Comparative in situ analyses of cell wall matrix polysaccharide dynamics in developing rice and wheat grain

Cell wall polysaccharides of wheat and rice endosperm are an important source of dietary fibre. Monoclonal antibodies specific to cell wall polysaccharides were used to determine polysaccharide dynamics during the development of both wheat and rice grain. Wheat and rice grain present near synchronous developmental processes and significantly different endosperm cell wall compositions, allowing the localisation of these polysaccharides to be related to developmental changes. Arabinoxylan (AX) and mixed-linkage glucan (MLG) have analogous cellular locations in both species, with deposition of AX and MLG coinciding with the start of grain filling. A glucuronoxylan (GUX) epitope was detected in rice, but not wheat endosperm cell walls. Callose has been reported to be associated with the formation of cell wall outgrowths during endosperm cellularisation and xyloglucan is here shown to be a component of these anticlinal extensions, occurring transiently in both species. Pectic homogalacturonan (HG) was abundant in cell walls of maternal tissues of wheat and rice grain, but only detected in endosperm cell walls of rice in an unesterified HG form. A rhamnogalacturonan-I (RG-I) backbone epitope was observed to be temporally regulated in both species, detected in endosperm cell walls from 12 DAA in rice and 20 DAA in wheat grain. Detection of the LM5 galactan epitope showed a clear distinction between wheat and rice, being detected at the earliest stages of development in rice endosperm cell walls, but not detected in wheat endosperm cell walls, only in maternal tissues. In contrast, the LM6 arabinan epitope was detected in both species around 8 DAA and was transient in wheat grain, but persisted in rice until maturity

Structure, chemical composition and enzymatic activities of pearlings and bran obtained from pearled wheat (Triticum aestivum L.) by roller milling

While abrasive pearling (also referred to as debranning) of wheat kernels prior to milling increases the quality of the resultant flour for producing bread, the potential applications of the co-products of pearling is largely unknown. We studied the impact of different degrees of pearling (0, 3, 6, 9 and 12% by weight) on the composition of pearlings and bran obtained when subsequently roller milling pearled wheat kernels. Pearling does not remove the kernel outer tissues homogeneously as abrasion affects especially the accessible parts of the kernels. Nevertheless, the first 3% removed consisted of mainly pericarp. With 6% or more removed, a significant amount of starchy endosperm ended up in the pearlings. The starting bran material and bran obtained by subsequent roller milling of pearled wheat kernels had similar compositions but the latter had a lower average particle size. Moreover, removal of the outermost kernel layers substantially decreased the enzyme activity levels in the bran.publisher: Elsevier articletitle: Structure, chemical composition and enzymatic activities of pearlings and bran obtained from pearled wheat (Triticum aestivum L.) by roller milling journaltitle: Journal of Cereal Science articlelink: http://dx.doi.org/10.1016/j.jcs.2014.12.009 content_type: article copyright: Copyright © 2015 Elsevier Ltd. All rights reserved.status: publishe

Wheat bran AX properties and choice of xylanase affect enzymic production of wheat bran-derived arabinoxylan-oligosaccharides

Wheat bran-derived arabinoxylan-oligosaccharides (AXOS) have recently been shown to potentially exert prebiotic effects. In this study, fifteen bran samples obtained by milling different wheat cultivars were treated with xylanases from Hypocrea jecorina (XHJ), Aspergillus aculeatus (XAA) and Pseudoalteromonas haloplanktis (XPH) to assess the impact of bran source and xylanase properties on the AXOS yield and structure. The total arabinoxylan (AX) extraction yield was higher with XHJ (8.2–10.7%) and XAA (8.2–10.8%) than with XPH (6.9–9.5%). Irrespective of the enzyme, a significant negative correlation was observed between extraction yield and arabinose to xylose (A/X) ratio of the bran AX (r = –0.7), but not between yield and bran AX level. The A/X ratio of the extracted material was in a 0.27–0.34 range for all bran samples and all enzymes, which, combined with yield data and microscopic analysis, indicated primary hydrolysis of aleurone and nucellar epidermis AX. The average degree of polymerization (avDP) of the extracted AX was very low for all enzymes (2–3), owing to the release of high levels of monomeric arabinose and xylose. The release of these monosaccharides could be ascribed to (1) the activity of wheat bran-associated enzymes (arabinofuranosidases and xylosidases), (2) the hydrolytic properties of the xylanases themselves, and (3) the presence of xylosidases as contaminations in the enzyme preparation, in that order of importance. Heat treatment of bran before xylanase treatment significantly decreased the levels of monomeric arabinose and xylose in the extract, without affecting the extraction yield, resulting in a higher avDP (3–7), thus yielding true AXOS. Overall, for AXOS production, wheat cultivars with a low bran A/X ratio of their AX are preferable as starting materials, and inactivation of bran-associated enzymes before incubation is desirable. The XHJ xylanase was the best enzyme for wheat bran-derived AXOS production.status: publishe

Accumulated evidence substantiates a role for three classes of wheat xylanase inhibitors in plant defence

Since their discovery in 1997, work on proteinaceous wheat xylanase inhibitors (XIs) led to the identification and thorough biochemical and structural characterization of three classes of XIs, namely TAXI-type (Triticum aestivum xylanase inhibitor), XIPtype (xylanase inhibiting protein), and TLXI-type (thaumatin-like xylanase inhibitor) XIs. Already early on, a role in plant defense has been put forward for these proteins,mainly based on the observation that these XIs are only active against xylanases of microbial origin and can not inhibit the plant endogenous xylanases known so far. Considerable effort has been devoted to substantiate this plant defense hypothesis. Data at the genomic, transcriptomic and proteomic levels increasingly provide evidence that XIs, occurring as large polymorphic families, do indeed participate in plant defense. This review summarizes the current knowledge on XIs and the accumulated evidence on their role in plant defense. TLXI and XIP can be classified as pathogenesis-related (PR) proteins of classes PR-5 and PR-8, respectively, based on their homology with thaumatin-like proteins and chitinases. For TAXI, more evidence on the induction of TAXI proteins by a pathogen in tissues that normally do not express TAXI is required before it can be categorized as a new class of PR proteins. At the moment, TAXI proteins should be referred to as inducible defense-related proteins.status: publishe