Grain development in Brachypodium and other grasses: possible interactions between cell expansion, starch deposition, and cell-wall synthesis

Abstract

To explain the low levels of starch, high levels of (1,3;1,4)-β-glucan, and thick cell walls in grains of Brachypodium distachyon L. relative to those in other Pooideae, aspects of grain development were compared between B. distachyon and barley (Hordeum vulgare L.). Cell proliferation, cell expansion, and endoreduplication were reduced in B. distachyon relative to barley and, consistent with these changes, transcriptional downregulation of the cell-cycle genes CDKB1 and cyclin A3 was observed. Similarly, reduced transcription of starch synthase I and starch-branching enzyme I was observed as well as reduced activity of starch synthase and ADP-glucose pyrophosphorylase, which are consistent with the lowered starch content in B. distachyon grains. No change was detected in transcription of the major gene involved in (1,3;1,4)-β-glucan synthesis, cellulose synthase-like F6. These results suggest that, while low starch content results from a reduced capacity for starch synthesis, the unusually thick cell walls in B. distachyon endosperm probably result from continuing (1,3;1,4)-β-glucan deposition in endosperm cells that fail to expand. This raises the possibility that endosperm expansion is linked to starch deposition.

Determining the Subcellular Location of Synthesis and Assembly of the Cell Wall Polysaccharide (1,3; 1,4)-β-d-Glucan in Grasses

“…It is the most highly expressed CSLF gene in most tissues of barley (Hordeum vulgare), wheat (Triticum aestivum), Brachypodium distachyon, and rice (Oryza sativa), including developing seedling leaf, coleoptiles, and endosperm (Burton et al, 2008;Kimpara et al, 2008;Doblin et al, 2009;Nemeth et al, 2010;Pellny et al, 2012;Vega-Sánchez et al, 2012;Suliman et al, 2013;Trafford et al, 2013;Schreiber et al, 2014). When CSLF6 expression is reduced either by knockdown or knockout via mutation or T-DNA insertion (Tonooka et al, 2009;Nemeth et al, 2010;Taketa et al, 2012;Vega-Sánchez et al, 2012;Hu et al, 2014), a significant reduction in MLG is observed in both vegetative and floral tissues, indicating its gene product is responsible for the synthesis of the majority of MLG in grasses.…”

The Dynamics of Transcript Abundance during Cellularization of Developing Barley Endosperm

“…These factors, together with the speed with which the reserves are released to support seedling growth after germination, are also important determinants of seedling vigor and hence crop establishment. The total number of cells in the endosperm is believed to influence grain size (Trafford et al, 2013).…”

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

“…From cellularization (8 DAF) to maximum fresh weight, which can be reached 19 DAF, the number of cells across the endosperm lobe varies little and the endosperm area only increased 2.5-fold on the mid-grain cross-section of Brachypodium. In comparison, over the same developmental period in barley, the cell number increased 2-fold and the area increased 15-fold (Trafford et al 2013). Much reduced expression of cell-cycle-related genes cyclin A3 and Fig.…”

“…Bar ¼ 100 μm (Top), 20 μm (Bottom). Reproduced with permission from Guillon et al (2012) and Journal of Experimental Botany CDKB1 indicated that the low rate of cell proliferation in Brachypodium endosperm is probably due to the blocking of mitosis (Trafford et al 2013).…”

“…The seed width ranges from 1.3 to 1.6 mm and the depth is less than 1 mm, thus a width: depth ratio of 1.7 makes a narrow and flat profile for Brachypodium seeds (Opanowicz et al 2011;Tyler et al 2014). The seed mass ranges from 3.5 to 6.0 mg per seed (Trafford et al 2013;Tyler et al 2014). The Bd21-3 genotype which was used to generate a large collection of T-DNA mutants (Bragg et al 2012) has the biggest seeds compared to other accessions investigated (Tyler et al 2014).…”