Hordein Accumulation in Developing Barley Grains.

Gregory J Tanner,Crispin A Howitt,Malcolm J Blundell,Michelle L Colgrave,Antony Bacic

doi:10.3389/fpls.2019.00649

Abstract

The temporal pattern of accumulation of hordein storage proteins in developing barley grains was studied by enzyme-linked immunosorbent assay (ELISA), western blot and liquid chromatography tandem mass spectrometry (LC-MS/MS). Hordein accumulation was compared to the pattern seen for two abundant control proteins, serpin Z4 (an early accumulator) and lipid transferase protein (LTP1, a late accumulator). Hordeins were detected from 6 days post-anthesis (DPA) and peaked at 30 DPA. Changes in fresh weight indicate that desiccation begins at 20 DPA and by 37 DPA fresh weight had decreased by 35%. ELISA analysis of hordein content, expressed on a protein basis, increased to a maximum at 30 DPA followed by a 17% decrease by 37 DPA. The accumulation of 39 tryptic and 29 chymotryptic hordein peptides representing all classes of hordein was studied by LC-MS/MS. Most peptides increased to a maximum at 30 DPA, and either remained at the maximum or did not decrease significantly. Only five tryptic peptides, members of the related B1- and γ1-hordeins decreased significantly by 21–51% at 37 DPA. Thus, the concentration of some specific peptides was reduced while remaining members of the same family were not affected. The N-terminal signal region was removed by proteolysis during co-translation. In addition to a suite of previously characterized hordeins, two novel barley B-hordein isoforms mapping to wheat low molecular weight glutenins (LMW-GS-like B-hordeins), and two avenin-like proteins (ALPs) sharing homology with wheat ALPs, were identified. These identified isoforms have not previously been mapped in the barley genome. Cereal storage proteins provide significant nutritional content for human consumption and seed germination. In barley, the bulk of the storage proteins comprise the hordein family and the final hordein concentration affects the quality of baked and brewed products. It is therefore important to study the accumulation of hordeins as this knowledge may assist plant breeding for improved health outcomes (by minimizing triggering of detrimental immune responses), nutrition and food processing properties.

Highlights

Prolamins, are rich in proline and glutamine residues, and this is a collective name given to the alcohol-soluble, water-insoluble storage proteins that exist in wheat gluten, barley, rye, and oat
The accumulation of hordeins was measured by enzyme-linked immunosorbent assay (ELISA), calibrated against an appropriate standard consisting of total hordein purified from barley cv Sloop
Total hordein content detected by ELISA and LC-MS/MS, increased co-ordinately from 6 days post-anthesis (DPA) to a peak level at 30 DPA

Summary

Introduction

Prolamins, are rich in proline and glutamine residues, and this is a collective name given to the alcohol-soluble, water-insoluble storage proteins that exist in wheat gluten (gliadin and glutenin), barley (hordein), rye (secalins), and oat (avenin). The hordeins are amongst the triggers of coeliac disease (CD), a well characterized T-cell mediated disorder suffered by approximately 1% of most populations (Fasano et al, 2003; Lebwohl et al, 2018). Other adverse reactions to gluten exist including gluten intolerance (Biesiekierski et al, 2011, 2013; Choung et al, 2017), which affects approximately 10% of the population (Golley et al, 2015), and gluten allergy, a serious, rapid, IgE mediated allergy impacting 1% of the population (Snegaroff et al, 2006)

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