Gsp-1 genes comprise a multigene family in wheat that exhibits a unique combination of sequence diversity yet conservation

Sequence Diversity and Identification of Novel Puroindoline and Grain Softness Protein Alleles in Elymus, Agropyron and Related Species

The puroindoline proteins, PINA and PINB, which are encoded by the Pina and Pinb genes located at the Ha locus on chromosome 5D of bread wheat, are considered to be the most important determinants of grain hardness. However, the recent identification of Pinb-2 genes on group 7 chromosomes has stressed the importance of considering the effects of related genes and proteins. Several species related to wheat (two diploid Agropyron spp., four tetraploid Elymus spp. and five hexaploid Elymus and Agropyron spp.) were therefore analyzed to identify novel variation in Pina, Pinb and Pinb-2 genes which could be exploited for the improvement of cultivated wheat. A novel sequence for the Pina gene was detected in Elymus burchan-buddae, Elymus dahuricus subsp. excelsus and Elymus nutans and novel PINB sequences in Elymus burchan-buddae, Elymus dahuricus subsp. excelsus, and Elymus nutans. A novel PINB-2 variant was also detected in Agropyron repens and Elymus repens. The encoded proteins detected all showed changes in the tryptophan-rich domain as well as changes in and/or deletions of basic and hydrophobic residues. In addition, two new AGP sequences were identified in Elymus nutans and Elymus wawawaiensis. The data presented therefore highlight the sequence diversity in this important gene family and the potential to exploit this diversity to modify grain texture and end-use quality in wheat.

A review of the occurrence of Grain softness protein-1 genes in wheat (Triticum aestivum L.)

Grain softness protein-1 (Gsp-1) is a small, 495-bp intronless gene found throughout the Triticeae tribe at the distal end of group 5 chromosomes. With the Puroindolines, it constitutes a key component of the Hardness locus. Gsp-1 likely plays little role in grain hardness, but has direct interest due to its utility in phylogeny and its role in arabinogalactan peptides. Further role(s) remain to be identified. In the polyploid wheats, Triticum aestivum and T. turgidum, the gene is present in a homoeologous series. Since its discovery, there have been conflicting reports and data as to the number of Gsp-1 genes and the level of sequence polymorphism. Little is known about allelic variation within a species. In the simplest model, a single Gsp-1 gene is present in each wheat and Aegilops tauschii genome. The present review critically re-examines the published and some unpublished data (sequence available in the NCBI nucleotide and MIPS Wheat Genome Databases). A number of testable hypotheses are identified, and include the level of polymorphism that may represent (and define) different Gsp-1 alleles, the existence of a fourth Gsp-1 gene, and the apparent, at times, high level of naturally-occurring or artifactual gene chimeras. In summary, the present data provide firm evidence for at most, three Gsp-1 genes in wheat, although there are numerous data that suggest a more complex model.

Discovery, distribution and diversity of Puroindoline-D1 genes in bread wheat from five countries (Triticum aestivum L.)

BACKGROUND

Grain texture is one of the most important characteristics in bread wheat (Triticum aestivum L.). Puroindoline-D1 genes play the main role in controlling grain texture and are intimately associated with the milling and processing qualities in bread wheat.

RESULTS

A series of diagnostic molecular markers and dCAPS markers were used to characterize Pina-D1 and Pinb-D1 in 493 wheat cultivars from diverse geographic locations. A primer walking strategy was used to characterize PINA-null alleles at the DNA level. Results indicated that Chinese landraces encompassing 12 different Puroindoline-D1 allelic combinations showed the highest diversity, while CIMMYT wheat cultivars containing 3 different Puroindoline-D1 allelic combinations showed the lowest diversity amongst wheat cultivars from the five countries surveyed. Two novel Pina-D1 alleles, designated Pina-D1s with a 4,422-bp deletion and Pina-D1u with a 6,460-bp deletion in the Ha (Hardness) locus, were characterized at the DNA level by a primer walking strategy, and corresponding molecular markers Pina-N3 and Pina-N4 were developed for straightforward identification of the Pina-D1s and Pina-D1u alleles. Analysis of the association of Puroindoline-D1 alleles with grain texture indicated that wheat cultivars with Pina-null/Pinb-null allele, possessing an approximate 33-kb deletion in the Ha locus, have the highest SKCS hardness index amongst the different genotypes used in this study. Moreover, wheat cultivars with the PINA-null allele have significantly higher SKCS hardness index than those of Pinb-D1b and Pinb-D1p alleles.

CONCLUSIONS

Molecular characterization of the Puroindoline-D1 allele was investigated in bread wheat cultivars from five geographic regions, resulting in the discovery of two new alleles - Pina-D1s and Pina-D1u. Molecular markers were developed for both alleles. Analysis of the association of the Puroindoline-D1 alleles with grain texture showed that cultivars with PINA-null allele possessed relatively high SKCS hardness index. This study can provide useful information for the improvement of wheat quality, as well as give a deeper understanding of the molecular and genetic processes controlling grain texture in bread wheat.

Stability of puroindoline peptides and effects on wheat rust

Peptides modelled on the tryptophan rich domain of puroindolines and the related grain softness protein-1 have a broad range of antibacterial and antifungal activities. With the aims of further investigating the activities of these antimicrobial peptides we studied their activity against wheat rust diseases and environmental stability. PINA-based peptides were found to have high pH and thermal stability in addition to being stable over long periods at room temperature. These properties could make them excellent candidates as preservatives in food. PuroA, Pina-R39G and PuroB peptides adversely affected the morphology of the stripe rust spores (Puccinia striiformis f. sp. tritici), while PuroA and PuroB showed moderate inhibition of their germination. Additionally, GSP-5D reduced the germination of leaf rust spores (P. triticina). PuroA and PuroB sprayed onto stripe rust infected plants effected a moderate reduction in the number of stripe rust uredinia on wheat seedlings, as did PuroB sprayed onto the seedlings and allowed to coat the leaves for 5 day prior to spore infection. The results suggest that the presence of the PIN-based peptides may lower frequency of initial infection foci.

The RNA-mediated silencing of one of the Pin genes in allohexaploid wheat simultaneously decreases the expression of the other, and increases grain hardness

The RNAi-mediated silencing of Pina and Pinb, the two genes responsible for the grain texture of allohexaploid wheat, was induced and analysed in two wheat cultivars, Kontesa and Torka. A characterization of the two genes in non-transgenic plants revealed that Pinb carries a point mutation, designated Pinb-D1c in both cultivars. This mutation does not influence transcript abundance or protein content. Two silencing cassettes of the hpRNA type were constructed and used for stable transformation via Agrobacterium. In total, 43 transgenic lines representing the two cultivars were obtained, transformed with the silencing cassettes for Pina or for Pinb or co-transformed with both cassettes. The relative transcript levels of the two genes in the same progeny plant were found to be similar, independent of the silencing cassette used. The reduction in the Pina and Pinb transcript levels in the segregating T(1) progeny of Kontesa and Torka transformed with one of the silencing cassettes exceeded 80%. Co-transformation with the silencing cassettes for both genes resulted in a reduction of over 91% of Pina and Pinb transcripts in some segregating T(1) progeny of Kontesa. The silencing was transmitted to the T(4) kernel generation of the T(3) lines. A significant reduction or lack of both puroindoline proteins in the silenced lines correlated with an essential increase in grain hardness. The discussion covers some new insights into the function of the Pin genes, including the simultaneous silencing of both, independent of the siRNA signal.

Sixty million years in evolution of soft grain trait in grasses: emergence of the softness locus in the common ancestor of Pooideae and Ehrhartoideae, after their divergence from Panicoideae

Together maize, Sorghum, rice, and wheat grass (Poaceae) species are the most important cereal crops in the world and exhibit different "grain endosperm texture." This trait has been studied extensively in wheat because of its pivotal role in determining quality of products obtained from wheat grain. Grain softness protein-1 and Puroindolines A and B (grain storage proteins), encoded by Ha-like genes: Gsp-1, Pina, and Pinb, of the Hardness (Ha) locus, are the main determinants of the grain softness/hardness trait in wheat. The origin and evolution of grain endosperm texture in grasses was addressed by comparing genomic sequences of the Ha orthologous region of wheat, Brachypodium, rice, and Sorghum. Results show that the Ha-like genes are present in wheat and Brachypodium but are absent from Sorghum bicolor. A truncated remnant of an Ha-like gene is present in rice. Synteny analysis of the genomes of these grass species shows that only one of the paralogous Ha regions, created 70 My by whole-genome duplication, contained Ha-like genes. The comparative genome analysis and evolutionary comparison with genes encoding grain reserve proteins of grasses suggest that an ancestral Ha-like gene emerged, as a new member of the prolamin gene family, in a common ancestor of the Pooideae (Triticeae and Brachypoidieae tribes) and Ehrhartoideae (rice), between 60 and 50 My, after their divergence from Panicoideae (Sorghum). It was subsequently lost in Ehrhartoideae. Recurring duplications, deletions, and/or truncations occurred independently and appear to characterize Ha-like gene evolution in the grass species. The Ha-like genes gained a new function in Triticeae, such as wheat, underlying the soft grain phenotype. Loss of these genes in some wheat species leads, in turn, to hard endosperm seeds.

Molecular genetics of puroindolines and related genes: allelic diversity in wheat and other grasses

The hardness or texture of cereal grains is a primary determinant of their technological and processing quality. Among members of the Triticeae, most notably wheat, much of the variation in texture is controlled by a single locus comprised of the Puroindoline a, Puroindoline b and Grain Softness Protein-1 (Gsp-1) genes. Puroindolines confer the three major texture classes of soft and hard common wheat and the very hard durum wheat. The protein products of these genes interact with lipids and are associated with the surface of isolated starch (as a protein fraction known as 'friabilin'). During the past ten years a great diversity of alleles of both Puroindoline genes have been discovered and significant advances made in understanding the relationship between the gene presence/absence, sequence polymorphism and texture of cereal grains. Efforts have also focussed on Puroindoline and Gsp-1 genes in diploid progenitors, other Triticeae grasses and synthetic wheats in order to understand the evolution of this gene family and find potentially useful variants. The puroindoline homologues in other cereals such as rye and barley are also receiving attention. This work summarises new developments in molecular genetics of puroindolines in wheat and related Triticeae grasses, and the related genes in other cereals.