A chromosome-level genome assembly of Plantago ovata.

Plantago ovata is cultivated for production of its seed husk (psyllium). When wet, the husk transforms into a mucilage with properties suitable for pharmaceutical industries, utilised in supplements for controlling blood cholesterol levels, and food industries for making gluten-free products. There has been limited success in improving husk quantity and quality through breeding approaches, partly due to the lack of a reference genome. Here we constructed the first chromosome-scale reference assembly of P. ovata using a combination of 5.98 million PacBio and 636.5 million Hi-C reads. We also used corrected PacBio reads to estimate genome size and transcripts to generate gene models. The final assembly covers ~ 500 Mb with 99.3% gene set completeness. A total of 97% of the sequences are anchored to four chromosomes with an N50 of ~ 128.87 Mb. The P. ovata genome contains 61.90% repeats, where 40.04% are long terminal repeats. We identified 41,820 protein-coding genes, 411 non-coding RNAs, 108 ribosomal RNAs, and 1295 transfer RNAs. This genome will provide a resource for plant breeding programs to, for example, reduce agronomic constraints such as seed shattering, increase psyllium yield and quality, and overcome crop disease susceptibility.

The novel features of Plantago ovata seed mucilage accumulation, storage and release.

Seed mucilage polysaccharide production, storage and release in Plantago ovata is strikingly different to that of the model plant Arabidopsis. We have used microscopy techniques to track the development of mucilage secretory cells and demonstrate that mature P. ovata seeds do not have an outer intact cell layer within which the polysaccharides surround internal columellae. Instead, dehydrated mucilage is spread in a thin homogenous layer over the entire seed surface and upon wetting expands directly outwards, away from the seed. Observing mucilage expansion in real time combined with compositional analysis allowed mucilage layer definition and the roles they play in mucilage release and architecture upon hydration to be explored. The first emergent layer of hydrated mucilage is rich in pectin, extremely hydrophilic, and forms an expansion front that functions to 'jumpstart' hydration and swelling of the second layer. This next layer, comprising the bulk of the expanded seed mucilage, is predominantly composed of heteroxylan and appears to provide much of the structural integrity. Our results indicate that the synthesis, deposition, desiccation, and final storage position of mucilage polysaccharides must be carefully orchestrated, although many of these processes are not yet fully defined and vary widely between myxospermous plant species.

A small-scale fractionation pipeline for rapid analysis of seed mucilage characteristics.

BACKGROUND: Myxospermy is a process by which the external surfaces of seeds of many plant species produce mucilage-a polysaccharide-rich gel with numerous fundamental research and industrial applications. Due to its functional properties the mucilage can be difficult to remove from the seed and established methods for mucilage extraction are often incomplete, time-consuming and unnecessarily wasteful of precious seed stocks. RESULTS: Here we tested the efficacy of several established protocols for seed mucilage extraction and then downsized and adapted the most effective elements into a rapid, small-scale extraction and analysis pipeline. Within 4 h, three chemically- and functionally-distinct mucilage fractions were obtained from myxospermous seeds. These fractions were used to study natural variation and demonstrate structure-function links, to screen for known mucilage quality markers in a field trial, and to identify research and industry-relevant lines from a large mutant population. CONCLUSION: The use of this pipeline allows rapid analysis of mucilage characteristics from diverse myxospermous germplasm which can contribute to fundamental research into mucilage production and properties, quality testing for industrial manufacturing, and progressing breeding efforts in myxospermous crops.