Spatiotemporal Deposition of Cell Wall Polysaccharides in Oat Endosperm during Grain Development.

Oat (Avena sativa) is a cereal crop whose grains are rich in (1,3; 1,4)-β-D-glucan (mixed linkage glucan or MLG), a soluble dietary fiber. In our study, we analyzed oat endosperm development in two Canadian varieties with differing MLG content and nutritional value. We confirmed that oat undergoes a nuclear type of endosperm development but with a shorter cellularisation phase than barley (Hordeum vulgare). Callose and cellulose were the first polysaccharides to be detected in the early anticlinal cell walls at 11 days post-emergence (DPE) of the panicle. Other polysaccharides such as heteromannan and homogalacturonan were deposited early in cellularisation around 12 DPE after the first periclinal walls are laid down. In contrast to barley, heteroxylan deposition coincided with completion of cellularisation and was detected from 14 DPE but was only detectable after demasking. Notably, MLG was the last polysaccharide to be laid down at 18 DPE within the differentiation phase, rather than during cellularisation. In addition, differences in the spatiotemporal patterning of MLG were also observed between the two varieties. The lower MLG-containing cultivar AC Morgan (3.5% w/w groats) was marked by the presence of a discontinuous pattern of MLG labelling, while labelling in the same walls in CDC Morrison (5.6% w/w groats) was mostly even and continuous. RNA-Seq analysis revealed higher transcript levels of multiple MLG biosynthetic Cellulose Synthase-like F (CSLF) and CSLH) genes during grain development in CDC Morrison compared to AC Morgan that likely contributes to the increased abundance of MLG at maturity in CDC Morrison. CDC Morrison was also observed to have smaller endosperm cells with thicker walls than AC Morgan from cellularisation onwards, suggesting the processes controlling cell size and shape are established early in development. This study has highlighted that the molecular processes influencing MLG content and deposition are more complex than previously imagined.