Dynamic development of starch granules and the regulation of starch biosynthesis in Brachypodium distachyon: comparison with common wheat and Aegilops peregrina

Abstract

BackgroundThorough understanding of seed starch biosynthesis and accumulation mechanisms is of great importance for agriculture and crop improvement strategies. We conducted the first comprehensive study of the dynamic development of starch granules and the regulation of starch biosynthesis in Brachypodium distachyon and compared the findings with those reported for common wheat (Chinese Spring, CS) and Aegilops peregrina.ResultsOnly B-granules were identified in Brachypodium Bd21, and the shape variation and development of starch granules were similar in the B-granules of CS and Bd21. Phylogenetic analysis showed that most of the Bd21 starch synthesis-related genes were more similar to those in wheat than in rice. Early expression of key genes in Bd21 starch biosynthesis mediate starch synthesis in the pericarp; intermediate-stage expression increases the number and size of starch granules. In contrast, these enzymes in CS and Ae. peregrina were mostly expressed at intermediate stages, driving production of new B-granules and increasing the granule size, respectively. Immunogold labeling showed that granule-bound starch synthase (GBSSI; related to amylose synthesis) was mainly present in starch granules: at lower levels in the B-granules of Bd21 than in CS. Furthermore, GBSSI was phosphorylated at threonine 183 and tyrosine 185 in the starch synthase catalytic domain in CS and Ae. peregrina, but neither site was phosphorylated in Bd21, suggesting GBSSI phosphorylation could improve amylose biosynthesis.ConclusionsBd21 contains only B-granules, and the expression of key genes in the three studied genera is consistent with the dynamic development of starch granules. GBSSI is present in greater amounts in the B-granules of CS than in Bd21; two phosphorylation sites (Thr183 and Tyr185) were found in Triticum and Aegilops; these sites were not phosphorylated in Bd21. GBSSI phosphorylation may reflect its importance in amylose synthesis.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-014-0198-2) contains supplementary material, which is available to authorized users.

Starch Biosynthesis in the Developing Endosperms of Grasses and Cereals

“…Evidence suggests that compound starch granules represent the ancestral state in the Poaceae, e.g., Sorghum italica is the earliest diverging lineage within the PACMAD clade, and is the only member of this group with compound granules in the endosperm ( [1,[402][403][404] (see Figure 1) and simple or bimodal granules occur in only more recently diverged lineages. Brachypodium is phylogenetically closer to wheat (bimodal starch granules) than maize (simple starch granules) [405], yet shows only small B-type (and C-type) granules in the endosperm [398,406]. In the grasses and cereals of the Festucoid family, large lenticular-shaped A-type granules (10-35 µm diameter) are formed early in endosperm development, whilst the smaller, near-spherical B-type granules (averaging 5-9 µm in diameter) are formed later [407][408][409].…”

“…Three major starch granule morphologies exist within the Poaceae: (1) compound granules, which are made up of tightly packed polygonal granules arranged in spherical-like structures with membranous cross walls termed septa, and are typically found in rice and other members of the Ehrhartoideae [258] and Bambusoideae [396]; (2) simple granules, discrete granules of varying size (2-30 µm) and shape, e.g., polyhedral, lenticelullar, or spherical, and found in maize and sorghum [392,397]; and, (3) granules with a bimodal size distribution, typically a mix of small spherical (B-type) and large lenticellular (A-type) granules, and are generally found in the Festucoid family of temperate grasses, such as goatgrass (Aegilops peregrina Hack. ), and cereals such as wheat, barley, and rye [398][399][400][401]. Evidence suggests that compound starch granules represent the ancestral state in the Poaceae, e.g., Sorghum italica is the earliest diverging lineage within the PACMAD clade, and is the only member of this group with compound granules in the endosperm ( [1,[402][403][404] (see Figure 1) and simple or bimodal granules occur in only more recently diverged lineages.…”

Brachypodium Seed: A Potential Model for Studying Grain Development of Cereal Crops

“…Several studies have shown the Brachypodium starch granules are small in size and are not bimodally distributed (Chen et al 2014a;Guillon et al 2012;Hands et al 2012;Opanowicz et al 2011), although Tanackovic et al (2014) reported a bimodal population of Brachypodium granules consisting of small B-type (2.5-10 μm) and very small C-type (0.5-2.5 μm). The large A type granules, a typical type in cereals, are absent in Brachypodium.…”

“…It has been observed that the starch is distributed along a gradient in the Brachypodium seed, being more abundant in the proximal end of the endosperm than at the distal end which is away from embryo, and is restricted in the central zone of the endosperm at the distal end (Trafford et al 2013). The starch transiently accumulated in the pericarp at an early developmental stage, and started to accumulate in the endosperm after cellularization (Chen et al 2014a;Guillon et al 2012;Trafford et al 2013). Starch granules appeared at 15 DAF and were apparently distributed around the periphery of endosperm cells at 17 DAF, and afterwards took over all the spaces excluding the fused vacuoles in mature seeds (Guillon et al 2012).…”

“…Analysis of genes involved in starch biosynthesis and metabolism suggested that the low content of starch in Brachypodium endosperm was possibly due to reduced expression of starch synthesis genes and the reduced catalytic activity compared to that of barley (Chen et al 2014a;Trafford et al 2013). It was further hypothesized that the grain filling with less starch granules represents one possible reason for the small sizes of Brachypodium endosperm cells (Trafford et al 2013).…”