John E. Flintham

‘Green revolution’ genes encode mutant gibberellin response modulators

World wheat grain yields increased substantially in the 1960s and 1970s because farmers rapidly adopted the new varieties and cultivation methods of the so-called ‘green revolution’. The new varieties are shorter, increase grain yield at the expense of straw biomass, and are more resistant to damage by wind and rain,. These wheats are short because they respond abnormally to the plant growth hormone gibberellin. This reduced response to gibberellin is conferred by mutant dwarfing alleles at one of two Reduced height-1 (Rht-B1 and Rht-D1) loci,. Here we show that Rht-B1/Rht-D1 and maize dwarf-8 (d8), are orthologues of the Arabidopsis Gibberellin Insensitive (GAI) gene,. These genes encode proteins that resemble nuclear transcription factors and contain an SH2-like domain, indicating that phosphotyrosine may participate in gibberellin signalling. Six different orthologous dwarfing mutant alleles encode proteins that are altered in a conserved amino-terminal gibberellin signalling domain. Transgenic rice plants containing a mutant GAI allele give reduced responses to gibberellin and are dwarfed, indicating that mutant GAI orthologues could be used to increase yield in a wide range of crop species.

Optimizing wheat grain yield: effects of Rht (gibberellin-insensitive) dwarfing genes

Four sets of near-isogenic lines carrying different combinations of the alleles Rht-B1b , Rht-D1b and Rht-B1c for gibberellin-insensitive dwarfism in hexaploid wheat ( Triticum aestivum L. ) were compared with tall controls in a series of yield trials in eastern England and central Germany. In all four varietal backgrounds the effects of Rht-B1b and Rht-D1b were similar (plant height ≈ 86 and 83% of tall controls respectively) and in combination reduced plant height to c. 58%. The Rht-B1c allele caused more severe dwarfism ( c. 50%) and, when combined with Rht-D1b , reduced plant height still further to c. 41%. Data from the trials were consistent with a model for height/yield relationships in which the pleiotropic effects of the Rht alleles on yield can be inferred from their primary function: insensitivity to gibberellin limits stem extension growth, decreasing assimilate demand for this organ and diverting it to the developing ear (which is not itself dwarfed). The net balance between the resulting increase in harvest index and the curvilinear relationship observed between plant height and total shoot yield results in optimum grain yields at intermediate plant heights. Yield advantages of shorter plants over tall controls were evident over several trials with mean grain yields ranging from 200 to 760 g m −2 . The optimum plant height for yield improvement inn different genetic backgrounds was achieved by different Rht alleles according to the background varietal height, such that intrinsically taller genotypes required more potent Rht alleles to achieve maximum potential grain yield. Ear yield components showed increases in grain number due to Rht pleiotropy, from which it is inferred that the number of grains per ear is limited by supply of assimilates pre-anthesis. Increases in grain number were associated with decreases in mean weightn per grain which varied according to severity of dwarfism and varietal background, so that the net effect on grain yield per ear was sometimesn positive, sometimes negative, and sometimes neutral in different Rht /variety combinations.

Different genetic components control coat-imposed and embryo-imposeddormancy in wheat

Wheat grain dormancy is a multigenic trait controlled both by R genes conferring red testa pigmentation and by other genes, at least one of which has a major effect in the embryo. Enhanced grain dormancy and red test colour are inherited as pleiotropic effects of dominant R alleles at triplicate loci in hexaploid wheat. However, polymorphism for R genes cannot account for the wide variation in dormancy observed among different redgrained varieties. A variety of different dominant R alleles all have equivalent effects on dormancy when introgressed into white-grained wheats, although the latter vary in dormancy both in the absence and in the presence of dominant R alleles. As a result, certain redgrained genotypes can exhibit intermediate dormancy, similar to that of some white-grained genotypes with different genetic backgrounds. A new major gene (Phs) was identified as controlling the difference between two red-grained cultivars with widely different dormancies. The Phs gene appeared to exert its effect in the embryo of the grain, in contrast to R gene expression in maternal testa tissue. Discrete genetic functions thus underlie physiologically distinct mechanisms of coatimposed dormancy and embryo-imposed dormancy in wheat

Transcripts of Vp-1 homeologues are misspliced in modern wheat and ancestral species

The maize (Zea mays) Viviparous 1 (Vp1) transcription factor has been shown previously to be a major regulator of seed development, simultaneously activating embryo maturation and repressing germination. Hexaploid bread wheat (Triticum aestivum) caryopses are characterized by relatively weak embryo dormancy and are susceptible to preharvest sprouting (PHS), a phenomenon that is phenotypically similar to the maize vp1 mutation. Analysis of Vp-1 transcript structure in wheat embryos during grain development showed that each homeologue produces cytoplasmic mRNAs of different sizes. The majority of transcripts are spliced incorrectly, contain insertions of intron sequences or deletions of coding region, and do not have the capacity to encode full-length proteins. Several VP-1-related lower molecular weight protein species were present in wheat embryo nuclei. Embryos of a closely related tetraploid species (Triticum turgidum) and ancestral diploids also contained misspliced Vp-1 transcripts that were structurally similar or identical to those found in modern hexaploid wheat, which suggests that compromised structure and expression of Vp-1 transcripts in modern wheat are inherited from ancestral species. Developing embryos from transgenic wheat grains expressing the Avena fatua Vp1 gene showed enhanced responsiveness to applied abscisic acid compared with the control. In addition, ripening ears of transgenic plants were less susceptible to PHS. Our results suggest that missplicing of wheat Vp-1 genes contributes to susceptibility to PHS in modern hexaploid wheat varieties and identifies a possible route to increase resistance to this environmentally triggered disorder.

Genetic map locations for orthologous Vp1 genes in wheat and rice

Abstractu2002Chromosome locations for gene orthologues of the dormancy-related maize transcription factor VIVIPAROUS-1, encoded by the Vp1 locus on maize chromosome 3, were determined in wheat (Triticum aestivum L.) and rice (Oryza sativa L.) via linkage to markers on existing molecular maps using a cDNA of a wheat Vp1 orthologue as a probe in genomic Southern analyses. Vp1-orthologous loci were detected on the long arms of wheat chromosomes 3A, 3B and 3D [Xlars10 (taVp1) loci] and rice chromosome 1 (osVp1), in line with previous evidence of synteny between these regions of the rice and wheat genomes and chromosome 3 of maize. The wheat loci mapped some 30u2005cM from the centromeres and some 30u2005cM proximal to the red grain (R) loci that control seed colour and coat-imposed dormancy. This unequivocal, genetic separation of the Vp1 and R loci may offer an opportunity for improving resistance to pre-harvest sprouting in wheat by combining the coat-imposed dormancy associated with red seed colour and true embryo dormancy regulated by Vp1.

Genetic Control of Storage Oil Synthesis in Seeds of Arabidopsis

Quantitative trait loci (QTL) that control seed oil content and fatty acid composition were studied using a recombinant inbred population derived from a cross between the Arabidopsis ecotypes Landsberg erecta and Cape Verdi Islands. Multiple QTL model mapping identified two major and two minor QTL that account for 43% of the variation in oil content in the population. The most significant QTL is at the bottom of chromosome 2 and accounts for 17% of the genetic variation. Two other significant QTL, located on the upper and lower arms of chromosome 1, account for a further 19% of the genetic variation. A QTL near to the top of chomosome 3 is epistatic to that on the upper arm of chromosome 1. There are strong QTL for linoleic (18:2) and linolenic (18:3) acids contents that colocate with the FAD3 locus, another for oleic acid (18:1) that colocates with FAD2 and other less significant QTL for palmitic (16:0), stearic (18:0), and eicosaenoic (20:1) acids. The presence of the QTL for seed oil content on chromosome 2 was confirmed by the generation of lines that contain a 22-cM region of Landsberg erecta DNA at the bottom of chromosome 2 in a background containing Cape Verdi Islands in other regions of the genome that had been shown to influence oil content in the QTL analysis.

Cereal comparative genetics and preharvest sprouting

Most genes in hexaploid bread wheat are triplicated. Knowledge of the relationships between the three genomes then allows us to build consensus maps of loci controlling any trait. In this paper we show such a map of some of the major genes and QTL effects that have been reported to be associated with pre-harvest sprouting. The result highlights regions of the genome that have featured in several studies and possible links between QTL and major genes. The same analysis can be extended to other economic grass crop species, where the comparative genome relationships are known in some detail. In this way, loci related to preharvest sprouting in wheat have been compared with some major genes affecting dormancy in maize and dormancy related QTLs in rice. This alignment identifies some candidate loci from maize and some regions of the rice genome that may relate to important wheat QTLs. In turn this approach will open up application of the emerging rice genomic DNA sequence to wheat pre-harvest sprouting research.

Heterosis, overdominance for grain yield, and alpha-amylase activity in F 1 hybrids between near-isogenic Rht dwarf and tall wheats

The Rht-B1b, Rht-D1b and Rht-B1c alleles for reduced height in wheat (the Norin 10 and Tom Thumb dwarfing genes previously known as Rht1, Rht2 and Rht3) were exploited in combinations to generate a near-continuous range of plant heights, from 53 cm to 123 cm, amongst near-isogenic homozygotes and F 1 hybrids. Pleiotropic yield effects of Rht genes were measured in both homozygous (intravarietal) and heterozygous (intervarietal) genetic backgrounds. Heterosis due to overdominance of Rht genes was detected among intravarietal hybrids. The effects of heterozygosity at other genetic loci (mean dominance) were determined, independently of Rht effects, from comparisons between intravarietal and intervarietal F 1 hybrids. Genotypes of intermediate plant heights gave maximum yields, in agreement with other trials of the homozygous lines, so that heterosis (hybrid exceeding best parent) for Rht yield effects was observed in crosses between tall and dwarf isogenic pairs. This heterosis combined additively with increased mean weight per grain in intervarietal crosses, generating the highest overall grain yields in hybrids with semi-dwarf stature in heterozygous genetic backgrounds. The Rht-B1c allele showed single-gene overdominance for grain yield, also the production of alpha-amylase in ripening grains of Maris Huntsman was effectively inhibited in the Rht-B1a/c intravarietal hybrid. The Rht-B1c allele thus offers advantages for both grain yield and grain quality in the heterozygous condition and should be considered as an alternative to the conventional semi-dwarfing genes Rht-B1b and Rht-D1b for F 1 varieties in environments conductive to preharvest sprouting.

Genetic control mechanisms regulating the initiation of germination

Summary Genetic analysis of the transition from embryogenesis to germination has shown that these processes are under strict sequential, mutually exclusive, control. Understanding the regulation of this phase transition should provide important approaches to new technologies that can be usedto improve seed quality traits in crop plants. The developmental disorder pre-harvest sprouting (PHS) in cereals and pre-germination of seeds in the pods of oilseed rape are significant agronomic problems, that occur when seeds develop under cool moist environmental conditions. Phenotypically, pre-germination of seeds on the mother plant is similar to the severe maize and Arabidopsis mutations viviparous1 ( vp1 ), and abscisic acid insensitive3 ( abi3 ).The corresponding loci encode homologous transcription factors thatsimultaneously activate embryo maturation and repress germination. We have analysed gene expression programmes in wheat embryos under conditions that induce PHS, and have analysed in detail the structure and expression of wheat Vp1 homeologues. These analyses show that both maturation and germination genes are expressed simultaneously in embryos grown under cool moist conditions, and that the majority of VP1 transcripts expressed in the cytoplasm during normal embryo maturation are not correctly spliced. These results suggest that under perturbed environmental conditions wheat may not express enough functional VP1 activity to repress germination. This hypothesis is currently being tested using transgenic approaches. Repression of germination by ABI3 andother loci in Arabidopsis indicates that these factors interact with loci that enhance germination potential. Using a novel genetic screen to search for regulators of germination, we have identified the COMATOSE ( CTS ) locus. Genetic and physiological analyses show that CTS regulates germination potential by enhancing after ripening, sensitivity to gibberellins and pre-chilling, and by repressing the activities of loci that activate embryo maturation.

Use of comparative molecular genetics to study pre harvest sprouting in wheat

The use of model genetic systems in plant and animal studies has allowed a greatly increased understanding of how genomes regulate phenotype. Arabidopsis thaliana (arabidopsis) has proved to be an extremely useful model for plant molecular genetic studies and there are now many examples of important agricultural genes that have been identified using this species as a model. The discovery and investigation of a large number of arabidopsis mutants associated with seed development and germination indicates that resources are available, using comparative molecular genetics approaches, to identify candidate loci that may play important roles in the regulation and aetiology of pre-harvest sprouting (PHS). Specific examples are discussed including the ABSCISIC ACID INSENSITIVE3 (ABI3) and FUSCA3 (FUS3) loci, that encode proteins containing a highly conserved DNA binding domain originally identified in the maize Viviparous-1 (Vp-1) transcription factor. The potential use of this gene family for the manipulation of PHS, either through marker-assisted breeding or transgenic approaches is discussed.