Wheat (Triticum aestivum L. and T. turgidum L. ssp. durum) Kernel Hardness: I. Current View on the Role of Puroindolines and Polar Lipids

Barriers and levers to enhance end-use functional properties of durum wheat (Triticum turgidum L.) grain: An agronomic implication

The current trends in population growth and consumption pattern remain to increase the demand for durum wheat grain. However, multiple biotic and abiotic challenges due to climate change coupled with crop management practices possess major concern to improve durum wheat production and storage proteins. Efforts on developing innovative agronomic and breeding strategies are essential to enhance productivity, and nutritional quality under the changing climate. Nitrogen is an important structural component of protein, and potentially reduce the adverse effect of drought stress through maintaining metabolic activities. Optimum nitrogen fertilization allows durum wheat producing farmers to attain high quality yield, brings economic benefit, and reduces environmental pollution. However, defining an optimum nitrogen fertilizer rate for specific location requires considering yield achievement and quality of the end products. If the producers interest is, geared towards production of high protein content, high nitrogen dose is required. If the interest gears towards grain yield improvement optimization of nitrogen fertilizer rate is important. This indicates that defining product-specific nitrogen application is required for sustainable durum wheat production. Therefore, future challenges of increasing production, productivity, and end-use functional properties of durum wheat will only be achieved through cooperation of multidisciplinary teams who are able to incorporate new technologies.

Molecular and physical characterization of grain hardness in European spring common wheat (Triticum aestivum L.)

Grain hardness is the single most important trait in determining technological properties and end-use quality of wheat product. This trait is controlled by two genes (Pina-D1 and Pinb-D1) at the Hardness (Ha) locus. Soft endosperm kernels are characterized by the presence of alleles 'a' in both genes (Pina-D1a and Pinb-D1a), while the medium hard and hard grain is the result of deletion in the Pina-D1 gene or single mutation of the Pinb-D1 gene. The aim of the current study was to determine the relationship between common wheat grain hardness and the presence of puroindoline genes. Eighty-one spring common wheat cultivars from Europe were analysed for grain hardness by SKCS (Single Kernel Characterization System) and Pin-D1 genes. The analysed genotypes were divided into three hardness classes: hard, medium and soft and they showed four allelic combinations in Pin-D1 genes. The SKCS results showed that hard wheat was the major type in European cultivars, whereas molecular analysis showed differential allelic combinations of puroindoline genes among these classes. The conducted analyses suggest that another major gene or other factors were influencing kernel texture.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02897-3.

Gas cell stabilization by aqueous-phase constituents during bread production from wheat and rye dough and oat batter: Dough or batter liquor as model system

Proper gas cell stability during fermentation and baking is essential to obtain high-quality bread. Gas cells in wheat dough are stabilized by the gluten network formed during kneading and, from the moment this network locally ruptures, by liquid films containing nonstarch polysaccharides (NSPs) and surface-active proteins and lipids. Dough liquor (DL), the supernatant after ultracentrifugation of dough, is a model system for these liquid films and has been extensively studied mostly in the context of wheat bread making. Nonwheat breads are often of lower quality (loaf volume and crumb structure) than wheat breads because their doughs/batters lack a viscoelastic wheat gluten network. Therefore, gas cell stabilization by liquid film constituents may be more important in nonwheat than in wheat bread making. This manuscript aims to review the knowledge on DL/batter liquor (BL) and its relevance for studying gas cell stabilization in wheat and nonwheat (rye and oat) bread making. To this end, the unit operations in wheat, rye, and oat bread making are described with emphasis on gas incorporation and gas cell (de)stabilization. A discussion of the knowledge on the recoveries and chemical structures of proteins, lipids, and NSPs in DLs/BLs is provided and key findings of studies dealing with foaming and air-water interfacial properties of DL/BL are discussed. Next, the extent to which DL/BL functionality can be related to bread properties is addressed. Finally, the extent to which DL/BL is a representative model system for the aqueous phase of dough/batter is discussed and related to knowledge gaps and further research opportunities.

Normal-Phase HPLC-ELSD to Compare Lipid Profiles of Different Wheat Flours

Normal-phase high-performance liquid chromatography (HPLC) is widely used in combination with evaporative light scattering detection (ELSD) for separating and detecting lipids in various food samples. ELSD responses of different lipids were evaluated to elucidate the possibilities and challenges associated with quantification by means of HPLC-ELSD. Not only the number and type of polar functional groups but also the chain length and degree of unsaturation of (free or esterified) fatty acids (FAs) had a significant effect on ELSD responses. Tripalmitin and trilinolein yielded notably different ELSD responses, even if their constituting free FAs produced identical responses. How FA structure impacts ELSD responses of free FAs is thus not predictive for those of triacylglycerols and presumably other lipids containing esterified FAs. Because ELSD responses of lipids depend on the identity of the (esterified) FA(s) which they contain, fully accurate lipid quantification with HPLC-ELSD is challenging and time-consuming. Nonetheless, HPLC-ELSD is a good and fast technique to semi-quantitatively compare the levels of different lipid classes between samples of comparable FA composition. In this way, lipid profiles of different flours from near-isogenic wheat lines could be compared.

Toward the Genetic Basis and Multiple QTLs of Kernel Hardness in Wheat

Kernel hardness is one of the most important single traits of wheat seed. It classifies wheat cultivars, determines milling quality and affects many end-use qualities. Starch granule surfaces, polar lipids, storage protein matrices and Puroindolines potentially form a four-way interaction that controls wheat kernel hardness. As a genetic factor, Puroindoline polymorphism explains over 60% of the variation in kernel hardness. However, genetic factors other than Puroindolines remain to be exploited. Over the past two decades, efforts using population genetics have been increasing, and numerous kernel hardness-associated quantitative trait loci (QTLs) have been identified on almost every chromosome in wheat. Here, we summarize the state of the art for mapping kernel hardness. We emphasize that these steps in progress have benefitted from (1) the standardized methods for measuring kernel hardness, (2) the use of the appropriate germplasm and mapping population, and (3) the improvements in genotyping methods. Recently, abundant genomic resources have become available in wheat and related Triticeae species, including the high-quality reference genomes and advanced genotyping technologies. Finally, we provide perspectives on future research directions that will enhance our understanding of kernel hardness through the identification of multiple QTLs and will address challenges involved in fine-tuning kernel hardness and, consequently, food properties.

Insights into the Genetic Architecture of Bran Friability and Water Retention Capacity, Two Important Traits for Whole Grain End-Use Quality in Winter Wheat

Bran friability (particle size distribution after milling) and water retention capacity (WRC) impact wheat bran functionality in whole grain milling and baking applications. The goal of this study was to identify genomic regions and underlying genes that may be responsible for these traits. The Hard Winter Wheat Association Mapping Panel, which comprised 299 lines from breeding programs in the Great Plains region of the US, was used in a genome-wide association study. Bran friability ranged from 34.5% to 65.9% (median, 51.1%) and WRC ranged from 159% to 458% (median, 331%). Two single-nucleotide polymorphisms (SNPs) on chromosome 5D were significantly associated with bran friability, accounting for 11-12% of the phenotypic variation. One of these SNPs was located within the Puroindoline-b gene, which is known for influencing endosperm texture. Two SNPs on chromosome 4A were tentatively associated with WRC, accounting for 4.6% and 4.4% of phenotypic variation. The favorable alleles at the SNP sites were present in only 15% (friability) and 34% (WRC) of lines, indicating a need to develop new germplasm for these whole-grain end-use quality traits. Validation of these findings in independent populations will be useful for breeding winter wheat cultivars with improved functionality for whole grain food applications.

Identification of a Wheat Thaumatin-like Protein That Inhibits Saccharomyces cerevisiae

Plants often produce antifungal peptides and proteins in response to infection. Also wheat, which is the main ingredient of bread dough, contains such components. Here, we show that while some industrial strains of the baker's yeast Saccharomyces cerevisiae can efficiently ferment dough, some other strains show much lower fermentation capacities because they are sensitive to a specific wheat protein. We purified and identified what turned out to be a thaumatin-like protein through a combination of activity-guided fractionation, cation exchange chromatography, reversed-phase HPLC, and LC-MS/MS. Recombinant expression of the corresponding gene and testing the activity confirmed the inhibitory activity of the protein.

Marker-assisted transfer of PinaD1a gene to develop soft grain wheat cultivars

Grain softness has been a major trait of interest in wheat because of its role in producing flour suitable for making high-quality biscuits, cookies, cakes and some other products. In the present study, marker-assisted backcross breeding scheme was deployed to develop advanced wheat lines with soft grains. The Australian soft-grained variety Barham was used as the donor parent to transfer the puroindoline grain softness gene Pina-D1a to the Indian variety, DBW14, which is hard grained and has PinaD1bPinbD1a genes. Foreground selection with allele-specific PCR-based primer for Pina-D1a (positive selection) was used to identify heterozygous BC1F1 plants. Background selection with 173 polymorphic SSR primers covering all the 21 chromosomes was also carried out, in the foreground-selected BC1F1 plants. BC1F2 plants were selected by ascertaining the presence of Pina-D1a (positive selection) and absence of Pina-D1b (negative selection). Using the approach of positive, negative and background selection with molecular markers, 15 BC1F2 and 31 BC2F1 plants were finally selected. The 15 BC1F2 plants were selfed and the 31 BC2F1 plants were further backcrossed and selfed to raise BC3F1 and BC2F2 progenies, respectively. A part of the BC2F2 seed of each of the 31 plants was analyzed for grain hardness index (GHI) with single-kernel characterization system. The GHI varied from 12.1 to 37.1 in the seeds borne on the 31 BC2F1 plants. The reasons for this variation and further course of action are discussed.

Effect of Radio Frequency Cold Plasma Treatment on Intermediate Wheatgrass (Thinopyrum intermedium) Flour and Dough Properties in Comparison to Hard and Soft Wheat (Triticum aestivum L.)

Cold plasma is an emerging technology to improve microbiological safety as well as functionality of foods. This study compared the effect of radio frequency cold plasma on flour and dough properties of three members of the Triticeae tribe, soft as well as hard wheat (Triticum aestivum L.) and intermediate wheatgrass (Thinopyrum intermedium, IWG). These three flour types differ in their protein content and composition and were evaluated for their solubility, solvent retention capacity, starch damage, GlutoPeak and Farinograph profiles, and protein secondary structures. Plasma treatment resulted in dehydration of flours but did not change protein content or solubility. Farinograph water absorption increased for all flours after plasma treatment (from 56.5–61.1 before to 71.0–81.6%) and coincided with higher solvent retention capacity for water and sodium carbonate. Plasma treatment under our conditions was found to cause starch damage to the extent of 3.46–6.62% in all samples, explaining the higher solvent retention capacity for sodium carbonate. However, Farinograph properties were changed differently in each flour type: dough development time and stability time decreased for hard wheat and increased for soft wheat but remained unchanged in intermediate wheatgrass. GlutoPeak parameters were also affected differently: peak torque for intermediate wheatgrass increased from 32 to 39.5 GlutoPeak units but was not different for the other two flours. Soft wheat did not always aggregate after plasma treatment, i.e., did not aggregate within the measurement time. It was also the only flour where protein secondary structures were changed after plasma treatment, exhibiting an increase from 15.2 to 27.9% in β-turns and a decrease from 59.4 to 47.9% in β-sheets. While this could be indicative of a better hydrated gluten network, plasma-treated soft wheat was the only flour where viscoelastic properties were changed and extensibility decreased. Further research is warranted to elucidate molecular changes underlying these effects.

Using RT-qPCR, Proteomics, and Microscopy to Unravel the Spatio-Temporal Expression and Subcellular Localization of Hordoindolines Across Development in Barley Endosperm

Hordeum vulgare (barley) hordoindolines (HINs), HINa, HINb1, and HINb2, are orthologous proteins of wheat puroindolines (PINs) that are small, basic, cysteine-rich seed-specific proteins and responsible for grain hardness. Grain hardness is, next to its protein content, a major quality trait. In barley, HINb is most highly expressed in the mid-stage developed endosperm and is associated with both major endosperm texture and grain hardness. However, data required to understand the spatio-temporal dynamics of HIN transcripts and HIN protein regulation during grain filling processes are missing. Using reverse transcription quantitative PCR (RT-qPCR) and proteomics, we analyzed HIN transcript and HIN protein abundance from whole seeds (WSs) at four [6 days after pollination (dap), 10, 12, and ≥20 dap] as well as from aleurone, subaleurone, and starchy endosperm at two (12 and ≥20 dap) developmental stages. At the WS level, results from RT-qPCR, proteomics, and western blot showed a continuous increase of HIN transcript and HIN protein abundance across these four developmental stages. Miroscopic studies revealed HIN localization mainly at the vacuolar membrane in the aleurone, at protein bodies (PBs) in subaleurone and at the periphery of starch granules in the starchy endosperm. Laser microdissetion (LMD) proteomic analyses identified HINb2 as the most prominent HIN protein in starchy endosperm at ≥20 dap. Additionally, our quantification data revealed a poor correlation between transcript and protein levels of HINs in subaleurone during development. Here, we correlated data achieved by RT-qPCR, proteomics, and microscopy that reveal different expression and localization pattern of HINs in each layer during barley endosperm development. This indicates a contribution of each tissue to the regulation of HINs during grain filling. The effect of the high protein abundance of HINs in the starchy endosperm and their localization at the periphery of starch granules at late development stages at the cereal-based end-product quality is discussed. Understanding the spatio-temporal regulated HINs is essential to improve barley quality traits for high end-product quality, as hard texture of the barley grain is regulated by the ratio between HINb/HINa.

Genotypic Variation in Wheat Flour Lysophospholipids

Lysophospholipids (LPLs) are the most abundant polar lipids in wheat endosperm and naturally complex with amylose, affecting starch physicochemical properties. We analyzed LPLs in wheat flour from 58 cultivars which differ by grain hardness using liquid chromatography mass spectrometry (LCMS). There were significant differences in LPL content between cultivars, demonstrating that genotype rather than environment contributes most to the total variance in wheat endosperm LPLs. Polar lipids such as LPLs may play a role in grain hardness through their interaction with puroindoline proteins, however, no strong correlation between kernel hardness and LPLs was detected. This may reflect the location of LPLs within the starch granule as opposed to the puroindoline proteins outside starch granules. LPLs may have an indirect relationship with kernel hardness as they could share the same origin as polar lipids that interact with puroindoline on the starch granule surface.

Bread wheat milling behavior: effects of genetic and environmental factors, and modeling using grain mechanical resistance traits

Genetic (Pinb-D1 alleles) and environment (through vitreousness) have important effects on bread wheat milling behavior. SKCS optimal values corresponding to soft vitreous or hard mealy grains were defined to obtain the highest total flour yield. Near-isogenic lines of bread wheat that differ in hardness, due to distinct puroindoline-b alleles (the wild type, Pinb-D1a, or the mutated forms, Pinb-D1b or Pinb-D1d), were grown in different environments and under two nitrogen fertilization levels, to study genetic and environmental effects on milling behavior. Milling tests used a prototype mill, equipped with two break steps, one sizing step, and two reduction steps, and this enabled 21 individual or aggregated milling fractions to be collected. Four current grain characters, thousand grain weight, test weight, grain diameter, and protein content, were measured, and three characters known to influence grain mechanical resistance, NIRS hardness, SKCS hardness index, and grain vitreousness (a character affecting the grain mechanical behavior but generally not studied). As expected, the wild type or mutated forms of Pinb-D1 alleles led to contrasted milling behavior: soft genotypes produced high quantities of break flour and low quantities of reduction flour, whereas reverse quantities were observed for hard genotypes. This different milling behavior had only a moderate influence on total flour production. NIRS hardness and vitreousness were, respectively, the most important and the second most important grain characters to explain milling behavior. However, contrary to NIRS hardness, vitreousness was only involved in endosperm reduction and not in the separation between the starchy endosperm and the outer layers. The highest flour yields were obtained for SKCS values comprised between 30 and 50, which corresponded either to soft vitreous or hard mealy grains. Prediction equations were defined and showed a good accuracy estimating break and reduction flours portions, but should be used more cautiously for total flour.

Artificial MicroRNA-Based Specific Gene Silencing of Grain Hardness Genes in Polyploid Cereals Appeared to Be Not Stable Over Transgenic Plant Generations

Gene silencing by RNA interference is a particularly important tool in the study of gene function in polyploid cereal species for which the collections of natural or induced mutants are very limited. Previously we have been testing small interfering RNA-based approach of gene silencing in wheat and triticale. In this research, artificial microRNAs (amiRs) were studied in the same species and the same target genes to compare effectiveness of both gene silencing pathways. amiR cassettes were designed to silence Puroindoline a (Pina) and Puroindoline b (Pinb) hardness genes in wheat and their orthologues Secaloindoline a (Sina) and Secaloindoline b (Sinb) genes in triticale. Each of the two cassettes contained 21 nt microRNA (miR) precursor derived from conserved regions of Pina/Sina or Pinb/Sinb genes, respectively. Transgenic plants were obtained with high efficiency in two cultivars of wheat and one cultivar of triticale after using the Pinb-derived amiR vector for silencing of Pinb or Sinb, respectively. Lack of transgenic plants in wheat or very low transformation efficiency in triticale was observed using the Pina-derived amiR cassette, despite large numbers of embryos attempted. Silencing of Pinb in wheat and Sinb in triticale was highly efficient in the T1 generation. The transcript level of Pinb in wheat was reduced up to 92% and Sinb in triticale was reduced up to 98%. Moreover, intended silencing of Pinb/Sinb with Pinb-derived amiR cassette was highly correlated with simultaneous silencing of Pina/Sina in the same transgenic plants. High downregulation of Pinb/Pina genes in T1 plants of wheat and Sinb/Sina genes in T1 plants of triticale was associated with strong expression of Pinb-derived amiR. Silencing of the target genes correlated with increased grain hardness in both species. Total protein content in the grains of transgenic wheat was significantly lower. Although, the Pinb-derived amiR cassette was stably inherited in the T2 generation of wheat and triticale the silencing effect including strongly decreased expression of silenced genes as well as strong expression of Pinb-derived amiR was not transmitted. Advantages and disadvantages of posttranscriptional silencing of target genes by means of amiR and siRNA-based approaches in polyploid cereals are discussed.

Production of bioactive wheat puroindoline proteins in Nicotiana benthamiana using a virus-based transient expression system

The emergence of antibiotic resistant pathogenic strains of bacteria has necessitated the development of novel antimicrobial agents. The puroindoline A and B (PINA and PINB) proteins of wheat, well-known for their roles in determining the important phenotype of grain texture, are also antimicrobial, making them attractive as natural bio-control agents. However, the biochemical basis of PIN functionality remains unclear due to limitations in expressing them at the required yield and purity and lack of accurate tertiary structure. This study focussed on rapid transient expression of PINs targeted to different subcellular compartments (chloroplast, apoplast, endoplasmic reticulum and cytosol) of Nicotiana benthamiana leaf cells using the deconstructed tobacco mosaic virus-based 'magnICON®' system. The expressed recombinant PINs were characterised by Western blot using the Durotest anti-friabilin antibody, enzyme-linked immunosorbent assays (ELISA) and antimicrobial activity tests. Maximum yield of the His-tagged PINs occurred when targeted to the chloroplast. Both PINs exhibited oligomeric and monomeric forms on gels, but Western blots with the widely used Durotest anti-friabilin antibody identified only oligomeric forms. Only the PINs purified by a hydrophobic interaction method exhibited monomeric forms with the anti-His tag antibody, indicating correct folding. Interestingly, the Durotest antibody did not bind to monomers, suggesting their epitope may be obscured. PINs purified by His-tag affinity purification under native conditions or by the hydrophobic method exhibited antimicrobial activities. The successful in planta expression and optimisation of purification will enable future studies to examine the detailed structure of the PINs and explore novel bio-control applications in health, food and/or agriculture.

Changes in the starch-protein interface depending on common wheat grain hardness revealed using atomic force microscopy

The atomic force microscope tip was used to progressively abrade the surface of non-cut starch granules embedded in the endosperm protein matrix in grain sections from wheat near-isogenic lines differing in the puroindoline b gene and thus, hardness. In the hard near-isogenic wheat lines, starch granules exhibited two distinct profiles corresponding either to abrasion in the surrounding protein layer or the starch granule. An additional profile, only identified in soft lines, revealed a marked stop in the abrasion at the protein-starch transition similar to a lipid interface playing a lubricant role. It was related to the presence of both wild-type puroindolines, already suggested to act at the starch-protein interface through their association with polar lipids. This study revealed, for the first time, in situ differences in the nano-mechanical properties at the starch-protein interface in the endosperm of wheat grains depending on the puroindoline allelic status.

Relationships between wheat grain physical characteristics studied through near-isogenic lines with distinct puroindoline-b allele

Genetic (different forms of puroindoline-b) and environment (through variations in vitreousness), have important effects on wheat grain mechanical properties. The two methods of hardness measurements (NIRS, SKCS) do not give the same information. Bread wheat near-isogenic lines differing in hardness, due to distinct puroindoline-b alleles (the wild type, Pinb-D1a, or the mutated forms, Pinb-D1b or Pinb-D1d), were grown for three years in seven sites and under two nitrogen fertilization levels, to study genetic and environmental effects on grain mechanical properties. Two methods, Near-Infrared Reflectance Spectroscopy (NIRS) and Single Kernel Characterization System (SKCS), currently used for grain hardness characterization, were carried out. Grain vitreousness, which is known to affect the grain mechanical behavior but is generally not studied, was also measured, as well as three other characters (Thousand Grain Weight, Test Weight and protein content). The relationships between the different characters were studied. Results revealed a clear effect of the different Pinb-D1 alleles on NIRS hardness, and a marked impact of the environmental conditions on vitreousness. SKCS hardness was influenced by both Pinb-D1 alleles and environmental conditions. The relationship between SKCS and NIRS hardness was strong when considering together soft and hard genotypes, but moderate within a class of genetical hardness. Vitreousness had only a weak effect on NIRS hardness, whereas vitreousness and SKCS values were strongly correlated, with two distinct regressions for soft and hard genotypes. Vitreousness was positively related to protein content, especially in the case of hard genotypes, which were able to reach high vitreousness values never observed for soft genotypes.

De Novo Assembly and Transcriptome Analysis of Wheat with Male Sterility Induced by the Chemical Hybridizing Agent SQ-1

Wheat (Triticum aestivum L.), one of the world's most important food crops, is a strictly autogamous (self-pollinating) species with exclusively perfect flowers. Male sterility induced by chemical hybridizing agents has increasingly attracted attention as a tool for hybrid seed production in wheat; however, the molecular mechanisms of male sterility induced by the agent SQ-1 remain poorly understood due to limited whole transcriptome data. Therefore, a comparative analysis of wheat anther transcriptomes for male fertile wheat and SQ-1-induced male sterile wheat was carried out using next-generation sequencing technology. In all, 42,634,123 sequence reads were generated and were assembled into 82,356 high-quality unigenes with an average length of 724 bp. Of these, 1,088 unigenes were significantly differentially expressed in the fertile and sterile wheat anthers, including 643 up-regulated unigenes and 445 down-regulated unigenes. The differentially expressed unigenes with functional annotations were mapped onto 60 pathways using the Kyoto Encyclopedia of Genes and Genomes database. They were mainly involved in coding for the components of ribosomes, photosynthesis, respiration, purine and pyrimidine metabolism, amino acid metabolism, glutathione metabolism, RNA transport and signal transduction, reactive oxygen species metabolism, mRNA surveillance pathways, protein processing in the endoplasmic reticulum, protein export, and ubiquitin-mediated proteolysis. This study is the first to provide a systematic overview comparing wheat anther transcriptomes of male fertile wheat with those of SQ-1-induced male sterile wheat and is a valuable source of data for future research in SQ-1-induced wheat male sterility.

Lipid partitioning in maize (Zea mays L.) endosperm highlights relationships among starch lipids, amylose, and vitreousness

Content and composition of maize endosperm lipids and their partition in the floury and vitreous regions were determined for a set of inbred lines. Neutral lipids, i.e., triglycerides and free fatty acids, accounted for more than 80% of endosperm lipids and are almost 2 times higher in the floury than in the vitreous regions. The composition of endosperm lipids, including their fatty acid unsaturation levels, as well as their distribution may be related to metabolic specificities of the floury and vitreous regions in carbon and nitrogen storage and to the management of stress responses during endosperm cell development. Remarkably, the highest contents of starch lipids were observed systematically within the vitreous endosperm. These high amounts of starch lipids were mainly due to lysophosphatidylcholine and were tightly linked to the highest amylose content. Consequently, the formation of amylose-lysophosphatidylcholine complexes has to be considered as an outstanding mechanism affecting endosperm vitreousness.

Sina and Sinb genes in triticale do not determine grain hardness contrary to their orthologs Pina and Pinb in wheat

BACKGROUND

Secaloindoline a (Sina) and secaloindoline b (Sinb) genes of hexaploid triticale (x Triticosecale Wittmack) are orthologs of puroindoline a (Pina) and puroindoline b (Pinb) in hexaploid wheat (Triticum aestivum L.). It has already been proven that RNA interference (RNAi)-based silencing of Pina and Pinb genes significantly decreased the puroindoline a and puroindoline b proteins in wheat and essentially increased grain hardness (J Exp Bot 62:4025-4036, 2011). The function of Sina and Sinb in triticale was tested by means of RNAi silencing and compared to wheat.

RESULTS

Novel Sina and Sinb alleles in wild-type plants of cv. Wanad were identified and their expression profiles characterized. Alignment with wheat Pina-D1a and Pinb-D1a alleles showed 95% and 93.3% homology with Sina and Sinb coding sequences. Twenty transgenic lines transformed with two hpRNA silencing cassettes directed to silence Sina or Sinb were obtained by the Agrobacterium-mediated method. A significant decrease of expression of both Sin genes in segregating progeny of tested T1 lines was observed independent of the silencing cassette used. The silencing was transmitted to the T4 kernel generation. The relative transcript level was reduced by up to 99% in T3 progeny with the mean for the sublines being around 90%. Silencing of the Sin genes resulted in a substantial decrease of secaloindoline a and secaloindoline b content. The identity of SIN peptides was confirmed by mass spectrometry. The hardness index, measured by the SKCS (Single Kernel Characterization System) method, ranged from 22 to 56 in silent lines and from 37 to 49 in the control, and the mean values were insignificantly lower in the silent ones, proving increased softness. Additionally, the mean total seed protein content of silenced lines was about 6% lower compared with control lines. Correlation coefficients between hardness and transcript level were weakly positive.

CONCLUSIONS

We documented that RNAi-based silencing of Sin genes resulted in significant decrease of their transcripts and the level of both secaloindoline proteins, however did not affect grain hardness. The unexpected, functional differences of Sin genes from triticale compared with their orthologs, Pin of wheat, are discussed.

The antimicrobial domains of wheat puroindolines are cell-penetrating peptides with possible intracellular mechanisms of action

The puroindoline proteins (PINA and PINB) of wheat display lipid-binding properties which affect the grain texture, a critical parameter for wheat quality. Interestingly, the same proteins also display antibacterial and antifungal properties, attributed mainly to their Tryptophan-rich domain (TRD). Synthetic peptides based on this domain also display selectivity towards bacterial and fungal cells and do not cause haemolysis of mammalian cells. However, the mechanisms of these activities are unclear, thus limiting our understanding of the in vivo roles of PINs and development of novel applications. This study investigated the mechanisms of antimicrobial activities of synthetic peptides based on the TRD of the PINA and PINB proteins. Calcein dye leakage tests and transmission electron microscopy showed that the peptides PuroA, Pina-M and Pina-W→F selectively permeabilised the large unilamellar vesicles (LUVs) made with negatively charged phospholipids mimicking bacterial membranes, but were ineffective against LUVs made with zwitterionic phospholipids mimicking eukaryotic membranes. Propidium iodide fluorescence tests of yeast (Saccharomyces cerevisiae) cells showed the peptides were able to cause loss of membrane integrity, PuroA and Pina-M being more efficient. Scanning electron micrographs of PINA-based peptide treated yeast cells showed the formation of pits or pores in cell membranes and release of cellular contents. Gel retardation assays indicated the peptides were able to bind to DNA in vitro, and the induction of filamental growth of E. coli cells indicated in vivo inhibition of DNA synthesis. Together, the results strongly suggest that the PIN-based peptides exert their antimicrobial effects by pore formation in the cell membrane, likely by a carpet-like mechanism, followed by intracellular mechanisms of activity.