Functional packaging of seeds

Comparative transcriptomics of seed nourishing tissues: uncovering conserved and divergent pathways in seed plants

The evolutionary and ecological success of spermatophytes is intrinsically linked to the seed habit, which provides a protective environment for the initial development of the new generation. This environment includes an ephemeral nourishing tissue that supports embryo growth. In gymnosperms this tissue originates from the asexual proliferation of the maternal megagametophyte, while in angiosperms it is a product of fertilization, and is called the endosperm. The emergence of these nourishing tissues is of profound evolutionary value, and they are also food staples for most of the world's population. Here, using Orthofinder to infer orthologue genes among newly generated and previously published datasets, we provide a comparative transcriptomic analysis of seed nourishing tissues from species of several angiosperm clades, including those of early diverging lineages, as well as of one gymnosperm. Our results show that, although the structure and composition of seed nourishing tissues has seen significant divergence along evolution, there are signatures that are conserved throughout the phylogeny. Conversely, we identified processes that are specific to species within the clades studied, and thus illustrate their functional divergence. With this, we aimed to provide a foundation for future studies on the evolutionary history of seed nourishing structures, as well as a resource for gene discovery in future functional studies.

Functional Complexity on a Cellular Scale: Why In Situ Analyses Are Indispensable for Our Understanding of Lignified Tissues

Lignins are a key adaptation that enables vascular plants to thrive in terrestrial habitats. Lignin is heterogeneous, containing upward of 30 different monomers, and its function is multifarious: It provides structural support, predetermined breaking points, ultraviolet protection, diffusion barriers, pathogen resistance, and drought resilience. Recent studies, carefully characterizing lignin in situ, have started to identify specific lignin compositions and ultrastructures with distinct cellular functions, but our understanding remains fractional. We summarize recent works and highlight where further in situ lignin analysis could provide valuable insights into plant growth and adaptation. We also summarize strengths and weaknesses of lignin in situ analysis methods.

Domestication has altered gene expression and secondary metabolites in pea seed coat

The mature seed in legumes consists of an embryo and seed coat. In contrast to knowledge about the embryo, we know relatively little about the seed coat. We analyzed the gene expression during seed development using a panel of cultivated and wild pea genotypes. Gene co-expression analysis identified gene modules related to seed development, dormancy, and domestication. Oxidoreductase genes were found to be important components of developmental and domestication processes. Proteomic and metabolomic analysis revealed that domestication favored proteins involved in photosynthesis and protein metabolism at the expense of seed defense. Seed coats of wild peas were rich in cell wall-bound metabolites and the protective compounds predominated in their seed coats. Altogether, we have shown that domestication altered pea seed development and modified (mostly reduced) the transcripts along with the protein and metabolite composition of the seed coat, especially the content of the compounds involved in defense. We investigated dynamic profiles of selected identified phenolic and flavonoid metabolites across seed development. These compounds usually deteriorated the palatability and processing of the seeds. Our findings further provide resources to study secondary metabolism and strategies for improving the quality of legume seeds which comprise an important part of the human protein diet.

Fruit dehiscence mechanism and release of dimorphic seeds with different germination properties in Commelina erecta

Commelina erecta is a successful weed species. The aims of this study were to analyse the morpho-anatomy of the fruit and dimorphic seeds of the weed C. erecta, the dynamics and type of dormancy, and water entry. Flowers and fruits at different development stages were processed using standard anatomical techniques. Besides, experiments of imbibition, germinability and water entry were performed on both seed types. In the fruit of C. erecta, free and coated seeds are developed within dehiscent and indehiscent carpels, respectively. Dehiscent carpels open through a region of mechanical weakness in the dorsal vascular bundle. This region does not form in the indehiscent carpel. The main anatomical differences between the two seed types were observed in the testa and in the number of covering layers. Imbibition experiments showed that the covering of both seed types is water permeable, so these seeds lack physical dormancy and may exhibit physiological dormancy. Germinability experiments showed that the dormancy in free seeds is variable throughout the reproductive season, whereas, in coated seeds, it is high throughout the reproductive season. The embryotega is an area where the hardness of the seed coat is interrupted and facilitates water entry. Differences in the morpho-anatomy of carpels result in the formation of dimorphic seeds with different covering layers and different germination properties. These different properties allow some seeds germinate immediately after falling from the mother plant, and others to be incorporated into the seed bank. These results are useful for designing weed management strategies in agroecosystems.

Wrack Burial Limits Germination and Establishment of Yellow Flag Iris (Iris pseudacorus L.)

Seed burial under wrack, mats of water-transported plant debris, can limit recruitment of seedlings in wetlands. In a greenhouse experiment, we studied the effects of wrack burial (0, 1, 2, 4, 8 cm depths) on germination and emergence of the macrophyte Iris pseudacorus, native to Europe, Mediterranean Basin, and western Asia, that has invaded wetlands in nearly every global ecozone. We recorded the percentages of germinating, senescent, and quiescent seeds and evaluated seedling establishment and growth relative to substrate environmental variables. Seedling emergence of I. pseudacorus was reduced from >80% in controls without burial to <40% even at minimal wrack depths of 1 cm. Few I. pseudacorus seedlings were able to emerge from wrack burial of up to 8 cm in depth. We also found greater numbers of both quiescent seeds and germinated seeds that did not emerge from wrack burial. Reduced seedling emergence and increased seed quiescence with wrack burial were primarily explained by a reduction in daily temperature variation within the substrate. No senescent seedlings were observed with any depth of wrack burial. In view of our results, the management of I. pseudacorus invasion will be a long-term challenge, requiring continued control due to persistent seeds in wrack-buried seed banks.

Ginkgo seed shell provides a unique model for bioinspired design

Natural structural materials typically feature complex hierarchical anisotropic architectures, resulting in excellent damage tolerance. Such highly anisotropic structures, however, also provide an easy path for crack propagation, often leading to catastrophic fracture as evidenced, for example, by wood splitting. Here, we describe the weakly anisotropic structure of Ginkgo biloba (ginkgo) seed shell, which has excellent crack resistance in different directions. Ginkgo seed shell is composed of tightly packed polygonal sclereids with cell walls in which the cellulose microfibrils are oriented in a helicoidal pattern. We found that the sclereids contain distinct pits, special fine tubes like a "screw fastener," that interlock the helicoidal cell walls together. As a result, ginkgo seed shell demonstrates crack resistance in all directions, exhibiting specific fracture toughness that can rival other highly anisotropic natural materials, such as wood, bone, insect cuticle, and nacre. In situ characterization reveals ginkgo's unique toughening mechanism: pit-guided crack propagation. This mechanism forces the crack to depart from the weak compound middle lamella and enter into the sclereid, where the helicoidal cell wall significantly inhibits crack growth by the cleavage and breakage of the fibril-based cell walls. Ginkgo's toughening mechanism could provide guidelines for a new bioinspired strategy for the design of high-performance bulk materials.

Biomimicking Nature-Inspired Design Structures-An Experimental and Simulation Approach Using Additive Manufacturing

Whether it is a plant- or animal-based bio-inspiration design, it has always been able to address one or more product/component optimisation issues. Today's scientists or engineers look to nature for an optimal, economically viable, long-term solution. Similarly, a proposal is made in this current work to use seven different bio-inspired structures for automotive impact resistance. All seven of these structures are derived from plant and animal species and are intended to be tested for compressive loading to achieve load-bearing capacity. The work may even cater to optimisation techniques to solve the real-time problem using algorithm-based generative shape designs built using CATIA V6 in unit dimension. The samples were optimised with Rhino 7 software and then simulated with ANSYS workbench. To carry out the comparative study, an experimental work of bioprinting in fused deposition modelling (3D printing) was carried out. The goal is to compare the results across all formats and choose the best-performing concept. The results were obtained for compressive load, flexural load, and fatigue load conditions, particularly the number of life cycles, safety factor, damage tolerance, and bi-axiality indicator. When compared to previous research, the results are in good agreement. Because of their multifunctional properties combining soft and high stiffness and lightweight properties of novel materials, novel materials have many potential applications in the medical, aerospace, and automotive sectors.

Seed-to-Seedling Transition: Novel Aspects

Transition from seed to seedling represents a critical stage in plants’ life cycles. This process includes three significant events in the seeds: (i) tissue hydration, (ii) the mobilization of reserve nutrients, and (iii) the activation of metabolic activity. Global metabolic rearrangements lead to the initiation of radicle growth and the resumption of vegetative development. It requires massive reprogramming of the transcriptome, proteome, metabolome, and attendant signaling pathways, resulting in the silencing of seed-maturation genes and the activation of vegetative growth genes. This Special Issue discusses the mechanisms of genetic, epigenetic, and hormonal switches during seed-to-seedling transitions. Detailed information has also been covered regarding the influence of germination features on seedling establishment.

The walnut shell network: 3D visualisation of symplastic and apoplastic transport routes in sclerenchyma tissue

High symplastic connectivity via pits was linked to the lignification of the developing walnut shell. With maturation, this network lessened, whereas apoplastic intercellular space remained and became relevant for shell drying. The shell of the walnut (Juglans regia) sclerifies within several weeks. This fast secondary cell wall thickening and lignification of the shell tissue might need metabolites from the supporting husk tissue. To reveal the transport capacity of the walnut shell tissue and its connection to the husk, we visualised the symplastic and apoplastic transport routes during shell development by serial block face-SEM and 3D reconstruction. We found an extensive network of pit channels connecting the cells within the shell tissue, but even more towards the husk tissue. Each pit channel ended in a pit field, which was occupied by multiple plasmodesmata passing through the middle lamella. During shell development, secondary cell wall formation progressed towards the interior of the cell, leaving active pit channels open. In contrast, pit channels, which had no plasmodesmata connection to a neighbouring cell, got filled by cellulose layers from the inner cell wall lamellae. A comparison with other nut species showed that an extended network during sclerification seemed to be linked to high cell wall lignification and that the connectivity between cells got reduced with maturation. In contrast, intercellular spaces between cells remained unchanged during the entire sclerification process, allowing air and water to flow through the walnut shell tissue when mature. The connectivity between inner tissue and environment was essential during shell drying in the last month of nut development to avoid mould formation. The findings highlight how connectivity and transport work in developing walnut shell tissue and how finally in the mature state these structures influence shell mechanics, permeability, conservation and germination.

Fire-released seed dormancy - a global synthesis

Seed dormancy varies greatly between species, clades, communities, and regions. We propose that fireprone ecosystems create ideal conditions for the selection of seed dormancy as fire provides a mechanism for dormancy release and postfire conditions are optimal for germination. Thus, fire‐released seed dormancy should vary in type and abundance under different fire regimes. To test these predictions, we compiled data from a wide range of fire‐related germination experiments for species in different ecosystems across the globe. We identified four dormancy syndromes: heat‐released (physical) dormancy, smoke‐released (physiological) dormancy, non‐fire‐released dormancy, and non‐dormancy. In fireprone ecosystems, fire, in the form of heat and/or chemical by‐products (collectively termed ‘smoke’), are the predominant stimuli for dormancy release and subsequent germination, with climate (cold or warm stratification) and light sometimes playing important secondary roles. Fire (heat or smoke)‐released dormancy is best expressed where woody vegetation is dense and fires are intense, i.e. in crown‐fire ecosystems. In such environments, seed dormancy allows shade‐intolerant species to take advantage of vegetation gaps created by fire and synchronize germination with optimal recruitment conditions. In grassy fireprone ecosystems (e.g. savannas), where fires are less intense but more frequent, seed dormancy is less common and dormancy release is often not directly related to fire (non‐fire‐released dormancy). Rates of germination, whether controls or postfire, are twice as fast in savannas than in mediterranean ecosystems. Fire‐released dormancy is rare to absent in arid ecosystems and rainforests. The seeds of many species with fire‐released dormancy also possess elaiosomes that promote ant dispersal. Burial by ants increases insulation of seeds from fires and places them in a suitable location for fire‐released dormancy. The distribution of these dormancy syndromes across seed plants is not random – certain dormancy types are associated with particular lineages (phylogenetic conservatism). Heat‐released dormancy can be traced back to fireprone floras in the ‘fiery’ mid‐Cretaceous, followed by smoke‐released dormancy, with loss of fire‐related dormancy among recent events associated with the advent of open savannas and non‐fireprone habitats. Anthropogenic influences are now modifying dormancy‐release mechanisms, usually decreasing the role of fire as exaptive effects. We conclude that contrasting fire regimes are a key driver of the evolution and maintenance of diverse seed dormancy types in many of the world's natural ecosystems.

Xyloglucan remodelling enzymes and the mechanics of plant seed and fruit biology

This article comments on: Di Marzo M, Ebeling Viana V, Banfi C, Cassina V, Corti R, Herrera-Ubaldo H, Babolin N, Guazzotti A, Kiegle E, Gregis V, de Folter S, Sampedro J, Mantegazza F, Colombo L, Ezquer I. 2022. Cell wall modifications by α-XYLOSIDASE1 are required for the control of seed and fruit size. Journal of Experimental Botany 73, 1499–1515.

Seed protection strategies of the brainy Elaeocarpus ganitrus endocarp: Gradient motif yields fracture tolerance

Be it animals or plants, most of the organism's offspring come into existence after their embryos develop inside a protective shell. In plants, these hard protective shells are called endocarps. They serve the function of nourishing and protecting the seeds from external mechanical damage. Through evolution, endocarps of plants have developed various structural strategies to protect the enclosed seeds from external threats, and these strategies can vary according to the habitat or lifestyle of a particular plant. One such intriguing hard plant shell is the endocarp of the Elaeocarpus ganitrus fruit. It mostly grows in South Asia's mountainous forests, and its endocarps are known in the local communities as unbreakable and everlasting prayer beads. We report an in-depth investigation on microstructure, tomography, and mechanical properties to cast light on its performance and the underlying structure-property relation. The 3D structural quantifications by micro-CT demonstrate that the endocarp has gradient microarchitecture. In addition, the endocarp also exhibits gradient hardness and stiffness. The toughening mechanisms arising from the layered cellular structure enable the endocarps to withstand higher loads up to 5000 N before they fracture. Our findings provide experimental evidence of outstanding fracture tolerance and seed protection strategies developed by Elaeocarpus ganitrus endocarp that encourage the design of synthetic fracture tolerant structures. STATEMENT OF SIGNIFICANCE: Endocarps are low-density plant shells that exhibit remarkable fracture resistance and energy absorption when they encounter impact by falling from high trees and prolonged compression and abrasion by the predators. Such outstanding mechanical performance originates through structural design strategies developed to protect their seeds. Here we demonstrate previously undiscovered structural features and mechanical properties of Elaeocarpus ganitrus endocarp. We scrutinize the microstructure using high-resolution x-ray tomography scans and the 3D structural quantifications reveal a gradient microstructure which is in agreement with the gradient hardness and stiffness. The multiscale hierarchical structures combined with the gradient motif yield impressive fracture tolerance in Elaeocarpus ganitrus endocarp. These findings advance the knowledge of the structure-property relation in hard plant shells, and the procured structural design strategies can be utilized to design fracture-resistant structures.