The deposition and characterization of starch in Brachypodium distachyon.

Vanja Tanackovic,Jan T. Svensson,Susanne L. Jensen,Alain Buléon,Andreas Blennow

doi:10.1093/jxb/eru276

Abstract

Brachypodium distachyon is a non-domesticated cereal. Nonetheless, Brachypodium was recently introduced as a model plant for temperate cereals. This study compares grain starch metabolism in Brachypodium and barley (Hordeum vulgare). In Brachypodium, we identified and annotated 28 genes involved in starch metabolism and identified important motifs including transit peptides and putative carbohydrate-binding modules (CBMs) of the families CBM20, CBM45, CBM48, and CBM53. Starch content was markedly lower in Brachypodium grains (12%) compared to barley grains (47%). Brachypodium starch granules were doughnut shaped and bimodally distributed into distinct small B-type (2.5-10 µm) and very small C-type (0.5-2.5 µm) granules. Large A-type granules, typical of cereals, were absent. Starch-bound phosphate, important for starch degradation, was 2-fold lower in Brachypodium compared with barley indicating different requirements for starch mobilization. The amylopectin branch profiles were similar and the amylose content was only slightly higher compared with barley cv. Golden Promise. The crystallinity of Brachypodium starch granules was low (10%) compared to barley (20%) as determined by wide-angle X-ray scattering (WAXS) and molecular disorder was confirmed by differential scanning calorimetry (DSC). The expression profiles in grain for most genes were distinctly different for Brachypodium compared to barley, typically showing earlier decline during the course of development, which can explain the low starch content and differences in starch molecular structure and granule characteristics. High transitory starch levels were observed in leaves of Brachypodium (2.8% after 14h of light) compared to barley (1.9% after 14h of light). The data suggest important pre-domesticated features of cereals.

Highlights

Starch is the most important source of carbohydrates for humans, and starch from cereals is of crucial value
In Brachypodium, we identified and annotated 28 genes involved in starch metabolism and identified important motifs including transit peptides and putative carbohydrate-binding modules (CBMs) of the families CBM20, CBM45, CBM48, and CBM53
The comparative genetic capacity for starch biosynthesis and degradation in Brachypodium and barley was investigated in silico and the enzyme functional gene structure deduced

Summary

Introduction

Starch is the most important source of carbohydrates for humans, and starch from cereals is of crucial value. In 2012, world production of cereals amounted to 2.3 billion tons Starch is the principal storage product of the majority of plants. It is stored in both photosynthetic and non-photosynthetic organs, among which the grass grain is of central importance, with starch forming its main constituent. Starch is synthesized in well-organised starch granules in amyloplasts in the grain tissue and is made up of two polysaccharides: amylose and amylopectin. Amylopectin typically comprises 70–75% of the starch granule; it is more than 100-fold larger than amylose and contains clustered α-1,6 linkages.

Methods

Results

Conclusion