Abstract
The transcriptome of the developing starchy endosperm of hexaploid wheat (Triticum aestivum) was determined using RNA-Seq isolated at five stages during grain fill. This resource represents an excellent way to identify candidate genes responsible for the starchy endosperm cell wall, which is dominated by arabinoxylan (AX), accounting for 70% of the cell wall polysaccharides, with 20% (1,3;1,4)-β-d-glucan, 7% glucomannan, and 4% cellulose. A complete inventory of transcripts of 124 glycosyltransferase (GT) and 72 glycosylhydrolase (GH) genes associated with cell walls is presented. The most highly expressed GT transcript (excluding those known to be involved in starch synthesis) was a GT47 family transcript similar to Arabidopsis (Arabidopsis thaliana) IRX10 involved in xylan extension, and the second most abundant was a GT61. Profiles for GT43 IRX9 and IRX14 putative orthologs were consistent with roles in AX synthesis. Low abundances were found for transcripts from genes in the acyl-coA transferase BAHD family, for which a role in AX feruloylation has been postulated. The relative expression of these was much greater in whole grain compared with starchy endosperm, correlating with the levels of bound ferulate. Transcripts associated with callose (GSL), cellulose (CESA), pectin (GAUT), and glucomannan (CSLA) synthesis were also abundant in starchy endosperm, while the corresponding cell wall polysaccharides were confirmed as low abundance (glucomannan and callose) or undetectable (pectin) in these samples. Abundant transcripts from GH families associated with the hydrolysis of these polysaccharides were also present, suggesting that they may be rapidly turned over. Abundant transcripts in the GT31 family may be responsible for the addition of Gal residues to arabinogalactan peptide.
Highlights
The transcriptome of the developing starchy endosperm of hexaploid wheat (Triticum aestivum) was determined using RNASeq isolated at five stages during grain fill
The starchy endosperm cell wall of the model grass Brachypodium distachyon has been shown to differ from wheat and barley starchy endosperm cell walls, having higher cellulose content and detectable pectin; it has much greater 1,3;1,4-b-D-glucan content than wheat (Guillon et al, 2011)
Two of the most abundant transcripts, granule-bound starch synthase I and high-molecular-weight 1Dx5 glutenin (Fig. 1C), indicate the dominance of storage product transcripts in this transcriptome and reflect a pattern of earlier increase for starch synthetic enzymes compared with storage protein (Shewry et al, 2009)
Summary
The transcriptome of the developing starchy endosperm of hexaploid wheat (Triticum aestivum) was determined using RNASeq isolated at five stages during grain fill This resource represents an excellent way to identify candidate genes responsible for the starchy endosperm cell wall, which is dominated by arabinoxylan (AX), accounting for 70% of the cell wall polysaccharides, with 20% (1,3;1,4)-b-D-glucan, 7% glucomannan, and 4% cellulose. The starchy endosperm cell walls constitute 2% to 3% of white flour, have important effects on physical properties such as viscosity (Saulnier et al, 2007a; Faik, 2010), and provide a source of dietary fiber (Topping, 2007) Due to their importance, the composition of the cell walls of mature wheat starchy endosperm has been extensively studied and reported to comprise approximately 70% arabinoxylan (AX), 20% 1,3;1,4-bD-glucan, 7% glucomannan, and 4% cellulose (Mares and Stone, 1973a; Bacic and Stone, 1980). There have been no reports of pectin or xyloglucan in the cell walls of either mature or developing starchy endosperm of wheat (Shibuya and Nakane, 1984)
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