TAMFT-3A and TAMFT-3B2 homeologs are associated with wheat preharvest sprouting

The phenomenon of preharvest sprouting (PHS), caused by rain after physiological maturity and prior to harvest, negatively affects wheat (Triticum aestivum L.) production and end use. Investigating the genetics that control PHS resistance may result in increased control of seed dormancy. Multiple genes involved in the development of seed dormancy are associated with PHS. In this study, the TaMFT (3A, 3B1, 3B2, 3D), TaMKK3-4A, and TaVP1-3B genes were assessed for association with PHS in a double-haploid line (DHL) hard red winter wheat population derived from a BC₁ cross between the cultivars Loma and Warhorse, where Loma was the recurrent and PHS susceptible parent. The 162 BC₁ DHL lines were grown over two field seasons and PHS susceptibility was assessed by measuring PHS resistance in physiologically mature heads. The PHS variation was associated with the TaMFT-A and the B2 homeolog with Loma carrying mutant forms of each gene. No sequence variation between Loma and Warhorse was detected in the exons of the TaMFT-B1 and D homeologs. No association between PHS resistance and TaMKK3-4A or TaVp1-3B variation was observed, though Loma and Warhorse vary for TaMKK3-4A and TaVp1-3B mutations reported to be PHS associated. Previous research has shown TaMFT-3A as having a large impact on PHS resistance. In the current study, the TaMFT-3A and TaMFT-3B2 alleles each explained 14% of observed PHS variation. Markers for both TaMFT-3A and TaMFT-3B2 should be used in selecting for increased wheat dormancy and PHS resistance.

Genes Impacting Grain Weight and Number in Wheat (Triticum aestivum L. ssp. aestivum)

The primary goal of common wheat (T. aestivum) breeding is increasing yield without negatively impacting the agronomic traits or product quality. Genetic approaches to improve the yield increasingly target genes that impact the grain weight and number. An energetic trade-off exists between the grain weight and grain number, the result of which is that most genes that increase the grain weight also decrease the grain number. QTL associated with grain weight and number have been identified throughout the hexaploid wheat genome, leading to the discovery of numerous genes that impact these traits. Genes that have been shown to impact these traits will be discussed in this review, including TaGNI, TaGW2, TaCKX6, TaGS5, TaDA1, WAPO1, and TaRht1. As more genes impacting the grain weight and number are characterized, the opportunity is increasingly available to improve common wheat agronomic yield by stacking the beneficial alleles. This review provides a synopsis of the genes that impact grain weight and number, and the most beneficial alleles of those genes with respect to increasing the yield in dryland and irrigated conditions. It also provides insight into some of the genetic mechanisms underpinning the trade-off between grain weight and number and their relationship to the source-to-sink pathway. These mechanisms include the plant size, the water soluble carbohydrate levels in plant tissue, the size and number of pericarp cells, the cytokinin and expansin levels in developing reproductive tissue, floral architecture and floral fertility.

Correction to: Genetic characterization and expression analysis of wheat (Triticum aestivum) line 07OR1074 exhibiting very low polyphenol oxidase (PPO) activity

The above-mentioned article was published in 2015 with an error in the reverse primer sequence for the PPOA2d1074 marker, which made amplification difficult. The reverse primer was missing a thymine nucleotide at the thirteenth position (GCGGTGCTTCACTTGGT).

Review: Revealing the genetic mechanisms of pre-harvest sprouting in hexaploid wheat (Triticum aestivum L.)

Pre-harvest sprouting (PHS) of wheat (Triticum aestivum L.) is an important phenomenon that results in weather dependent reductions in grain yield and quality across the globe. Due to the large annual losses, breeding PHS resistant varieties is of great importance. Many quantitative trait loci have been associated with PHS and a number of specific genes have been proven to impact PHS. TaPHS1, TaMKK3, Tamyb10, and TaVp1 have been shown to have a large impact on PHS susceptibility while many other genes such as TaSdr, TaQSd, and TaDOG1 have been shown to account for smaller, but significant, proportions of variation. These advances in understanding the genetics behind PHS are making molecular selection and loci stacking viable methods for affecting this quantitative trait. The current review article serves to provide a brief synthesis of recent advances regarding PHS, as well as provide unique insight into the genetic mechanisms governing PHS in bread wheat.

The Impact of the Wheat Rht-B1b Semi-Dwarfing Allele on Photosynthesis and Seed Development Under Field Conditions

The Reduced Height (Rht) genes formed the basis for the green revolution in wheat by decreasing plant height and increasing productive tillers. There are two current widely used Rht mutant alleles, Rht-B1b and Rht-D1b. Both reduce plant height by 20% and increase seed yield by 5-10%. They are also associated with decreased seed size and protein content. Here, we tested the degree to which Rht-B1b impacts flag leaf photosynthetic rates and carbon and nitrogen partitioning to the flag leaf and grain during grain fill under field conditions using near isogenic lines (NILs) that were either standard height (Rht-B1a) or semi-dwarf (Rht-B1b). The results demonstrate that at anthesis, Rht-B1b reduces flag leaf photosynthetic rate per unit area by 18% and chlorophyll A content by 23%. Rht-B1b significantly reduced grain protein beginning at 14 days post anthesis (DPA) with the greatest difference seen at 21 DPA (12%). Rht-B1b also significantly decreased individual seed weight beginning at 21 DPA and by 15.2% at 28 DPA. Global expression analysis using RNA extracted from developing leaves and stems demonstrated that genes associated with carbon and nitrogen metabolism are not substantially altered by Rht-B1b. From this study, we conclude that Rht-B1b reduces flag leaf photosynthetic rate at flowering while changes in grain composition begin shortly after anthesis.

Identification of Candidate Genes Responsible for Stem Pith Production Using Expression Analysis in Solid-Stemmed Wheat

The wheat stem sawfly (WSS) is an economically important pest of wheat in the Northern Great Plains. The primary means of WSS control is resistance associated with the single quantitative trait locus (QTL) , which controls most stem solidness variation. The goal of this study was to identify stem solidness candidate genes via RNA-seq. This study made use of 28 single nucleotide polymorphism (SNP) makers derived from expressed sequence tags (ESTs) linked to contained within a 5.13 cM region. Allele specific expression of EST markers was examined in stem tissue for solid and hollow-stemmed pairs of two spring wheat near isogenic lines (NILs) differing for the QTL. Of the 28 ESTs, 13 were located within annotated genes and 10 had detectable stem expression. Annotated genes corresponding to four of the ESTs were differentially expressed between solid and hollow-stemmed NILs and represent possible stem solidness gene candidates. Further examination of the 5.13 cM region containing the 28 EST markers identified 260 annotated genes. Twenty of the 260 linked genes were up-regulated in hollow NIL stems, while only seven genes were up-regulated in solid NIL stems. An -methyltransferase within the region of interest was identified as a candidate based on differential expression between solid and hollow-stemmed NILs and putative function. Further study of these candidate genes may lead to the identification of the gene(s) controlling stem solidness and an increased ability to select for wheat stem solidness and manage WSS.

A wheat COP9 subunit 5-like gene is negatively involved in host response to leaf rust

The COP9 (constitutive photomorphogenesis 9) signalosome (CSN) is a protein complex involved in the ubiquitin proteasome system and a common host target of diverse pathogens in Arabidopsis. The known derubylation function of the COP9 complex is carried out by subunit 5 encoded by AtCSN5A or AtCSN5B in Arabidopsis. A single CSN5-like gene (designated as TaCSN5) with three homeologues was identified on the long arms of wheat (Triticum aestivum L.) group 2 chromosomes. In this study, we identified and characterized the function of TaCSN5 in response to infection by the leaf rust pathogen. Down-regulation of all three TaCSN5 homeologues or mutations in the homeologues on chromosomes 2A or 2D resulted in significantly enhanced resistance to leaf rust. Enhanced leaf rust resistance corresponded to a seven-fold increase in PR1 (pathogenesis-related gene 1) expression. Collectively, the data indicate that the wheat COP9 subunit 5-like gene acts as a negative regulator of wheat leaf rust resistance.

Overexpression of ADP-glucose pyrophosphorylase in both leaf and seed tissue synergistically increase biomass and seed number in rice (Oryza sativa ssp. japonica)

Increased expression of leaf or seed ADPglucose pyrophosphorylase activity (AGPase) has been shown to increase plant growth. However, no study has directly compared AGPase overexpression in leaves and/or seeds. In the present study, transgenic rice overexpressing AGPase in leaves or in seeds were crossed, resulting in four F2:3 homozygous genotypes with AGPase overexpression in leaves, seeds, both leaves and seeds, or neither tissue. The impact of AGPase overexpression in these genotypic groups was examined at the metabolic, transcriptomic, and plant growth levels. Leaf-specific AGPase overexpression increased flag leaf starch up to five times that of the wild type (WT) whereas overexpression of AGPase in both leaves and seeds conferred the greatest productivity advantages. Relative to the WT, AGPase overexpression in both leaves and seeds increased plant biomass and panicle number by 61% and 51%, respectively while leaf-specific AGPase overexpression alone only increased plant biomass and panicle number by 24 and 32% respectively. Extraction and analysis of RNA and leaf-specific metabolites demonstrated that carbon metabolism was broadly increased by AGPase overexpression in seeds and leaves. These findings indicate that stimulation of whole-plant growth and productivity can be best achieved by upregulation of starch biosynthesis in both leaves and seeds.

Genetic characterization and expression analysis of wheat (Triticum aestivum) line 07OR1074 exhibiting very low polyphenol oxidase (PPO) activity

Characterized novel mutations present at Ppo loci account for the substantial reduction of the total kernel PPO activity present in a putative null Ppo - A1 genetic background. Wheat (Triticum aestivum) polyphenol oxidase (PPO) contributes to the time-dependent discoloration of Asian noodles. Wheat contains multiple paralogous and orthologous Ppo genes, Ppo-A1, Ppo-D1, Ppo-A2, Ppo-D2, and Ppo-B2, expressed in wheat kernels. To date, wheat noodle color improvement efforts have focused on breeding cultivars containing Ppo-D1 and Ppo-A1 alleles conferring reduced PPO activity. A major impediment to wheat quality improvement is a lack of additional Ppo alleles conferring reduced kernel PPO. In this study, a previously reported very low PPO line, 07OR1074, was found to contain a novel allele at Ppo-A2 and null alleles at the Ppo-A1 and Ppo-D1 loci. To examine the impact of each mutation upon kernel PPO, populations were generated from crosses between 07OR1074 and the hard white spring wheat cultivars Choteau and Vida. Expression analysis using RNA-seq demonstrated no detectable Ppo-A1 transcripts in 07OR1074 while Ppo-D1 transcripts were present at less than 10% of that seen in Choteau and Vida. Novel markers specific for the Ppo-D1 and Ppo-A2 mutations discovered in 07OR1074, along with the Ppo-A1 STS marker, were used to screen segregating populations. Evaluation of lines indicated a substantial genotypic effect on PPO with Ppo-A1 and Ppo-D1 alleles contributing significantly to total PPO in both populations. These results show that the novel mutations in Ppo-A1 and Ppo-D1 present in 07OR1074 are both important to lowering overall wheat seed PPO activity and may be useful to produce more desirable and marketable wheat-based products.

A mutagenesis-derived broad-spectrum disease resistance locus in wheat

Wheat leaf rust, stem rust, stripe rust, and powdery mildew caused by the fungal pathogens Puccinia triticina, P. graminis f. sp. tritici, P. striiformis f. sp. tritici, and Blumeria graminis f. sp. tritici, respectively, are destructive diseases of wheat worldwide. Breeding durable disease resistance cultivars rely largely on continually introgressing new resistance genes, especially the genes with different defense mechanisms, into adapted varieties. Here, we describe a new resistance gene obtained by mutagenesis. The mutant, MNR220 (mutagenesis-derived new resistance), enhances resistance to three rusts and powdery mildew, with the characteristics of delayed disease development at the seedling stage and completed resistance at the adult plant stage. Genetic analysis demonstrated that the resistance in MNR220 is conferred by a single semidominant gene mapped on the short arm of chromosome 2B. Gene expression profiling of several pathogenesis-related genes indicated that MNR220 has an elevated and rapid pathogen-induced response. In addition to its potential use in breeding for resistance to multiple diseases, high-resolution mapping and cloning of the disease resistance locus in MNR220 may lead to a better understanding of the regulation of defense responses in wheat.

The ectopic expression of the wheat Puroindoline genes increase germ size and seed oil content in transgenic corn

Plant oil content and composition improvement is a major goal of plant breeding and biotechnology. The Puroindoline a and b (PINA and PINB) proteins together control whether wheat seeds are soft or hard textured and share a similar structure to that of plant non-specific lipid-transfer proteins. Here we transformed corn (Zea mays L.) with the wheat (Triticum aestivum L.) puroindoline genes (Pina and Pinb) to assess their effects upon seed oil content and quality. Pina and Pinb coding sequences were introduced into corn under the control of a corn Ubiquitin promoter. Three Pina/Pinb expression positive transgenic events were evaluated over two growing seasons. The results showed that Pin expression increased germ size significantly without negatively impacting seed size. Germ yield increased 33.8% while total seed oil content was increased by 25.23%. Seed oil content increases were primarily the result of increased germ size. This work indicates that higher oil content corn hybrids having increased food or feed value could be produced via puroindoline expression.

Seed-specific expression of the wheat puroindoline genes improves maize wet milling yields

The texture of maize (Zea mays L.) seeds is important to seed processing properties, and soft dent maize is preferred for both wet-milling and livestock feed applications. The puroindoline genes (Pina and Pinb) are the functional components of the wheat (Triticum aestivum L.) Hardness locus and together function to create soft grain texture in wheat. The PINs (PINA and PINB) are believed to act by binding to lipids on the surface of starch granules, preventing tight adhesion between starch granules and the surrounding protein matrix during seed maturation. Here, maize kernel structure and wet milling properties were successfully modified by the endosperm-specific expression of wheat Pins (Pina and Pinb). Pins were introduced into maize under the control of a maize gamma-Zein promoter. Three Pina/Pinb expression positive transgenic lines were evaluated over two growing seasons. Textural analysis of the maize seeds indicated that the expression of PINs decreased adhesion between starch and protein matrix and reduced maize grain hardness significantly. Reduction in pressure required to fracture kernels ranged from 15.65% to 36.86% compared with control seeds. Further, the PINs transgenic maize seeds had increased levels of extractable starch as characterized by a small scale wet milling method. Starch yield was increased by 4.86% on average without negatively impacting starch purity. The development of softer maize hybrids with higher starch extractability would be of value to maize processors.

Creation and functional analysis of new Puroindoline alleles in Triticum aestivum

The Hardness (Ha) locus controls grain texture and affects many end-use properties of wheat (Triticum aestivum L.). The Ha locus is functionally comprised of the Puroindoline a and b genes, Pina and Pinb, respectively. The lack of Pin allelic diversity is a major factor limiting Ha functional analyses and wheat quality improvement. In order to create new Ha alleles, a 630 member M(2) population was produced in the soft white spring cultivar Alpowa using ethylmethane sulfonate mutagenesis. The M(2) population was screened to identify new alleles of Pina and Pinb. Eighteen new Pin alleles, including eight missense alleles, were identified. F(2) populations for four of the new Pin alleles were developed after crossing each back to non-mutant Alpowa. Grain hardness was then measured on F(2:3) seeds and the impact of each allele on grain hardness was quantified. The tested mutations were responsible for between 28 and 94% of the grain hardness variation and seed weight and vigor of all mutation lines was restored among the F(2) populations. Selection of new Pin alleles following direct phenotyping or direct sequencing is a successful approach to identify new Ha alleles useful in improving wheat product quality and understanding Ha locus function.

Expression of a modified ADP-glucose pyrophosphorylase large subunit in wheat seeds stimulates photosynthesis and carbon metabolism

ADP-glucose pyrophosphorylase (AGP) is the rate-limiting step in seed starch biosynthesis. Expression of an altered maize AGP large subunit (Sh2r6hs) in wheat (Triticum aestivum L.) results in increased AGP activity in developing seed endosperm and seed yield. The yield phenotype involves increases in both seed number and total plant biomass. Here we describe stimulation of photosynthesis by the seed-specific Sh2r6hs transgene. Photosynthetic rates were increased in Sh2r6hs-expressing plants under high light but not low light growth conditions, peaking at roughly 7 days after flowering (DAF). In addition, there were significant increases in levels of fructose, glucose, and sucrose in flag leaves at both 7 and 14 DAF. In seeds, levels of carbon metabolites at 7 and 14 DAF were relatively unchanged but increases in glucose, ADP-glucose, and UDP-glucose were observed in seeds from Sh2r6hs positive plants at maturity. Increased photosynthetic rates relatively early in seed development appear to be key to the Sh2r6hs enhanced yield phenotype as no yield increase or photosynthetic rate changes were found when plants were grown in a suboptimal light environment. These findings demonstrate that stimulation of biochemical events in both source and sink tissues is associated with Sh2r6hs expression.

Expression of a modified ADP-glucose pyrophosphorylase large subunit in wheat seeds stimulates photosynthesis and carbon metabolism

ADP-glucose pyrophosphorylase (AGP) is the rate-limiting step in seed starch biosynthesis. Expression of an altered maize AGP large subunit (Sh2r6hs) in wheat (Triticum aestivum L.) results in increased AGP activity in developing seed endosperm and seed yield. The yield phenotype involves increases in both seed number and total plant biomass. Here we describe stimulation of photosynthesis by the seed-specific Sh2r6hs transgene. Photosynthetic rates were increased in Sh2r6hs-expressing plants under high light but not low light growth conditions, peaking at roughly 7 days after flowering (DAF). In addition, there were significant increases in levels of fructose, glucose, and sucrose in flag leaves at both 7 and 14 DAF. In seeds, levels of carbon metabolites at 7 and 14 DAF were relatively unchanged but increases in glucose, ADP-glucose, and UDP-glucose were observed in seeds from Sh2r6hs positive plants at maturity. Increased photosynthetic rates relatively early in seed development appear to be key to the Sh2r6hs enhanced yield phenotype as no yield increase or photosynthetic rate changes were found when plants were grown in a suboptimal light environment. These findings demonstrate that stimulation of biochemical events in both source and sink tissues is associated with Sh2r6hs expression.

Complementation of the pina (null) allele with the wild type Pina sequence restores a soft phenotype in transgenic wheat

The tightly linked puroindoline genes, Pina and Pinb, control grain texture in wheat, with wild type forms of both giving soft, and a sequence alteration affecting protein expression or function in either giving rise to hard wheat. Previous experiments have shown that addition of wild type Pina in the presence of mutated Pinb gave intermediate grain texture but addition of wild type Pinb gave soft grain. This raises questions as to whether Pina may be less functional than Pinb. Our goal here was to develop and characterize wheat lines expressing the wild type Pina-D1a sequence in hard wheat with the null mutation (Pina-D1b) for Pina. Three transgenic lines plus Bobwhite were evaluated in two environments. Grain texture, grain protein, and kernel weight were determined for the transgenic lines and Bobwhite. The three transgenic lines had soft phenotype, and none of the transgenic lines differed from Bobwhite for grain protein or kernel weight. The soft phenotype was accompanied by increases in Pina transcript accumulation. Total Triton X-114 extractable PINA and PINB increased from 2.5 to 5.5 times those from a soft wheat reference sample, and friabilin, PINA and PINB bound to starch, increased from 3.8 to 7.8 times those of the soft wheat reference. Bobwhite showed no starch bound PINA, but transgenic lines had levels from 5.3 to 13.7 times those of the soft wheat reference sample. Starch bound PINB in transgenic lines also increased from 0.9 to 2.5 times that for the soft wheat reference sample. The transgenic expression of wild type Pina sequence in the Pina null genotype gave soft grain with the characteristics of soft wheat including increased starch bound friabilin. The results support the hypothesis that both wild type Pin genes need to be present for friabilin formation and soft grain.

Increased puroindoline levels slow ruminal digestion of wheat (Triticum aestivum L.) starch by cattle

Starch is the primary nutrient in ruminant diets used to promote high levels of performance. The site of starch digestion alters the nature of digestive end products (VFA in the rumen vs. glucose in the small intestine) and the efficiency of use. Cereal grain endosperm texture plays a major role in the rate and extent of starch degradation in ruminants. Wheat grain texture is regulated by the starch surface protein complex friabilin that consists primarily of puroindoline (PIN) A and B. Soft kernel texture in wheat is a result of both PIN genes being in the wild type active form and bound to starch. The objective of this study was to investigate the effect of varying PIN content in wheat on the rate of starch digestion in the rumen of beef cattle. In Exp. 1, 6 transgenic soft pin a/b isolines created in a hard wheat background, and 2 hard wheat controls were milled to yield a wide range of mean particle sizes across all lines. Milled samples were incubated in situ for 3 h. Increased expression of both PINA and PINB decreased DM digestibility (DMD) by 29.2% (P < 0.05) and decreased starch digestibility by 30.8% (P < 0.05). Experiment 2 separated the effects of particle size and total PIN content on digestion by milling the hardest and softest lines such that the mean particle size was nearly identical. Increased PIN decreased DMD by 21.7% (P < 0.05) and starch digestibility by 19.9% (P < 0.05) across particle sizes smaller than whole kernel. Experiment 3 addressed the time course of PIN effects in the rumen by observing ground samples of the hardest and softest lines over a 12-h in situ period. Increased PIN decreased DMD by 10.4% (P < 0.05) and starch digestibility by 11.0% (P < 0.05) across all time points. Dry matter and starch digestibility results demonstrated that increased expression of PIN was associated with a decreased rate of ruminal digestion independent of particle size. Puroindolines seem to aid in the protection of starch molecules from microbial digestion in the rumen, potentially increasing the amount of starch entering the small intestine.

Wheat puroindolines interact to form friabilin and control wheat grain hardness

Wheat grain is sold based upon several physiochemical characteristics, one of the most important being grain texture. Grain texture in wheat directly affects many end use qualities such as milling yield, break flour yield, and starch damage. The hardness (Ha) locus located on the short arm of chromosome 5D is known to control grain hardness in wheat. This locus contains the puroindoline A ( pina) and puroindoline B ( pinb) genes. All wheats to date that have mutations in pina or pinb are hard textured, while wheats possessing both the 'soft type' pina-D1a and pinb-D1a sequences are soft. Furthermore, it has been shown that complementation of the pinb-D1b mutation in hard spring wheat can restore a soft phenotype. Here, our objective was to identify and characterize the effect the puroindoline genes have on grain texture independently and together. To accomplish this we transformed a hard red spring wheat possessing a pinb-D1b mutation with 'soft type' pina and pinb, creating transgenic isolines that have added pina, pinb, or pina and pinb. Northern blot analysis of developing control and transgenic lines indicated that grain hardness differences were correlated with the timing of the expression of the native and transgenically added puroindoline genes. The addition of PINA decreased grain hardness less than the reduction seen with added PINB. Seeds from lines having more 'soft type' PINB than PINA were the softest. Friabilin abundance was correlated with the presence of both 'soft type' PINA and PINB and did not correlate well with total puroindoline abundance. The data indicates that PINA and PINB interact to form friabilin and together affect wheat grain texture.

Puroindoline A-gene expression is involved in association of puroindolines to starch

Puroindolines largely influence cereal grain hardness. In order to understand how they exert this influence, we carried out a molecular analysis of the pina and pinb genes of many Italian wheat cultivars. On the basis of their pin genotypes they could be divided into three groups: Pina-D1a/Pinb-D1a; Pina-D1a/Pinb-D1b; and Pina-D1b/Pinb-D1a. Five cultivars from each group were chosen to be studied to examine the quantity of puroindolines associated with starch (friabilin) and the amount not associated with starch. In addition, the level of pina expression was measured using RT-PCR. Soft cultivars ( Pina-D1a/Pinb-D1a) exhibited the highest level of expression of pina; among the hard cultivars, those with the Pina-D1a/Pinb-D1b genotype showed a lower level of expression, while those with the Pina-D1b/Pinb-D1a genotype did not express pina. Total puroindoline and friabilin content was then measured by flow cytometry. Soft Pina-D1a/Pinb-D1a cultivars displayed high puroindoline content that was primarily starch associated. Hard Pina-D1b/Pinb-D1a cultivars had very low puroindoline content with no puroindoline bound to starch. Hard Pina-D1a/Pinb-D1b cultivars were highly heterogeneous with respect to both the content of puroindolines and the level of association with starch. The accurate quantification of puroindolines in starch-bound and not starch-bound forms in association with molecular analysis, indicates that pina expression and presence controls the abundance of total puroindoline and its association with starch.

Seed yield and plant biomass increases in rice are conferred by deregulation of endosperm ADP-glucose pyrophosphorylase

In this work we test the hypothesis that yield of rice ( Oryza sativa L.) can be enhanced by increasing endosperm activity of ADP-glucose pyrophosphorylase (AGP), a key enzyme in starch biosynthesis. The potential for increases in yield exist because rice initiates more seeds than are taken to maturity and possesses excess photosynthetic capacity that could be utilized if there were more demand for assimilate. Following an approach already shown to be successful in wheat, experiments were designed to increase demand for assimilate by increasing the capacity for starch synthesis in endosperm. This was accomplished by transforming rice with a modified maize AGP large subunit sequence ( Sh2r6hs) under control of an endosperm-specific promoter. This altered subunit confers upon AGP decreased sensitivity to allosteric inhibition by inorganic phosphate (Pi) and enhanced heat stability, potentially leading to higher AGP activity in vivo. The Sh2r6hs transgene increased AGP activity in developing endosperm by 2.7-fold in the presence of Pi. Increases in AGP activity in transgenic seeds compared with controls were maximal between 10-15 days after anthesis. Starch content of individual seeds at harvest was not increased, but seed weight per plant and total plant biomass were each increased by more than 20%. Increased endosperm AGP activity thus stimulates setting of additional seeds and overall plant growth rather than increasing yield of seeds already set. Results demonstrate that deregulation of endosperm AGP increases overall plant sink strength, leading to larger, more productive plants in a manner similar to that in wheat having similar genetic modification.

The maize genome contains a helitron insertion

The maize mutation sh2-7527 was isolated in a conventional maize breeding program in the 1970s. Although the mutant contains foreign sequences within the gene, the mutation is not attributable to an interchromosomal exchange or to a chromosomal inversion. Hence, the mutation was caused by an insertion. Sequences at the two Sh2 borders have not been scrambled or mutated, suggesting that the insertion is not caused by a catastrophic reshuffling of the maize genome. The insertion is large, at least 12 kb, and is highly repetitive in maize. As judged by hybridization, sorghum contains only one or a few copies of the element, whereas no hybridization was seen to the Arabidopsis genome. The insertion acts from a distance to alter the splicing of the sh2 pre-mRNA. Three distinct intron-bearing maize genes were found in the insertion. Of most significance, the insertion bears striking similarity to the recently described DNA helicase-bearing transposable elements termed HELITRONS: Like Helitrons, the inserted sequence of sh2-7527 is large, lacks terminal repeats, does not duplicate host sequences, and was inserted between a host dinucleotide AT. Like Helitrons, the maize element contains 5' TC and 3' CTRR termini as well as two short palindromic sequences near the 3' terminus that potentially can form a 20-bp hairpin. Although the maize element lacks sequence information for a DNA helicase, it does contain four exons with similarity to a plant DEAD box RNA helicase. A second Helitron insertion was found in the maize genomic database. These data strongly suggest an active Helitron in the present-day maize genome.

Hordoindolines are associated with a major endosperm-texture QTL in barley (Hordeum vulgare)

Endosperm texture has a tremendous impact on the end-use quality of wheat (Triticum aestivum L.). Cultivars of barley (Hordeum vulgare L.), a close relative of wheat, also vary measurably in grain hardness. However, in contrast to wheat, little is known about the genetic control of barley grain hardness. Puroindolines are endosperm-specific proteins found in wheat and its relatives. In wheat, puroindoline sequence variation controls the majority of wheat grain texture variation. Hordoindolines, the puroindoline homologs of barley, have been identified and mapped. Recently, substantial allelic variation was found for hordoindolines among commercial barley cultivars. Our objective was to determine the influence of hordoindoline allelic variation upon grain hardness and dry matter digestibility in the 'Steptoe' x 'Morex' mapping population. This population is segregating for hordoindoline allele type, which was measured by a HinA/HinB/Gsp composite marker. One-hundred and fifty lines of the 'Steptoe' x 'Morex' population were grown in a replicated field trial. Grain hardness was estimated by near-infrared reflectance (NIR) and measured using the single kernel characterization system (SKCS). Variation attributable to the HinA/HinB/Gsp locus averaged 5.7 SKCS hardness units (SKCS U). QTL analysis revealed the presence of several areas of the genome associated with grain hardness. The largest QTL mapped to the HinA/HinB/Gsp region on the short arm of chomosome 7 (5H). This QTL explains 22% of the SKCS hardness difference observed in this study. The results indicate that the Hardness locus is present in barley and implicates the hordoindolines in endosperm texture control.

Enhanced ADP-glucose pyrophosphorylase activity in wheat endosperm increases seed yield

Yield in cereals is a function of seed number and weight; both parameters are largely controlled by seed sink strength. The allosteric enzyme ADP-glucose pyrophosphorylase (AGP) plays a key role in regulating starch biosynthesis in cereal seeds and is likely the most important determinant of seed sink strength. Plant AGPs are heterotetrameric, consisting of two large and two small subunits. We transformed wheat (Triticum aestivum L.) with a modified form of the maize (Zea mays L.) Shrunken2 gene (Sh2r6hs), which encodes an altered AGP large subunit. The altered large subunit gives rise to a maize AGP heterotetramer with decreased sensitivity to its negative allosteric effector, orthophosphate, and more stable interactions between large and small subunits. The Sh2r6hs transgene was still functional after five generations in wheat. Developing seeds from Sh2r6hs transgenic wheat exhibited increased AGP activity in the presence of a range of orthophosphate concentrations in vitro. Transgenic Sh2r6hs wheat lines produced on average 38% more seed weight per plant. Total plant biomass was increased by 31% in Sh2r6hs plants. Results indicate increased availability and utilization of resources in response to enhanced seed sink strength, increasing seed yield, and total plant biomass.

Wheat puroindolines enhance fungal disease resistance in transgenic rice

Antimicrobial peptides play a role in the immune systems of animals and plants by limiting pathogen infection and growth. The puroindolines, endosperm-specific proteins involved in wheat seed hardness, are small proteins reported to have in vitro antimicrobial properties. Rice, the most widely used cereal crop worldwide, normally does not contain puroindolines. Transgenic rice plants that constitutively express the puroindoline genes pinA and/or pinB throughout the plants were produced. PIN extracts of leaves from the transgenic plants reduced in vitro growth of Magnaporthe grisea and Rhizoctonia solani, two major fungal pathogens of rice, by 35 to 50%. Transgenic rice expressing pinA and/or pinB showed significantly increased tolerance to M. grisea (rice blast), with a 29 to 54% reduction in symptoms, and R. solani (sheath blight), with an 11 to 22% reduction in symptoms. Puroindolines are effective in vivo in antifungal proteins and could be valuable new tools in the control of a wide range of fungal pathogens of crop plants.

Maize genes encoding the small subunit of ADP-glucose pyrophosphorylase

Plant ADP-glucose pyrophosphorylase (AGP) is a heterotetrameric enzyme composed of two large and two small subunits. Here, we report the structures of the maize (Zea mays) genes encoding AGP small subunits of leaf and endosperm. Excluding exon 1, protein-encoding sequences of the two genes are nearly identical. Exon 1 coding sequences, however, possess no similarity. Introns are placed in identical positions and exhibit obvious sequence similarity. Size differences are primarily due to insertions and duplications, hallmarks of transposable element visitation. Comparison of the maize genes with other plant AGP small subunit genes leads to a number of noteworthy inferences concerning the evolution of these genes. The small subunit gene can be divided into two modules. One module, encompassing all coding information except that derived from exon 1, displays striking similarity among all genes. It is surprising that members from eudicots form one group, whereas those from cereals form a second group. This implies that the duplications giving rise to family members occurred at least twice and after the separation of eudicots and monocot cereals. One intron within this module may have had a transposon origin. A different evolutionary history is suggested for exon 1. These sequences define three distinct groups, two of which come from cereal seeds. This distinction likely has functional significance because cereal endosperm AGPs are cytosolic, whereas all other forms appear to be plastid localized. Finally, whereas barley (Hordeum vulgare) reportedly employs only one gene to encode the small subunit of the seed and leaf, maize utilizes the two genes described here.

Expression of wheat puroindoline genes in transgenic rice enhances grain softness

The puroindoline genes (pinA and pinB) are believed to play critical roles in wheat (Triticum aestivum L.) grain texture. Mutations in either gene are associated with hard wheat. No direct evidence exists for the ability of puroindolines to modify cereal grain texture. Interestingly, puroindolines appear to be absent in cereal species outside of the tribe Triticeae, in which the dominant form of grain texture is hard. To assess the ability of the puroindolines to modify cereal grain texture, the puroindolines were introduced into rice (Oryzae sativa L.) under the control of the maize ubiquitin promoter. Textural analysis of transgenic rice seeds indicated that expression of PINA and/or PINB reduced rice grain hardness. After milling, flour prepared from these softer seeds had reduced starch damage and an increased percentage of fine flour particles. Our data support the hypothesis that puroindolines play important roles in controlling wheat grain texture and may be useful in modifying grain texture of other cereals.

Wheat grain hardness results from highly conserved mutations in the friabilin components puroindoline a and b

"Soft" and "hard" are the two main market classes of wheat (Triticum aestivum L.) and are distinguished by expression of the Hardness gene. Friabilin, a marker protein for grain softness (Ha), consists of two proteins, puroindoline a and b (pinA and pinB, respectively). We previously demonstrated that a glycine to serine mutation in pinB is linked inseparably to grain hardness. Here, we report that the pinB serine mutation is present in 9 of 13 additional randomly selected hard wheats and in none of 10 soft wheats. The four exceptional hard wheats not containing the serine mutation in pinB express no pinA, the remaining component of the marker protein friabilin. The absence of pinA protein was linked inseparably to grain hardness among 44 near-isogenic lines created between the soft variety Heron and the hard variety Falcon. Both pinA and pinB apparently are required for the expression of grain softness. The absence of pinA protein and transcript and a glycine-to-serine mutation in pinB are two highly conserved mutations associated with grain hardness, and these friabilin genes are the suggested tightly linked components of the Hardness gene. A previously described grain hardness related gene termed "GSP-1" (grain softness protein) is not controlled by chromosome 5D and is apparently not involved in grain hardness. The association of grain hardness with mutations in both pinA or pinB indicates that these two proteins alone may function together to effect grain softness. Elucidation of the molecular basis for grain hardness opens the way to understanding and eventually manipulating this wheat endosperm property.

A single mutation that increases maize seed weight

The maize endosperm-specific gene shrunken2 (Sh2) encodes the large subunit of the heterotetrameric starch synthetic enzyme adenosine diphosphoglucose pyrophosphorylase (AGP; EC 2.7.7.27). Here we exploit an in vivo, site-specific mutagenesis system to create short insertion mutations in a region of the gene known to be involved in the allosteric regulation of AGP. The site-specific mutagen is the transposable element dissociation (Ds). Approximately one-third (8 of 23) of the germinal revertants sequenced restored the wild-type sequence, whereas the remaining revertants contained insertions of 3 or 6 bp. All revertants retained the original reading frame 3' to the insertion site and involved the addition of tyrosine and/or serine. Each insertion revertant reduced total AGP activity and the amount of the SH2 protein. The revertant containing additional tyrosine and serine residues increased seed weight 11-18% without increasing or decreasing the percentage of starch. Other insertion revertants lacking an additional serine reduced seed weight. Reduced sensitivity to phosphate, a long-known inhibitor of AGP, was found in the high seed-weight revertant. This alteration is likely universally important since insertion of tyrosine and serine in the potato large subunit of AGP at the comparable position and expression in Escherichia coli also led to a phosphate-insensitive enzyme. These results show that single gene mutations giving rise to increased seed weight, and therefore perhaps yield, are clearly possible in a plant with a long history of intensive and successful breeding efforts.

De novo synthesis of an intron by the maize transposable element Dissociation

The mechanisms by which introns are gained or lost in the evolution of eukaryotic genes remain poorly understood. The discovery that transposable elements sometimes alter RNA splicing to allow partial or imperfect removal of the element from the primary transcripts suggests that transposons are a potential and continuing source of new introns. To date, splicing events that precisely restore the wild-type RNA sequence at the site of insertion have not been detected. Here we describe alternative RNA splicing patterns that result in precise removal of a Dissociation (Ds) insertion and one copy of its eight-nucleotide host site duplication from an exon sequence of the maize shrunken2-mutabe1 (sh2-m1) mutant. In one case, perfect splicing of Ds was associated with aberrant splicing of an intron located 32 bp upstream of the insertion site. The second transcript type was indistinguishable from wild-type mRNA, indicating that Ds was spliced like a normal intron in about 2% of the sh2-m1 transcripts. Our results suggest that the transposition of Ds into sh2 in 1968, in effect, marked the creation of a new intron in a modern eukaryotic gene. The possibility of precise intron formation by a transposable element demonstrated here may be a general phenomenon of intron formation, since consensus intron splice sites can be explained by insertions that duplicate host sequences upon integration. A model is presented.

ADP-glucose pyrophosphorylase in shrunken-2 and brittle-2 mutants of maize

The Shrunken-2 (Sh2) and Brittle-2 (Bt2) genes of maize encode subunits of the tetrameric maize endosperm ADPglucose pyrophosphorylase. However, in all sh2 and bt2 mutants so far examined, measurable ADPglucose pyrophosphorylase activity remains. We have investigated the origin of the residual activity found in various sh2 and bt2 mutants as well as tissue specific expression and post-translational modification of the Sh2 and Bt2 proteins. Sh2 and Bt2 cDNAs were expressed in Escherichia coli and antibodies were prepared against the resulting proteins SH2 and BT2 specific antibodies were used to demonstrate that SH2 and BT2 are endosperm specific, are altered or missing in various sh2 or bt2 mutants, and have a mol. wt. of 54 and 51 kDa respectively in the wild type. The Sh2 and Bt2 transcripts are also endosperm specific. Ten sh2 and eight bt2 mutants show varying severity of phenotypes expressed at transcript, protein subunit and kernel level. Synthesis of multiple transcripts and proteins commonly occurs as a result of sh2 or bt2 mutation. While all mutants produce detectable enzymic activity, not all produce detectable transcripts and proteins. To examine the origin of the apparent non-SH2/BT2 endosperm enzymic activity, homologs of Sh2 and Bt2, designated Agp1 and Agp2 respectively, were isolated from an embryo cDNA library and found to hybridize to endosperm transcripts distinct from those of Sh2 and Bt2. Thus Agp1 and Agp2 or closely related genes may be responsible for the residual activity in some sh2 and bt2 mutants. Surprisingly, no evidence of post-translational modification of the SH2 and BT2 protein subunits was detected.