Two plant puroindolines colocalize in wheat seed and in vitro synergistically fight against pathogens

Antifungal activity of puroindoline protein from soft wheat against grain molds and its potential as a biocontrol agent

Mold growth reduces the quality of stored grains, besides producing toxins that pose a potential threat to human health. Therefore, prevention of grain mold growth during storage is important to ensure a safe and high-quality product, preferably using an eco-friendly antifungal agent. The Puroindoline (PIN) protein was extracted by Triton X-114, and identified by QE mass spectrometry. A. flavus has attracted much attention because of its toxic secondary metabolites, and PIN protein showed a significant inhibition on A. flavus growth. Scanning electron microscopy revealed altered spore morphology of A. flavus following PIN protein treatment, and propidium iodide staining showed incomplete spore cell membranes. The disruption and deformation of A. flavus spores suggest that the cell walls and cell membranes were compromised. Decreased mitochondrial membrane potential and increased levels of intracellular reactive oxygen species (ROS) were detected using JC-1 and 2,7-dichlorodihydrofluorescein diacetate staining, respectively. PIN protein could effectively inhibit the growth and aflatoxins B1 production of A. flavus in stored grains, such as wheat and rice. PIN proteins can inhibit the growth of many common grain storage molds, including Penicillium, Aspergillus spp. (A. flavus, A. glaucus, A. kawachii, A. ochraceus, A. niger), Alternaria, and Fusarium graminearum, in a dose-dependent manner. PIN protein has a significant inhibitory effect on the growth of grain molds, with a stronger inhibitory effect noted in wheat and rice. Our study provides a novel and simple theoretical basis for the selection and storage of mold-resistant in grains and food during storage.

Inhibitory Effect of Puroindoline Peptides on Campylobacter jejuni Growth and Biofilm Formation

Puroindolines are small, amphipathic, wheat proteins that determine the hardness of the wheat kernel and protect crops from different pathogens. Puroindoline A (PinA) and puroindoline B (PinB) are two major isoforms of puroindolines. These proteins have antibacterial and antifungal properties mainly attributed to their characteristic tryptophan-rich domains (TRDs). In this in vitro study, we investigated the antimicrobial effect of PinA and PinB synthetic peptides against the growth and biofilm formation of Campylobacter jejuni. C. jejuni is an important microaerobic, foodborne pathogen that causes gastrointestinal and neurological diseases in humans. Our results showed that: (1) PinA, but not PinB, has strong antimicrobial activity against C. jejuni clinical strains 81-176 and F38011, Escherichia coli O157:H7, methicillin-resistant Staphylococcus aureus, Salmonella enterica serovar Typhimurium, and Listeria monocytogenes; (2) The substitution of two tryptophan residues to glycine (W→G) in the TRD of PinA abolishes its antimicrobial activity against these microorganisms; (3) PinA functions additively with two common antibiotics (ciprofloxacin and erythromycin) to inhibit or inactivate C. jejuni strains; (4) PinA damages the C. jejuni cellular membrane, (5) PinA is cytotoxic to human INT 407 cells at high concentrations; and (6) PinA inhibits C. jejuni biofilm formation. In summary, this study demonstrates the antimicrobial activity of PinA against C. jejuni growth and biofilm formation and further confirms the potential use of PinA as a therapeutic agent in health care or as preservatives in the agri-food industry.

Antifungal Effects of Fusion Puroindoline B on the Surface and Intracellular Environment of Aspergillus flavus

Aspergillus flavus infection is a major issue for safe food storage. In this study, we constructed an efficient prokaryotic expression system for puroindoline B (PINB) protein to detect its antifungal activity. The Puroindoline b gene was cloned into pET-28a (+) vector and expressed in Escherichia coli. Treatment with fusion PINB revealed that it inhibits mycelial growth of A. flavus, a common grain mold. Moreover, fusion PINB-treated A. flavus mycelium withered and exhibited a sunken spore head. As fusion PINB concentration increased, electrical conductivity in mycelium also increased, indicative of cell membrane damage. Furthermore, intracellular malate dehydrogenase and succinate dehydrogenase activity decreased, revealing a disruption in the tricarboxylic acid cycle. Moreover, the dampened activity of the ion pump Na⁺K⁺-ATPase negatively affected the intracellular regulation of both ions. Catalase and superoxide dismutase activity decreased, thus reducing antioxidant capacity, a result confirmed with an increase in malondialdehyde content. Changes to the GSH/GSSG ratio indicated a shift to an intracellular oxidative state. At the same time, laser scanning confocal microscopy assay showed the accumulation of reactive oxygen species and nuclear damage. Therefore, the PINB fusion protein may have the potential to control A. flavus in grain storage and food preservation.

Evaluation of Anthocyanin Content, Antioxidant Potential and Antimicrobial Activity of Black, Purple and Blue Colored Wheat Flour and Wheat-Grass Juice against Common Human Pathogens

The present study aimed to analyze the antioxidant and antimicrobial activity of anthocyanins extracted from colored wheat flour and wheat-grass juice against human pathogens. The total anthocyanin content and antioxidant potential in colored wheat flour and wheat-grass juice extracts were significantly higher than white flour and wheat-grass juice extracts. Ultra-performance liquid chromatography showed the maximum number of anthocyanin peaks in black wheat, with delphinidin-3-o-galactoside chloride, delphinidin-3-o-glucoside chloride, and cyanindin-3-o-glucoside chloride as the major contributors. Among flour extracts, maximum zones of inhibition against Staphylococcus aureus (MTCC 1934), Pseudomonas aeruginosa (MTCC 1434), Escherichia coli, and Candida albicans (MTCC 227) were produced by black flour extract, having the highest anthocyanin content. It exhibited a minimum microbicidal concentration (MMC) of 200 mg/mL against E. coli and C. albicans; and 100 and 150 mg/mL against S. aureus and P. aeruginosa, respectively. Black and purple flour extracts exhibited a minimum inhibitory concentration (MIC) of 50 mg/mL against S. aureus and P. aeruginosa. White flour extracts did not show MMC against E. coli and C. albicans. Among wheat-grass juice extracts, black wheat-grass was most effective and showed an MIC of 100-150 mg/mL against all pathogens. It exhibited an MMC of 200 mg/mL against S. aureus and P. aeruginosa. Hence, anthocyanin-rich colored wheat could be of nutraceutical importance.

Expression and purification of recombinant puroindoline A protein in Escherichia coli and its antifungal effect against Aspergillus flavus

Aspergillus flavus is the main cause of postharvest agricultural commodity loss. In this study, puroindoline A (PINA) protein was expressed in Escherichia coli, purified, and its antifungal properties against A. flavus were characterized. Sodium dodecyl sulfate polyacrylamide gel electrophoresis showed that the molecular weight of the recombinant PINA protein was approximately 44 kDa. PINA exerted a powerful antifungal effect against A. flavus at 42.42 μg/mL on potato dextrose agar culture medium. Flow cytometry and scanning electron microscopy revealed that the spore morphology was damaged by PINA exposure; spores were depressed and broken, suggesting that the cell wall was impaired. Transmission electron microscopy and propidium iodide staining illustrated significant changes in intracellular spore structure, indicating cell membrane damage. 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolocarbocyanine iodide staining indicated decreased mitochondrial membrane potential. Large nuclear condensation and DNA fragmentation were detected by 4',6-diamidino-2-phenylindole staining. The expression of genes related to the cell wall, cell membrane, and spore germination significantly changed in PINA-treated cells; this illustrated the probable mode of PINA action on A. flavus through cell wall destruction and triggered cell membrane, mitochondrial, and DNA damage leading to cell death. The antifungal mechanism of wheat PINA protein on A. flavus has been demonstrated in this study, and has potential application in preventing postharvest loss in the agricultural industry.

The antimicrobial properties of the puroindolines, a review

Antimicrobial proteins, and especially antimicrobial peptides (AMPs) hold great promise in the control of animal and plant diseases with low risk of pathogen resistance. The two puroindolines, a and b, from wheat control endosperm softness of the wheat caryopsis (grain), but have also been shown to inhibit the growth and kill various bacteria and fungi, while showing little toxicity to erythrocytes. Puroindolines are small (~ 13 kDa) amphipathic proteins with a characteristic tryptophan-rich domain (TRD) that is part of an 18 or 19 amino acid residue loop subtended by a disulfide bond. This review presents a brief history of the puroindolines, their physical-chemical characteristics, their interaction with lipids and membranes, and their activity as antimicrobial proteins and AMPs. In this latter context, the use of the TRDs of puroindoline a and b in puroindoline AMP function is reviewed. The activity of puroindoline a and b and their AMPs appear to act through similar but somewhat different modes, which may involve membrane binding, membrane disruption and ion channel formation, and intra-cellular nucleic acid binding and metabolic disruption. Natural and synthetic mutants have identified key elements of the puroindolines for antimicrobial activity.

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.

Exotic QTL improve grain quality in the tri-parental wheat population SW84

Developing the tri-parental exotic wheat population SW84

Genetic diversity of cultivated wheat was markedly reduced, first, during domestication and, second, since the onset of modern elite breeding. There is an increasing demand for utilizing genetic resources to increase genetic diversity and, simultaneously, to improve agronomic performance of cultivated wheat. To locate favorable effects of exotic wheat alleles, we developed the tri-parental wheat population SW84. The population was derived from crossing the hexaploid spring wheat cultivars Triso and Devon with one synthetic exotic donor accession, Syn084L, followed by two rounds of backcrossing and three rounds of selfing. SW84 consists of 359 BC₂F₄ lines, split into two families, D84 (Devon*Syn084L) and T84 (Triso*Syn084L).

Studying the genetic control of grain quality in SW84

As a case study, grain quality of SW84 was studied in replicated field trials. Transgressive segregation was observed for all studied grain quality traits by evaluating SW84 for two years at two locations under low and high nitrogen supply. Subsequently, a genome-wide association study (GWAS) was carried out based on genomic data derived from a 90k Infinium iSELECT single nucleotide polymorphism (SNP) array. In total, GWAS yielded 37 marker-trait associations, summarized to 16 quantitative trait loci (QTL). These SNPs indicate genetic regulators of grain protein content, grain hardness, sedimentation value and sedimentation ratio. The majority of exotic QTL alleles (75%) exerted favorable effects, increasing grain protein content and sedimentation value in ten and two cases, respectively. For instance, two exotic QTL alleles were associated with a substantial increase of grain protein content and sedimentation value by 1.09% and 7.31 ml, respectively. This finding confirms the potential of exotic germplasm to improve grain quality in cultivated wheat. So far, the molecular nature of most of the detected QTL is unknown. However, two QTL correspond to known genes controlling grain quality: The major QTL on chromosome 6B, increasing grain protein content by 0.70%, on average, co-localizes with the NAM-B1 gene, known to control grain protein content as well as iron and zinc content. Likewise, the major QTL on chromosome 5D, reducing grain hardness by 8.98%, on average, co-localizes with the gene for puroindoline b (Pinb-D1) at the Ha locus. In total, 13 QTL were detected across families, whereas one and three QTL were exclusively detected in families D84 and T84, respectively. Likewise, ten QTL were detected across nitrogen treatments, whereas one and five QTL were exclusively detected under low and high N treatments, respectively. Our data indicate that most effects in SW84 act across families and N levels. Merging of data from two families or two N treatments may, thus, be considered in association studies to increase sample size and, as a result, QTL detection power.

Utilizing favorable exotic QTL alleles in wheat breeding

Our study serves as a model how favorable exotic QTL alleles can be located in exotic germplasm of wheat. In future, the localized favorable exotic QTL alleles will be utilized in wheat breeding programs to simultaneously improve grain quality and selectively expand genetic diversity of the elite wheat gene pool.

Functional gold nanoparticle-based antibacterial agents for nosocomial and antibiotic-resistant bacteria

AIM

Medical treatments for bacterial-infections have become challenging because of the emergence of antibiotic-resistant bacterial strains. Thus, new therapeutics and antibiotics must be developed.

MATERIALS & METHODS

Arginine and tryptophan can target negatively-charged bacteria and penetrate bacterial cell membrane, respectively. Synthetic-peptides containing arginine, tryptophan and cysteine termini, in other words, (DVFLG)2REEW4C and (DVFLG)2REEW2C, as starting materials were mixed with aqueous tetrachloroauric acid to generate peptide-immobilized gold nanoparticles (i.e., [DVFLG]2REEW4C-AuNPs and [DVFLG]2REEW2C-AuNPs) through one-pot reactions.

RESULTS & DISCUSSION

The peptide immobilized AuNPs exhibit targeting capacity and antibacterial activity. Furthermore, (DVFLG)2REEW4C-AuNPs immobilized with a higher number of tryptophan molecules possess more effective antibacterial capacity than (DVFLG)2REEW2C-AuNPs. Nevertheless, they are not harmful for animal cells. The feasibility of using the peptide-AuNPs to inhibit the cell growth of bacterium-infected macrophages was demonstrated.

CONCLUSION

These results suggested that the proposed antibacterial AuNPs are effective antibacterial agents for Staphylococci, Enterococci and antibiotic-resistant bacterial strains. [Formula: see text].

Anti-biofilm and sporicidal activity of peptides based on wheat puroindoline and barley hordoindoline proteins

The broad-spectrum activity of antimicrobial peptides (AMPs) and low probability of development of host resistance make them excellent candidates as novel bio-control agents. A number of AMPs are found to be cationic, and a small proportion of these are tryptophan-rich. The puroindolines (PIN) are small, basic proteins found in wheat grains with proposed roles in biotic defence of seeds and seedlings. Synthetic peptides based on their unique tryptophan-rich domain (TRD) display antimicrobial properties. Bacterial endospores and biofilms are highly resistant cells, with significant implications in both medical and food industries. In this study, the cationic PIN TRD-based peptides PuroA (FPVTWRWWKWWKG-NH2 ) and Pina-M (FSVTWRWWKWWKG-NH2 ) and the related barley hordoindoline (HIN) based Hina (FPVTWRWWTWWKG-NH2 ) were tested for effects on planktonic cells and biofilms of the common human pathogens including Pseudomonas aeruginosa, Listeria monocytogenes and the non-pathogenic Listeria innocua. All peptides showed significant bactericidal activity. Further, PuroA and Pina-M at 2 × MIC prevented initial biomass attachment by 85-90% and inhibited >90% of 6-h preformed biofilms of all three organisms. However Hina, with a substitution of Lys-9 with uncharged Thr, particularly inhibited Listeria biofilms. The PIN based peptides were also tested against vegetative cells and endospores of Bacillus subtilis. The results provided evidence that these tryptophan-rich peptides could kill B. subtilis even in sporulated state, reducing the number of viable spores by 4 log units. The treated spores appeared withered under scanning electron microscopy. The results establish the potential of these tryptophan-rich peptides in controlling persistent pathogens of relevance to food industries and human health. Copyright © 2016 European Peptide Society and John Wiley & Sons, Ltd.

Wheat seed embryo excision enables the creation of axenic seedlings and Koch's postulates testing of putative bacterial endophytes

Early establishment of endophytes can play a role in pathogen suppression and improve seedling development. One route for establishment of endophytes in seedlings is transmission of bacteria from the parent plant to the seedling via the seed. In wheat seeds, it is not clear whether this transmission route exists, and the identities and location of bacteria within wheat seeds are unknown. We identified bacteria in the wheat (Triticum aestivum) cv. Hereward seed environment using embryo excision to determine the location of the bacterial load. Axenic wheat seedlings obtained with this method were subsequently used to screen a putative endophyte bacterial isolate library for endophytic competency. This absence of bacteria recovered from seeds indicated low bacterial abundance and/or the presence of inhibitors. Diversity of readily culturable bacteria in seeds was low with 8 genera identified, dominated by Erwinia and Paenibacillus. We propose that anatomical restrictions in wheat limit embryo associated vertical transmission, and that bacterial load is carried in the seed coat, crease tissue and endosperm. This finding facilitates the creation of axenic wheat plants to test competency of putative endophytes and also provides a platform for endophyte competition, plant growth, and gene expression studies without an indigenous bacterial background.

Role of Lipid Composition on the Interaction between a Tryptophan-Rich Protein and Model Bacterial Membranes

The interaction between tryptophan-rich puroindoline proteins and model bacterial membranes at the air-liquid interface has been investigated by FTIR spectroscopy, surface pressure measurements, and Brewster angle microscopy. The role of different lipid constituents on the interactions between lipid membrane and protein was studied using wild type (Pin-b) and mutant (Trp44 to Arg44 mutant, Pin-bs) puroindoline proteins. The results show differences in the lipid selectivity of the two proteins in terms of preferential binding to specific lipid head groups in mixed lipid systems. Pin-b wild type was able to penetrate mixed layers of phosphatidylethanolamine (PE) and phosphatidylglycerol (PG) head groups more deeply compared to the mutant Pin-bs. Increasing saturation of the lipid tails increased penetration and adsorption of Pin-b wild type, but again the response of the mutant form differed. The results provide insight as to the role of membrane architecture, lipid composition, and fluidity on antimicrobial activity of proteins. Data show distinct differences in the lipid binding behavior of Pin-b as a result of a single residue mutation, highlighting the importance of hydrophobic and charged amino acids in antimicrobial protein and peptide activity.

Biological activity of lactoferrin-functionalized biomimetic hydroxyapatite nanocrystals

The emergence of bacterial strains resistant to antibiotics is a general public health problem. Progress in developing new molecules with antimicrobial properties has been made. In this study, we evaluated the biological activity of a hybrid nanocomposite composed of synthetic biomimetic hydroxyapatite surface-functionalized by lactoferrin (LF-HA). We evaluated the antimicrobial, anti-inflammatory, and antioxidant properties of LF-HA and found that the composite was active against both Gram-positive and Gram-negative bacteria, and that it modulated proinflammatory and anti-inflammatory responses and enhanced antioxidant properties as compared with LF alone. These results indicate the possibility of using LF-HA as an antimicrobial system and biomimetic hydroxyapatite as a candidate for innovative biomedical applications.

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.

Wheat (Triticum aestivum L. and T. turgidum L. ssp. durum) Kernel Hardness: II. Implications for End-Product Quality and Role of Puroindolines Therein

 Wheat kernel hardness is a major quality characteristic used in classifying wheat cultivars. Differences in endosperm texture among Triticum aestivum L. or between T. aestivum and T. turgidum L. ssp. durum cultivars profoundly affect their milling behavior, the properties of the obtained flour or semolina particles, as well as the quality of products made thereof. It is now widely accepted that the presence, sequence polymorphism, or absence of the basic and cysteine-rich puroindolines a and b are responsible for differences in endosperm texture. These proteins show features in vitro, including foaming and lipid-binding properties, which provide them with a potential impact in the production of wheat-based food products, where they may improve gas cell stabilization or modulate interactions between starch, proteins, and/or lipids. We here summarize the impact of wheat hardness on milling properties and bread, cookie, cake, and pasta quality and discuss the role of puroindolines therein.

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

Wheat hardness has major consequences for the entire wheat supply chain from breeders and millers over manufacturers to, finally, consumers of wheat-based products. Indeed, differences in hardness among Triticum aestivum L. or between T. aestivum L. and T. turgidum L. ssp. durum wheat cultivars determine not only their milling properties, but also the properties of flour or semolina endosperm particles, their preferential use in cereal-based applications, and the quality of the latter. Although the mechanism causing differences in wheat hardness has been subject of research more than once, it is still not completely understood. It is widely accepted that differences in wheat hardness originate from differences in the interaction between the starch granules and the endosperm protein matrix in the kernel. This interaction seems impacted by the presence of either puroindoline a and/or b, polar lipids on the starch granule surface, or by a combination of both. We focus here on wheat hardness and its relation to the presence of puroindolines and polar lipids. More in particular, the structure, properties, and genetics of puroindolines and their interactions with polar lipids are critically discussed as is their possible role in wheat hardness. We also address future research needs as well as the presence of puroindoline-type proteins in other cereals.

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.

Role of non-prolamin proteins and low molecular weight redox agents in protein folding and polymerization in wheat grains and influence on baking quality parameters

The various enzyme systems and low molecular weight (LMW) redox agents are related to the folding and polymerization of prolamins in the ripening wheat grains and the formation of baking quality. Protein disulfide isomerases (PDIs) and cyclophylins accelerate "correct" folding of prolamins, which is most likely necessary for the subsequent formation of the macromolecular structure of the gluten protein matrix. PDIs are also involved in the polymerization of prolamins, catalyzing the oxidation of protein sulfhydryl groups. Molecular chaperone binding BiP protein facilitates folding of prolamins, with its role increasing in the stressful conditions. Reducing systems of thioredoxin and glutaredoxin, LMW redox pairs GSH/GSSG and Asc/DHAsc, thiol oxidases, and lipoxygenases (LOXs) regulate redox balance and the rate of polymerization of prolamins at the different stages of grain ripening. Additionally, LOX is probably involved in the protein-starch-lipid interactions between the starch granule and the protein matrix, mediated by puroindolines, determining the formation of grain texture. It is assumed that the high variability of baking quality in different environmental conditions is due to the interaction of labile enzyme systems with the storage proteins in the developing wheat caryopsis.

Expression, purification and antimicrobial activity of puroindoline A protein and its mutants

Wheat puroindoline proteins, PINA and PINB, play key roles in determining wheat grain hardness as well as in defending the plant against pathogens. PINA has much greater membrane-binding property and antimicrobial activity because it contains more tryptophan residues in the unique tryptophan-rich domain (TRD). In order to obtain proteins with higher antimicrobial activity, mutants of PINA containing two or three copies of TRD, designated ABBC and ABBBC, respectively, were constructed and expressed in E. coli Rosetta-gami (DE3). Metal affinity chromatography was used to purify the soluble affinity-tagged recombinant proteins. The secondary structures of the recombinant proteins were predicted by the online program Protein Homology/analog Y Recognition Engine v2.0 and experimentally assessed using circular dichroism. Minimum inhibition concentration tests and fluorescence microscope analyses were employed to evaluate the antimicrobial activities of the mutants. The results showed that the purified recombinant ABBC was correctly folded and presented significantly higher antimicrobial activities against E. coli and S. aureus than wild-type PINA, suggesting its potential use as an antimicrobial agent. The results also confirmed that TRD is a determinant of the antimicrobial activity of PINA and demonstrated that it is feasible to enhance the antimicrobial activity of PINA by adding one copy of TRD.

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.

Puroindoline-a, a lipid binding protein from common wheat, spontaneously forms prolate protein micelles in solution

The self-assembly in solution of puroindoline-a (Pin-a), an amphiphilic lipid binding protein from common wheat, was investigated by small angle neutron scattering, dynamic light scattering and size exclusion chromatography. Pin-a was found to form monodisperse prolate ellipsoidal micelles with a major axial radius of 112 ± 4.5 Å and minor axial radius of 40.4 ± 0.18 Å. These protein micelles were formed by the spontaneous self-assembly of 38 Pin-a molecules in solution and were stable over a wide pH range (3.5-11) and at elevated temperatures (20-65 °C). Pin-a micelles could be disrupted upon addition of the non-ionic surfactant dodecyl-β-maltoside, suggesting that the protein self-assembly is driven by hydrophobic forces, consisting of intermolecular interactions between Trp residues located within a well-defined Trp-rich domain of Pin-a.

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.

Experimental antibacterial therapy with puroindolines, lactoferrin and lysozyme in Listeria monocytogenes-infected mice

Puroindoline A and puroindoline B from plant seeds, bovine lactoferrin and chicken eggs lysozyme are antimicrobial proteins of innate immune system that lyse invading organisms. We investigate their potential antibacterial activity against Listeria monocytogenes in a mouse model. Bacteria were isolated from various organs for 7 days after challenge. Livers displayed consistently higher bacterial count (up to 10(7)cfu/g) than spleens, kidneys and brains. The efficacy of the AMPs was therefore established by measuring the infection level (cfu number) of these organs. Puroindoline A and puroindoline B (5mg/mouse), lactoferrin and lysozyme (1.25mg/mouse), intravenously injected individually, inhibited bacterial growth completely. Puroindoline A, puroindoline B and lactoferrin were effective when administered 24h before infection; lysozyme was effective at the time of infection or 5 days after. Their combined use resulted in the enhancement of individual antibacterial activities. Complete inhibition of bacterial growth was observed using concurrently 0.059mg/mouse of puroindoline A and 0.019mg/mouse of puroindoline B, lactoferrin and lysozyme. Individual antimicrobial proteins reduced significantly the expression level of pro-inflammatory cytokines (IL-6, IL-8, INF-gamma and TNF-alpha), acute phase proteins (C-reactive protein and fibrinogen) and the T lymphocyte antigens CD4, CD8a, CD8b and CD25. These results suggest their potential use for the control of L. monocytogenes infections.

The determinants of grain texture in cereals

Kernel hardness is an important agronomic trait that influences end-product properties. In wheat cultivars, this trait is determined by the Puroindoline a (Pina) and Puroindoline b (Pinb) genes, located in the Hardness locus (Ha) on chromosome 5DS of the D genome. Wild type alleles code puroindoline a (PINA) and puroindoline b (PINB) proteins, which form a 15-kDa friabilin present on the surface of water-washed starch granules. Both the proteins are accumulated in the starch endosperm cells and aleurone of the mature kernels. Puroindoline-like genes coding puroindoline-like proteins in the starch endosperm occur in some of the genomes of Triticeae and Aveneae cereals. Orthologs are present in barley, rye and oats. However, some genomes of these diploid and polyploid cereals, like that of Triticum turgidum var. durum (AABB) lack the puroindoline genes, having a very hard kernel texture. The two wild type alleles in opposition (dominant loci) control the soft phenotype. Mutation either in Pina or Pinb or in both leads to a medium-hard or hard kernel texture. The most frequent types of Pin mutations are point mutations within the coding sequence resulting in the substitution of a single amino acid or a null allele. The latter is the result of a frame shift determined by base deletion or insertion or a one-point mutation to the stop codon. The lipid-binding properties of the puroindolines affect not only the dough quality but also the plants' resistance to pathogens. Genetic modification of cereals with Puroindoline genes and/or their promoters enable more detailed functional analyses and the production of plants with the desired characteristics.

In planta mutagenesis determines the functional regions of the wheat puroindoline proteins

In planta analysis of protein function in a crop plant could lead to improvements in understanding protein structure/function relationships as well as selective agronomic or end product quality improvements. The requirements for successful in planta analysis are a high mutation rate, an efficient screening method, and a trait with high heritability. Two ideal targets for functional analysis are the Puroindoline a and Puroindoline b (Pina and Pinb, respectively) genes, which together compose the wheat (Triticum aestivum L.) Ha locus that controls grain texture and many wheat end-use properties. Puroindolines (PINs) together impart soft texture, and mutations in either PIN result in hard seed texture. Studies of the PINs' mode of action are limited by low allelic variation. To create new Pin alleles and identify critical function-determining regions, Pin point mutations were created in planta via EMS treatment of a soft wheat. Grain hardness of 46 unique PIN missense alleles was then measured using segregating F(2):F(3) populations. The impact of individual missense alleles upon PIN function, as measured by grain hardness, ranged from neutral (74%) to intermediate to function abolishing. The percentage of function-abolishing mutations among mutations occurring in both PINA and PINB was higher for PINB, indicating that PINB is more critical to overall Ha function. This is contrary to expectations in that PINB is not as well conserved as PINA. All function-abolishing mutations resulted from structure-disrupting mutations or from missense mutations occurring near the Tryptophan-rich region. This study demonstrates the feasibility of in planta functional analysis of wheat proteins and that the Tryptophan-rich region is the most important region of both PINA and PINB.

Molecular genetics of puroindolines and related genes: regulation of expression, membrane binding properties and applications

Kernel texture of wheat is a primary determinant of its technological properties. Soft kernel texture phenotype results when the Puroindoline a and Puroindoline b genes are present and encode the wild-type puroindolines PINA and PINB, respectively, and various mutations in either or both gene(s) result in hard phenotypes. A wealth of information is now available that furthers our understanding regarding the spatial and temporal regulation of expression of Puroindoline genes. Through the use of model membranes and synthetic peptides we also have a clearer understanding of the significance of the cysteine backbone, the tryptophan-rich domain (TRD) and the helicoid tertiary structures of PIN proteins in relation to their membrane-active properties. Many studies suggest individual yet co-operative modes of action of the PIN proteins in determining kernel texture, and significant evidence is accumulating that the proteins have in vivo and in vitro antimicrobial activities, shedding light on the biological roles of this unique ensemble of proteins. The puroindolines are now being explored for grain kernel texture modifications as well as antimicrobial activities.

Promoter analysis and immunolocalisation show that puroindoline genes are exclusively expressed in starchy endosperm cells of wheat grain

The purolindolines are small cysteine-rich proteins which are present in the grain of wheat. They have a major impact on the utilisation of the grain as they are the major determinants of grain texture, which affects both milling and baking properties. Bread and durum wheats were transformed with constructs comprising the promoter regions of the Puroindoline a (Pina) and Puroindoline b (Pinb) genes fused to the uidA (GUS) reporter gene. Nine lines showing 3:1 segregation for the transgene and comprising all transgene/species combinations were selected for detailed analysis of transgene expression during grain development. This showed that transgene expression occurred only in the starchy endosperm cells and was not observed in any other seed or vegetative tissues. The location of the puroindoline proteins in these cells was confirmed by tissue printing of developing grain, using a highly specific monoclonal antibody for detection and an antibody to the aleurone-localised 8S globulin as a control. This provides clear evidence that puroindolines are only synthesised and accumulated in the starchy endosperm cells of the wheat grain.

The promoter of the wheat puroindoline-a gene (PinA) exhibits a more complex pattern of activity than that of the PinB gene and is induced by wounding and pathogen attack in rice

Puroindolines form the molecular basis of wheat grain hardness. However, little is known about puroindoline gene regulation. We previously reported that the Triticum aestivum puroindoline-b gene (PinB) promoter directs beta-glucuronidase gene (uidA) seed-specific expression in transgenic rice. In this study, we isolated a puroindoline-a gene (PinA), analyzed PinA promoter activity by 5' deletions and compared PinA and PinB promoters in transgenic rice. Seeds of PinA-1214 and PinB-1063 transgenic plants strongly expressed uidA in endosperm, in the aleurone layer and in epidermis cells in a developmentally regulated manner. The GUS activity was also observed in PinA-1214 embryos. Whereas the PinB promoter is seed specific, the PinA promoter also directed, but to a lower level, uidA expression in roots of seedlings and in the vascular tissues of palea and pollen grains of dehiscent anthers during flower development. In addition, the PinA promoter was induced by wounding and by Magnaporthe grisea. By deletion analysis, we showed that the "390-bp" PinA promoter drives the same expression pattern as the "1214-bp" promoter. Moreover, the "214-bp" PinA promoter drives uidA expression solely in pollen grains of dehiscent anthers. The presence of putative cis-regulatory elements that may be related to PinA expression is discussed from an evolutionary point of view. By electrophoretic mobility shift assay, we showed that putative cis-elements (WUN-box, TCA motifs and as-1-like binding sites) whose presence in the PinA promoter may be related to wounding and/or the pathogen response form complexes with nuclear extracts isolated from wounded wheat leaves.

Tryptophan- and arginine-rich antimicrobial peptides: Structures and mechanisms of action

Antimicrobial peptides encompass a number of different classes, including those that are rich in a particular amino acid. An important subset are peptides rich in Arg and Trp residues, such as indolicidin and tritrpticin, that have broad and potent antimicrobial activity. The importance of these two amino acids for antimicrobial activity was highlighted through the screening of a complete combinatorial library of hexapeptides. These residues possess some crucial chemical properties that make them suitable components of antimicrobial peptides. Trp has a distinct preference for the interfacial region of lipid bilayers, while Arg residues endow the peptides with cationic charges and hydrogen bonding properties necessary for interaction with the abundant anionic components of bacterial membranes. In combination, these two residues are capable of participating in cation-pi interactions, thereby facilitating enhanced peptide-membrane interactions. Trp sidechains are also implicated in peptide and protein folding in aqueous solution, where they contribute by maintaining native and nonnative hydrophobic contacts. This has been observed for the antimicrobial peptide from human lactoferrin, possibly restraining the peptide structure in a suitable conformation to interact with the bacterial membrane. These unique properties make the Arg- and Trp-rich antimicrobial peptides highly active even at very short peptide lengths. Moreover, they lead to structures for membrane-mimetic bound peptides that go far beyond regular alpha-helices and beta-sheet structures. In this review, the structures of a number of different Trp- and Arg-rich antimicrobial peptides are examined and some of the major mechanistic studies are presented.

Cloning and expression of two plant proteins: similar antimicrobial activity of native and recombinant form

Antimicrobial peptides and proteins are being studied with increasing interest because of their broad range antimicrobial activity. Among plant antimicrobial proteins, the wheat seed polypeptides, puroindoline a and puroindoline b, are particularly interesting because of their established antibacterial activity. In this paper we describe different strategies used to clone His tagged and GST tagged puroindolines obtaining 1.5 mg recombinant protein from 1 l culture. The antimicrobial activity of recombinant and native puroindolines was comparable.