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

Abstract

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. Abbreviations DPA Days post-anthesis ML Mucilage layer MSC Mucilage secretory cell SEM Scanning electron microscopy Upon exposure to aqueous environments, seeds from myxospermous species extrude a polysaccharide-rich gel from their seed surface, often called mucilage. Numerous species display myxospermy and there are a range of possible evolutionary advantages of synthesising such a carbon-rich and energy-expensive substance 1. Of all myxospermous species, the seed mucilage system of Arabidopsis is the best characterised. Arabidopsis seed mucilage has been used extensively as a proxy for the study of plant cell wall polysaccharide biosynthesis, enabling increased molecular characterisation of pectin biosynthesis, its main polysaccharide component 2 , as well as the biosynthesis of cellulose 3,4 and several hemicelluloses 5-8 , which are minor but integral components. Mucilage from other species can be highly diverse 1 and while P. ovata mucilage is also a complex mixture of polymers, it is predominantly heteroxylan with only a minor pectin component. While the pectin component is a near-linear rhamnogalacturonan 9-11 , the P. ovata heteroxylan (accounting for around 90% of the mucilage polysaccharides) is highly complex with the current scientific consensus defining P. ovata heteroxylan comprising a β-(1,4)-linked-d-xylopyranose backbone, heavily substituted at O-2 and/or O-3 positions with various mono-, di-and oligosaccharide substitutions of α-l-arabinofuranose and β-d-xylopyranose 9,11,12. It is likely that, as with other eudicots, the β-(1,4)-linked-d-xylopyranose backbone is synthesised by several members of

The goo‐d stuff: Plantago as a myxospermous model with modern utility

“…3a and 3b). Several Plantago species display ejected seed coat cells within expanded mucilage (Gutterman & Shem-Tov, 1996;Phan et al, 2016Phan et al, , 2020 with occurrence and features also appearing clade-specific ( Fig. 3c).…”

“…Arabidopsis mucilage‐related processes were the first to be closely studied (Western et al ., 2000) and more recently flax ( Linum usitatissimum ) processes were defined (Miart et al ., 2019). A recent review compared events in Arabidopsis and flax (Viudes et al ., 2020) and now mucilage‐related processes of psyllium ( P. ovata ) (Phan et al ., 2020) have been described in suitable detail for direct comparison of all three (Fig. 2).…”

“…The purpose of ejected seed coat cells is a significant unanswered question. Seed development of P. cunninghamii, which produces ejected seed coat cells (Phan et al, 2016) follows a similar sequence of events to P. ovata (Phan et al, 2020) and it is unclear how/where the developmental sequence subtly differs between them. An investigation of Plantago species development, representing each of (Shehata & Loutfy, 2006;Phan et al, 2016) and publicly available ITS sequences (Rønsted et al, 2002;Tay et al, 2010).…”

“…At maturity, the mucilage polysaccharides are stored in discrete cells in Arabidopsis and flax, while in psyllium the MSC layer has been compressed and disintegrated to form a uniform mucilage layer. Upon hydration, swelling of the mucilage ruptures the outer MSC walls in Arabidopsis and flax, while in psyllium there are no intact cells left and the mucilage passively expands away from the seed surface as it hydrates (models adapted fromVoiniciuc et al, 2015;Miart et al, 2019;Phan et al, 2020).For ease of comparison, MSCs are not depicted to scale. (c) Summary of similarities and differences of MSC features.…”

Review of structure and bioactivity of the Plantago (Plantaginaceae) polysaccharides

“…The extraction optimization, structural identification, and biological activity of polysaccharides from Plantago have been investigated extensively. It is worth mentioning that P. ovata seed husk (as known as psyllium husk, ispaghula husk, isabgol, and mucilage) contains more than 85% polysaccharide and has been recognized as a dietary fiber by the FDA ( Guo et al, 2008 , Phan et al, 2020 ). However, the structures and bioactivity of psyllium husk were rarely mentioned in the available reviews related to Plantago polysaccharides.…”