and wheat). Thus, the aim of this work was to test the utility of the H. chilense genome for investigating the YPC in the Triticeae.

RESULTS

Twelve genes related to endosperm carotenoid content and/or YPC in grasses (Dxr, Hdr [synonym ispH], Ggpps1, Psy2, Psy3, Pds, Zds, e-Lcy, b-Lcy, Hyd3, Ccd1 and Ppo1) were identified, and mapped in H. chilense using rice genes to identify orthologs from barley, wheat, sorghum and maize. Macrocolinearity studies revealed that gene positions were in agreement in H. vulgare and H. chilense. Additionally, three main regions associated with YPC were identified in chromosomes 2Hch, 3Hch and 7Hch in H. chilense, the former being the most significant one.

CONCLUSIONS

The results obtained are consistent with previous findings in wheat and suggest that Ggpps1, Zds and Hyd3 on chromosome 2Hch may be considered candidate genes in wheat for further studies in YPC improvement. Considering the syntenic location of carotenoid genes in H. chilense, we have concluded that the Hch genome may constitute a valuable tool for YPC studies in the Triticeae.

The polyphenol oxidase gene family in land plants: Lineage-specific duplication and expansion

BACKGROUND

Plant polyphenol oxidases (PPOs) are enzymes that typically use molecular oxygen to oxidize ortho-diphenols to ortho-quinones. These commonly cause browning reactions following tissue damage, and may be important in plant defense. Some PPOs function as hydroxylases or in cross-linking reactions, but in most plants their physiological roles are not known. To better understand the importance of PPOs in the plant kingdom, we surveyed PPO gene families in 25 sequenced genomes from chlorophytes, bryophytes, lycophytes, and flowering plants. The PPO genes were then analyzed in silico for gene structure, phylogenetic relationships, and targeting signals.

RESULTS

Many previously uncharacterized PPO genes were uncovered. The moss, Physcomitrella patens, contained 13 PPO genes and Selaginella moellendorffii (spike moss) and Glycine max (soybean) each had 11 genes. Populus trichocarpa (poplar) contained a highly diversified gene family with 11 PPO genes, but several flowering plants had only a single PPO gene. By contrast, no PPO-like sequences were identified in several chlorophyte (green algae) genomes or Arabidopsis (A. lyrata and A. thaliana). We found that many PPOs contained one or two introns often near the 3' terminus. Furthermore, N-terminal amino acid sequence analysis using ChloroP and TargetP 1.1 predicted that several putative PPOs are synthesized via the secretory pathway, a unique finding as most PPOs are predicted to be chloroplast proteins. Phylogenetic reconstruction of these sequences revealed that large PPO gene repertoires in some species are mostly a consequence of independent bursts of gene duplication, while the lineage leading to Arabidopsis must have lost all PPO genes.

CONCLUSION

Our survey identified PPOs in gene families of varying sizes in all land plants except in the genus Arabidopsis. While we found variation in intron numbers and positions, overall PPO gene structure is congruent with the phylogenetic relationships based on primary sequence data. The dynamic nature of this gene family differentiates PPO from other oxidative enzymes, and is consistent with a protein important for a diversity of functions relating to environmental adaptation.