A mathematical model of mucilage expansion in myxospermous seeds of Capsella bursa-pastoris (shepherd's purse)

Production, Composition, and Ecological Function of Sweet-Basil-Seed Mucilage during Hydration

The sweet-basil (Ocimum basilicum L.) fruit/pericarp produces mucilage that engulfs the fruit and seed within minutes of hydration. Seed mucilage is produced by plant species that have adapted to arid, sandy soils. This study was conducted to determine how basil-seed mucilage improves ecological fitness. A second objective was to find ways to remove mucilage, which may interfere with commercial planting. Basil fruit/seeds were examined using light and environmental scanning electron microscopy. Columnar structures of basil mucilage rapidly unfolded from the pericarp upon initial hydration. Dilute hydrochloric acid removed the mucilage, which decreased the water content four-fold but did not inhibit seed germination in a laboratory test. Nondestructive Fourier-transform mid-infrared (FTIR) spectroscopy confirmed that the mucilage was primarily composed of hemicellulose that anchored the basil seed to resist movement. The fully hydrated seeds approached zero water potential, so the mucilage did not interfere with hydration. The seeds that were planted in growing media with mucilage had from 12 to 28% higher seedling emergence and survival percentages after 10 days than seeds without mucilage. Basil-fruit/seed mucilage provides a reservoir of loosely bound water at high water potential for seed germination and early seedling development, thus improving survivability under low moisture.

Seed coat mucilages: Structural, functional/bioactive properties, and genetic information

Seed coat mucilages are mainly polysaccharides covering the outer layer of the seeds to facilitate seed hydration and germination, thereby improving seedling emergence and reducing seedling mortality. Four types of polysaccharides are found in mucilages including xylan, pectin, glucomannan, and cellulose. Recently, mucilages from flaxseed, yellow mustard seed, chia seed, and so on, have been used extensively in the areas of food, pharmaceutical, and cosmetics contributing to stability, texture, and appearance. This review, for the first time, addresses the similarities and differences in physicochemical properties, molecular structure, and functional/bioactive properties of mucilages among different sources; highlights their structure and function relationships; and systematically summarizes the related genetic information, aiming with the intent to explore the potential functions thereby extending their future industrial applications.

Seed mucilage evolution: Diverse molecular mechanisms generate versatile ecological functions for particular environments

Plant myxodiasporous species have the ability to release a polysaccharidic mucilage upon imbibition of the seed (myxospermy) or the fruit (myxocarpy). This is a widespread capacity in angiosperms providing multiple ecological functions including higher germination efficiency under environmental stresses. It is unclear whether myxodiaspory has one or multiple evolutionary origins and why it was supposedly lost in several species. Here, we summarize recent advances on three main aspects of myxodiaspory. (a) It represents a combination of highly diverse traits at different levels of observation, ranging from the dual tissular origin of mucilage secretory cells to diverse mucilage polysaccharidic composition and ultrastructural organization. (b) An asymmetrical selection pressure is exerted on myxospermy-related genes that were first identified in Arabidopsis thaliana. The A. thaliana and the flax intra-species mucilage variants show that myxospermy is a fast-evolving trait due to high polymorphism in a few genes directly acting on mucilage establishment. In A. thaliana, these actors are downstream of a master regulatory complex and an original phylogenetic overview provided here illustrates that this complex has sequentially evolved after the common ancestor of seed plants and was fully established in the common ancestor of the rosid clade. (c) Newly identified myxodiaspory ecological functions indicate new perspectives such as soil microorganism control and plant establishment support.

What kind of seed dormancy occurs in the legume genus Cassia?

Cassia is a diverse legume genus widespread in the (sub-)tropical zone of the world. Several studies have been done on this genus; however, significant changes have occurred at the taxonomic level over the years. This has led to inaccurate information about seed dormancy in Cassia since many species are no longer included in the genus. Thus, our work aims to investigate and update the information about the kind of dormancy that occurs in seeds of Cassia species and also look into two notorious species in this group (C. fistula and C. javanica) to compare myxospermous vs. non-myxospermous seeds regarding dormancy and germination traits. Seed dormancy reports were found for 53 Cassia species, and the only kind of seed dormancy found for these species was physical dormancy (PY). Non-dormancy was not found, and all seeds had a blockage to water uptake during the dormant state, that is, all have PY. Of these 53 species, only 18 are currently included in the genus Cassia. C. fistula and C. javanica have fully developed embryos, and dormancy is only conferred by the (water-impermeable) seed coat. The lens in the seed coat is the only structure that creates a water pathway to break PY in C. fistula. Myxospermous seeds came out of dormancy faster than non-myxospermous ones. PY seems to be the only kind of seed dormancy that has evolved in Cassia. The extent of this kind of dormancy in all subtribe Cassiinae is also discussed.

Cytological Approaches Combined With Chemical Analysis Reveals the Layered Nature of Flax Mucilage

The external seed coat cell layer of certain species is specialized in the production and extrusion of a polysaccharide matrix called mucilage. Variations in the content of the released mucilage have been mainly associated with genetically regulated physiological modifications. Understanding the mucilage extrusion process in crop species is of importance to gain deeper insight into the complex cell wall biosynthesis and dynamics. In this study, we took advantage of the varying polysaccharide composition and the size of the flax mucilage secretory cells (MSCs) to study mucilage composition and extrusion in this species of agricultural interest. We demonstrate herein that flax MSCs are structured in four superimposed layers and that rhamnogalacturonans I (RG I) are firstly synthesized, in the upper face, preceding arabinoxylan and glucan synthesis in MSC lower layers. Our results also reveal that the flax mucilage release originates from inside MSC, between the upper and deeper layers, the latter collaborating to trigger polysaccharide expansion, radial cell wall breaking and mucilage extrusion in a peeling fashion. Here, we provide evidence that the layer organization and polysaccharide composition of the MSCs regulate the mucilage release efficiency like a peeling mechanism. Finally, we propose that flax MSCs may represent an excellent model for further investigations of mucilage biosynthesis and its release.

Regulation of Carbon Partitioning in the Seed of the Model Legume Medicago truncatula and Medicago orbicularis: A Comparative Approach

The proportion of starch, protein and oil in legume seeds is species dependent. The model legume, Medicago truncatula, has predominantly oil and protein stores. To investigate the regulation of seed oil production we compared M. truncatula with M. orbicularis, which has less oil and protein. The types of protein and fatty acids are similar between the two species. Electron microscopy indicated that the size and distribution of the oil bodies in M. orbicularis, is consistent with reduced oil production. M. orbicularis has more extruded endosperm mucilage compared to M. truncatula. The cotyledons have a greater cell wall content, visualized as thicker cell walls. The reduced oil content in M. orbicularis is associated with increased expression of the MtGLABRA2-like (MtGL2) transcription factor, linked to an inverse relationship between mucilage and oil content in Arabidopsis. The expression of the pectin biosynthesis GALACTURONOSYLTRANSFERASE (GAUT) genes, is also increased in M. orbicularis. These increases in extruded mucilage and cell wall storage components in M. orbicularis are accompanied by reduced expression of transcriptional regulators of oil biosynthesis, MtLEAFY COTYLEDON1-LIKE (MtL1L), MtABSCISIC ACID-INSENSITIVE3 (MtABI3), and MtWRINKLED-like (MtWRI), in M. orbicularis. The reduced oil in M. orbicularis, is consistent with increased synthesis of cell wall polysaccharides and decreased expression of master transcription factors regulating oil biosynthesis and embryo maturation. Comparative investigations between these two Medicago species is a useful system to investigate the regulation of oil content and carbon partitioning in legumes.

Differences in glycosyltransferase family 61 accompany variation in seed coat mucilage composition in Plantago spp

Xylans are the most abundant non-cellulosic polysaccharide found in plant cell walls. A diverse range of xylan structures influence tissue function during growth and development. Despite the abundance of xylans in nature, details of the genes and biochemical pathways controlling their biosynthesis are lacking. In this study we have utilized natural variation within the Plantago genus to examine variation in heteroxylan composition and structure in seed coat mucilage. Compositional assays were combined with analysis of the glycosyltransferase family 61 (GT61) family during seed coat development, with the aim of identifying GT61 sequences participating in xylan backbone substitution. The results reveal natural variation in heteroxylan content and structure, particularly in P. ovata and P. cunninghamii, species which show a similar amount of heteroxylan but different backbone substitution profiles. Analysis of the GT61 family identified specific sequences co-expressed with IRREGULAR XYLEM 10 genes, which encode putative xylan synthases, revealing a close temporal association between xylan synthesis and substitution. Moreover, in P. ovata, several abundant GT61 sequences appear to lack orthologues in P. cunninghamii. Our results indicate that natural variation in Plantago species can be exploited to reveal novel details of seed coat development and polysaccharide biosynthetic pathways.

Differentiation of the seed coat and composition of the mucilage of Lepidium perfoliatum L.: a desert annual with typical myxospermy

Myxospermy is an important feature in seeds of many plant species grown in desert region. Fertilization can initiate differentiation of the seed coat epidermis into a specialized cell type with mucilage production. In the present study, comprehensive analyses were performed on the seed coat differentiation, mucilage production and composition, and seed germination in Lepidium perfoliatum (Brassicaceae), a desert annual with typical myxospermy in China. First, results indicated that mucilage was secreted uniformly at the outer tangential wall, resulting in compression of the cytoplasm to the bottom of the epidermal cells. Secondly, the inner tangential wall and two radial walls of the subepidermal cells were apparently thickened by production of a secondary cell wall material, which resulted in a 'typical' palisade appearance. Thirdly, immunohistochemical staining combined with the enzymatic digestion and infrared spectrum analysis of the mucilage indicated that, while one important component of the seed coat mucilage in L. perfoliatum was pectin, it also contained β-1,3-d-glucan and xyloglucan. Finally, seed germination showed that seeds with mucilage displayed significantly higher germination percentage than that of demucilaged seeds in abundant or excess water conditions. These results suggest that the possible ecological role of mucilage in L. perfoliatum is in the adaptation to habitats with well-watered and water-logged conditions, rather than water stress.

Arabidopsis seed mucilage secretory cells: regulation and dynamics

Seeds from various angiosperm species produce polysaccharide mucilage facilitating germination and, therefore, conferring major evolutionary advantages. The seed epidermal mucilage secretory cells (MSCs) undergo numerous tightly controlled changes of their extracellular matrixes (ECMs) throughout seed development. Recently, major progress based on the model species Arabidopsis thaliana was published, including the identification of 54 genes necessary for mucilage synthesis and release. Here, we review these genes that constitute the so-called 'MSC toolbox', within which transcription factors and proteins related to polysaccharide production, secretion, modification, and stabilization are the most abundant and belong to complex regulatory networks. We also discuss how seed coat 'omics data-mining, comparative genomics, and operon-like gene cluster studies will provide means to identify new members of the MSC toolbox.

Medicinal plants contain mucilage used in traditional Persian medicine (TPM)

CONTEXT

Conventional therapies using mucilage plants greatly used by practitioners in Iran. The usage of mucilages is rooted in traditional knowledge with a history of more than 4000 years. Scientific assessment of these historical documents could be valuable for finding new potential usage in the current medicine.

OBJECTIVE

This study assembled an inventory of mucilage plants considered important therapeutic aids for alleviating the ailments in ancient Persian medicine and compared therapeutic applications of ancient times with current findings of medicinal mucilages in the same plant species.

METHODS

A literature search compiled some main traditional manuscripts of Persian medicine, including the book of AlHavi, Canon of Medicine, Zakhireh-iKharazmshahi, Qarabadine-kabir, Tohfat ol Moemenin, and Makhzan-ol-advieh, and select mucilage plants used in treating the mouth and respiratory system disorders. Also, current investigations on related subjects were considered through a search of the Pub Med and Google Scholar databases.

RESULTS

In Iran, the application of medicinal plants contains mucilage date back to ancient times. In mentioned medieval Persian books, 20 medicinal plants containing mucilage were identified. Mucilages have been traditionally used via oral or topical routes for a variety of disorders. According to this study, most of the cited medicinal plant species were used for their mucilaginous, anti-inflammatory, and anti-oxidant effects.

CONCLUSIONS

A scientific evaluation of these historical documents can give an insight into the ideas of the past and be valuable in finding new data on clinical use of the mucilages that should lead to future opportunities to investigate their potential medicinal use.

Expression of PRX36, PMEI6 and SBT1.7 is controlled by complex transcription factor regulatory networks for proper seed coat mucilage extrusion

Mucilage secretory cells (MSC) form an intriguing cell layer important for seed germination. In Arabidopsis thaliana, several master transcription factors (TFs) and "actor" proteins have already been identified as key players for seed coat differentiation including epidermal cell formation, mucilage production and extrusion. The regulation of the genes coding for MSC cell wall "actor" proteins by TFs needs to be better established. Here, the expression and the regulation of 3 known actors (PRX36, PMEI6, SBT1.7) and 2 additional putative actors (PRX56, DIR12) have been analyzed in T-DNA mutants affected in master TFs (ap2, egl3/gl3, gl2, myb5, tt8, ttg1, ttg2 and luh1/mum1). Genes with somehow similar function are differentially regulated and conversely, genes with different functions are regulated in similar manner.

The microbiome: stress, health and disease

Bacterial colonisation of the gut plays a major role in postnatal development and maturation of key systems that have the capacity to influence central nervous system (CNS) programming and signaling, including the immune and endocrine systems. Individually, these systems have been implicated in the neuropathology of many CNS disorders and collectively they form an important bidirectional pathway of communication between the microbiota and the brain in health and disease. Regulation of the microbiome-brain-gut axis is essential for maintaining homeostasis, including that of the CNS. Moreover, there is now expanding evidence for the view that commensal organisms within the gut play a role in early programming and later responsivity of the stress system. Research has focused on how the microbiota communicates with the CNS and thereby influences brain function. The routes of this communication are not fully elucidated but include neural, humoral, immune and metabolic pathways. This view is underpinned by studies in germ-free animals and in animals exposed to pathogenic bacterial infections, probiotic agents or antibiotics which indicate a role for the gut microbiota in the regulation of mood, cognition, pain and obesity. Thus, the concept of a microbiome-brain-gut axis is emerging which suggests that modulation of the gut microflora may be a tractable strategy for developing novel therapeutics for complex stress-related CNS disorders where there is a huge unmet medical need.

Co-adaptation of seed dormancy and flowering time in the arable weed Capsella bursa-pastoris (shepherd's purse)

BACKGROUND AND AIMS

The duration of the plant life cycle is an important attribute that determines fitness and coexistence of weeds in arable fields. It depends on the timing of two key life-history traits: time from seed dispersal to germination and time from germination to flowering. These traits are components of the time to reproduction. Dormancy results in reduced and delayed germination, thus increasing time to reproduction. Genotypes in the arable seedbank predominantly have short time to flowering. Synergy between reduced seed dormancy and reduced flowering time would create stronger contrasts between genotypes, offering greater adaptation in-field. Therefore, we studied differences in seed dormancy between in-field flowering time genotypes of shepherd's purse.

METHODS

Genotypes with early, intermediate or late flowering time were grown in a glasshouse to provide seed stock for germination tests. Secondary dormancy was assessed by comparing germination before and after dark-incubation. Dormancy was characterized separately for seed myxospermy heteromorphs, observed in each genotype. Seed carbon and nitrogen content and seed mass were determined as indicators of seed filling and resource partitioning associated with dormancy.

KEY RESULTS

Although no differences were observed in primary dormancy, secondary dormancy was weaker among the seeds of early-flowering genotypes. On average, myxospermous seeds showed stronger secondary dormancy than non-myxospermous seeds in all genotypes. Seed filling was similar between the genotypes, but nitrogen partitioning was higher in early-flowering genotypes and in non-myxospermous seeds.

CONCLUSIONS

In shepherd's purse, early flowering and reduced seed dormancy coincide and appear to be linked. The seed heteromorphism contributes to variation in dormancy. Three functional groups of seed dormancy were identified, varying in dormancy depth and nitrate response. One of these groups (FG-III) was distinct for early-flowering genotypes. The weaker secondary dormancy of early-flowering genotypes confers a selective advantage in arable fields.