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

Abstract

I. Introduction to seed mucilage II. Plantago seed mucilage composition and properties III. Same but different: mucilage-related features of key model species and beyond IV. Where next? V. Conclusion and outlook Acknowledgements References Summary Mucilage, a gel-like layer formed around wetted seeds in a process called myxospermy, has importance as a proxy for studying cell wall polysaccharide biosynthesis and interactions and as a source of valuable health supplements and hydrocolloids. Arabidopsis thaliana has provided unrivalled insight into mucilage/cell wall synthesis, but its lack of commercial utility presents an opportunity to develop an alternative myxospermous model linking genetics, chemistry and functionality. Here, we discuss recent advances in the understanding of mucilage production, composition, and properties of Plantago, a promising candidate as an alternative model with economic relevance. We outline how genomic/transcriptomic and chemical analysis advances could be made to strengthen Plantago's use as a model system, through challenging but achievable approaches.

The composition of Australian Plantago seeds highlights their potential as nutritionally-rich functional food ingredients

“…More recently, Plantago seed mucilage has gained great industrial and medical significance. Commonly known as psyllium husk, the milled seed mucilage of P. ovata contains highly-hydrophilic heteroxylan and pectin polysaccharides 9 , 10 that are used in food technology to texturally mimic fat 11 and gluten 12 , 13 , and in health applications as a dietary fibre supplement to aid laxation 14 , treat hypercholesterolemia 15 , diabetes 16 , and irritable bowel syndrome 17 . While the clinical benefits of psyllium husk (generally attributed to the high viscosity of the heteroxylan) are not in doubt, pre-clinical and in vitro studies showing free radical scavenging 18 – 23 , immunomodulation 24 – 26 , and treatment of metabolic disorders 27 – 30 by extracts of whole, unhusked Plantago seeds demonstrate that the non-husk/non-mucilage seed components may have further beneficial effects.…”

“…While the clinical benefits of psyllium husk (generally attributed to the high viscosity of the heteroxylan) are not in doubt, pre-clinical and in vitro studies showing free radical scavenging 18 – 23 , immunomodulation 24 – 26 , and treatment of metabolic disorders 27 – 30 by extracts of whole, unhusked Plantago seeds demonstrate that the non-husk/non-mucilage seed components may have further beneficial effects. However, current production of psyllium husk from P. ovata (the only commercially important Plantago species) mostly discards these non-husk/non-mucilage components which account for around 75% of the seed yield 10 . In India (the world’s largest psyllium producer) this equates to over 160,000 tonnes of non-husk material that was wasted from the 56,000 tonnes of psyllium husk produced in 2019/2020 31 , 32 despite the material likely containing many beneficial nutrients.…”

“…These studies show the potential of WSF from different Plantago species to be a less wasteful, more nutritious alternative to psyllium husk for selectively manipulating food qualities based on interspecific differences in functionality. Therefore, we hypothesise that different Plantago species may yield correspondingly diverse functionalities and Australia may boast valuable untapped resources, with the added advantage of already being adapted to demanding climatic constraints that limit production of psyllium from P. ovata 10 .…”

Myxospermy Evolution in Brassicaceae: A Highly Complex and Diverse Trait with Arabidopsis as an Uncommon Model

“…The mucilage trait of the other studied species belonging to lineage 2 is highly diverse (Figure 2). Some do not display any AM, such as Thlaspi arvense (22) and Noccaea caerulescens (24), or show a very thin layer of AM that requires a higher resolution to be observed, such as for Stanleya pinnata (20). Other species of lineage 2 show the extrusion of several conical structures at the seed surface, such as Sisymbrium irio (19) and Eutrema salsugineum (23), or display large pieces of poorly cohesive mucilage for Schrenkiela parvula (21) (Figure 2).…”

“…In Angiosperms, several so-called myxospermous species spread among a majority of orders were reported to release SM with highly diverse compositions and contrasted MSC morphology [ 15 , 17 ]. Great variation in mucilage amount and/or compositions also occurs at the genus level, such as in the Arabidopsis species [ 18 ] or Plantago species [ 19 , 20 ] and even among populations, such as in A. thaliana [ 21 , 22 ] and flax ( Linum usitatissimum ) [ 23 , 24 ]. Those large variations make it difficult to trace back the evolutionary scenario of myxospermy only based on the global biochemical or morphological description.…”