Early expression of grain hardness in the developing wheat endosperm

The endosperm microstructure, physical, thermal properties and specific milling energy of spelt (Triticum aestivum ssp. spelta) grain and flour

Previous research has shown that the endosperm microstructure and physical properties of grain have significance in grain processing and in the development of processing machines. The aim of our study was to analyze the endosperm microstructure, physical, thermal properties, and specific milling energy of organic spelt (Triticum aestivum ssp. spelta) grain and flour. Image analysis combined with fractal analysis was used to describe the microstructural differences of the endosperm of spelt grain. The endosperm morphology of spelt kernels was monofractal, isotropic, and complex. A higher proportion of Type-A starch granules resulted in an increased proportion of voids and interphase boundaries in the endosperm. Changes in the fractal dimension were correlated with kernel hardness, specific milling energy, the particle size distribution of flour, and the starch damage rate. Spelt cultivars varied in size and shape of the kernels. Kernel hardness was a property that differentiated specific milling energy, particle size distribution of flour, and starch damage rate. Fractal analysis may be considered as a useful tool for evaluating milling processes in the future.

Influence of Gene Expression on Hardness in Wheat

Puroindoline (Pina and Pinb) genes control grain texture or hardness in wheat. Wild-type/soft alleles lead to softer grain while a mutation in one or both of these genes results in a hard grain. Variation in hardness in genotypes with identical Pin alleles (wild-type or mutant) is known but the molecular basis of this is not known. We now report the identification of wheat genotypes with hard grain texture and wild-type/soft Pin alleles indicating that hardness in wheat may be controlled by factors other than mutations in the coding region of the Pin genes. RNA-Seq analysis was used to determine the variation in the transcriptome of developing grains of thirty three diverse wheat genotypes including hard (mutant Pin) and soft (wild type) and those that were hard without having Pin mutations. This defined the role of pin gene expression and identified other candidate genes associated with hardness. Pina was not expressed in hard wheat with a mutation in the Pina gene. The ratio of Pina to Pinb expression was generally lower in the hard non mutant genotypes. Hardness may be associated with differences in Pin expression and other factors and is not simply associated with mutations in the PIN protein coding sequences.

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.

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.

Proteomes of hard and soft near-isogenic wheat lines reveal that kernel hardness is related to the amplification of a stress response during endosperm development

Wheat kernel texture, a major trait determining the end-use quality of wheat flour, is mainly influenced by puroindolines. These small basic proteins display in vitro lipid binding and antimicrobial properties, but their cellular functions during grain development remain unknown. To gain an insight into their biological function, a comparative proteome analysis of two near-isogenic lines (NILs) of bread wheat Triticum aestivum L. cv. Falcon differing in the presence or absence of the puroindoline-a gene (Pina) and kernel hardness, was performed. Proteomes of the two NILs were compared at four developmental stages of the grain for the metabolic albumin/globulin fraction and the Triton-extracted amphiphilic fraction. Proteome variations showed that, during grain development, folding proteins and stress-related proteins were more abundant in the hard line compared with the soft one. These results, taken together with ultrastructural observations showing that the formation of the protein matrix occurred earlier in the hard line, suggested that a stress response, possibly the unfolded protein response, is induced earlier in the hard NIL than in the soft one leading to earlier endosperm cell death. Quantification of the albumin/globulin fraction and amphiphilic proteins at each developmental stage strengthened this hypothesis as a plateau was revealed from the 500 °Cd stage in the hard NIL whereas synthesis continued in the soft one. These results open new avenues concerning the function of puroindolines which could be involved in the storage protein folding machinery, consequently affecting the development of wheat endosperm and the formation of the protein matrix.

Transgenic rice plants harboring the grain hardness-locus region of Aegilops tauschii

Grain hardness of wheat is determined by the hardness (Ha)-locus region, which contains three friabilin-related genes: puroindoline-a (Pina), puroindoline-b (Pinb) and GSP-1. In our previous study, we produced the transgenic rice plants harboring the large genomic fragment of the Ha-locus region of Aegilops tauschii containing Pina and GSP-1 genes by Agrobacterium-mediated transformation. To examine the effects of the transgenes in the rice endosperms, we firstly confirmed the homozygosity of the T-DNAs in four independent T2 lines by using fluorescence in situ hybridization (FISH) and DNA gel blot analyses. The transgenes, Pina and GSP-1, were stably expressed in endosperms of the T3 and T4 seeds at RNA and protein levels, indicating that the promoters and other regulatory elements on the wheat Ha-locus region function in rice, and that multigene transformation using a large genomic fragment is a useful strategy. The functional contribution of the transgene-derived friabilins to the rice endosperm structure was considered as an increase of spaces between compound starch granules, resulting in a high proportion of white turbidity seeds.

Impact of low hydration of barley grain on β-glucan degradation and lipid transfer protein (LTP1) modifications during the malting process

One of the objectives of the malting industry is to reduce the energy cost during kilning without major effect on malt quality. In this study, the impact of a low hydration steeping process on lipid transfer protein (LTP1) modifications and β-glucan breakdown was evaluated in low (LH) and high (HH) hydrated malts. LTP1 modifications analyzed by MS/MS revealed acylation, glycation, and disulfide bond breakage in both LH and HH malts. LTP1 free amine content measurement and fluorescence of Maillard protein adducts revealed no significant difference between LH and HH malts. Immunolabeling of LTP1 during malting highlighted the diffusion of the protein from the aleurone layer to the endosperm at the end of steeping in both LH and HH malts. By contrast, a significant higher amount of β-glucans was measured in LH malts after five days of germination, whereas no significant difference between LH and HH malts was revealed through immunostaining of β-glucans or evaluation of the endosperm integrity after seven days of germination. The possibility to reduce the effects of a low hydration steeping process on β-glucan hydrolysis by increasing germination time was discussed.

Grain hardness: a major determinant of wheat quality

Wheat quality, a complex term, depends upon intentional use for unambiguous products. The foremost determinants of wheat quality are endosperm texture (grain hardness), protein content and gluten strength. Endosperm texture in wheat is the single most important and defining quality characteristic, as it facilitates wheat classification and affects milling, baking and end-use quality. Various techniques used for grain hardness measurement are classified into diverse groups according to grinding, crushing and abrasion. The most extensively used methods for texture measurement are PSI, NIR hardness, SKCS, pearling index, SDS-PAGE and PCR markers. Friabilin is a 15 kDa endosperm specific protein associated with starch granules of wheat grain and is unswervingly related to grain softness. Chemically, it is a concoction of different polypeptides, primarily puroindolines; Pin a and Pin b. Hardness (Ha) locus of chromosome 5DS makes the distinction between soft and hard classes of wheat. Some additional modifying genes are also present which contribute to the disparity within wheat classes. Numerous allelic mutations in Pin have been reported and their relation to end product quality has been established. This treatise elaborates the consequence of grain hardness in wheat eminence.

Physicochemical and Dough-handling Characteristics of Indian Wheat and Triticale Cultivars

Four bread wheat (PBW-138, PBW-299, PBW-343 and PBW-373), two durum wheat (PDW-215 and PDW-233) and two triticale cultivars (TL-419 and TL-1210) were investigated for physicochemical, milling and dough-handling properties for predicting end-use quality. Physical properties of durum wheat (PDW-215) and bread wheat (PBW-138, PBW-299 and PBW-343) cultivars were better than other wheat, durum and triticale cultivars. The compositional analysis revealed nonsignificant differences between the different cultivars; however, starch observed significant variation for different varieties. Particle size distribution indicated that triticale flours showed lower particle size than wheat and durum wheat. Dough-handling studies revealed triticale flours to be the weakest, while bread wheat flours were observed to be intermediate between durum and triticale. Among all the varieties, the bread wheat (PBW-138) variety was observed to be best, followed by PDW-215 durum wheat variety. Strong correlations were observed between physicochemical and dough-handling parameters, which can be used as quality parameter for suitable end-use.

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.

Grain characterization and milling behaviour of near-isogenic lines differing by hardness

Wheat grain hardness is a major factor affecting the milling behaviour and end-product quality although its exact structural and biochemical basis is still not understood. This study describes the development of new near-isogenic lines selected on hardness. Hard and soft sister lines were characterised by near infrared reflectance (NIR) and particle size index (PSI) hardness index, grain protein content, thousand kernel weight and vitreousness. The milling behaviour of these wheat lines was evaluated on an instrumented micromill which also measures the grinding energy and flour particle size distribution was investigated by laser diffraction. Endosperm mechanical properties were measured using compression tests. Results pointed out the respective effect of hardness and vitreousness on those characteristics. Hardness was shown to influence both the mode of fracture and the mechanical properties of the whole grain and endosperm. Thus, this parameter also acts on milling behaviour. On the other hand, vitreousness was found to mainly play a role on the energy required to break the grain. This study allows us to distinguish between consequences of hardness and vitreousness. Hardness is suggested to influence the adhesion forces between starch granules and protein matrix whereas vitreousness would rather be related to the endosperm microstructure.

A microarray analysis of wheat grain hardness

Grain hardness is an important quality characteristic of wheat grain, and considerable research effort has focused on characterising the genetic and biochemical basis underlying the hardness phenotype. Previous research has shown that the predominant difference between hard and soft seeds is linked to the puroindoline (PIN) proteins. In this study the near-isogenic lines of Heron and Falcon, which differ only in the grain hardness character, were compared using a cDNA microarray consisting of approximately 5,000 unique cDNA clones that were isolated from wheat and barley endosperm tissue. Our analysis showed that major differences in gene expression were evident for puroindoline-a (Pina), with a minor but not consistent change in the expression of puroindoline-b (Pinb). These observations were confirmed using a 16,000 unique cDNA microarray in a comparison of hard wheats with either the Pina null or Pinb mutation.

FTIR imaging of wheat endosperm cell walls in situ reveals compositional and architectural heterogeneity related to grain hardness

Endosperm cell walls of cultivars of wheat (Triticum aestivum L.) selected for their endosperm texture (two soft and two hard) were analysed in situ by Fourier transform infrared (FTIR) microspectroscopy. FTIR imaging coupled with statistical analysis was used to map the compositional and structural heterogeneity within transverse sections from which cell contents had been removed by sonication. In the majority of grains analysed, two distinct populations of endosperm cells could be identified by spectral features that were related to cell morphology and age, regardless of cultivar. The main cell-wall component responsible for these differences was the polysaccharide arabinoxylan. In a few samples, this heterogeneity was absent, for reasons that are not understood, but this was not correlated to endosperm texture or growth conditions. Within the same population of endosperm cells, cell walls of hard endosperm could be distinguished from those of soft endosperm by their spectral features. Compared to hard cultivars, the peripheral endosperm of soft cultivars was characterised by a higher amount of polymer, whose spectral feature was similar to water-extractable arabinoxylan. In contrast, no specific compound has been identified in the central endosperm: structural differences within the polysaccharides probably contribute to the distinction between hard and soft cultivars. In developing grain, a clear difference in the composition of the endosperm cell walls of hard and soft wheat cultivars was observed as early as 15 days after anthesis.

Genetic and biochemical analysis of common wheat cultivars lacking puroindoline a

Puroindoline a (Pin-a) and puroindoline b (Pin-b), two basic isoforms encoded by the Pina-D1 and Pinb-D1 loci respectively, involved in controlling grain texture in wheat, were isolated from starch granules of soft wheat cultivars using three different extraction procedures, and fractionated by acidic polyacrylamide gel electrophoresis (A-PAGE). Tris buffer containing 1% Triton X-114 extracted Pin-a and small amounts of Pin-b, whereas 1% SDS preferably extracted Pin-b. Large amounts of both puroindolines were isolated by a solution containing 50% propan-2-ol and 50 mM NaCl. This solution extracted reduced amounts of Pin-b and no traces of Pin-a from starch granules of 20 hard common wheats containing the null allele Pina-D1b. The absence of Pin-a was confirmed by immunostaining with an anti-Pin-a antiserum. With the exception of two cultivars, null Pin-a cultivars gave no PCR fragment with three primer pairs specific to either the coding region or the promoter region of Pina-D1a, suggesting that major changes had occurred at the Pina-D1 locus in these genotypes. Cultivars Fortuna and Glenman were unique in giving size-specific PCR fragments with all primer pairs for the allele Pina-D1a and showed a cytosine deletion at position 267 in the coding region of the Pin-a gene, which resulted in a TGA stop codon at position 361. However, there was no evidence of a mutated protein in the A-PAGE or SDS-PAGE patterns of Fortuna and Glenman. The novel gene, provisionally named Pina-D1c, is the first null allele due to a point mutation that has been identified at the Pina-D1 locus.

Real Time RT-PCR and flow cytometry to investigate wheat kernel hardness: role of puroindoline genes and proteins

Developing seeds from Triticum aestivum (wheat) cultivars were collected after flowering and analysed for puroindoline a and b gene expression by Real Time RT-PCR. Mature seeds were investigated for the presence and the amount of starch-associated puroindoline a and b proteins by flow cytometry. Puroindoline a gene and protein were found to have a predominant role in controlling wheat kernel hardness.