The dead seed coat functions as a long-term storage for active hydrolytic enzymes

A method to determine antifungal activity in seed exudates by nephelometry

BACKGROUND

One of the levers towards alternative solutions to pesticides is to improve seed defenses against pathogens, but a better understanding is needed on the type and regulation of existing pathways during germination. Dormant seeds are able to defend themselves against microorganisms during cycles of rehydration and dehydration in the soil. During imbibition, seeds leak copious amounts of exudates. Here, we developed a nephelometry method to assay antimicrobial activity (AA) in tomato seed exudates as a proxy to assess level of defenses.

RESULTS

A protocol is described to determine the level of AA against the nonhost filamentous fungus Alternaria brassicicola in the exudates of tomato seeds and seedlings. The fungal and exudate concentrations can be adjusted to modulate the assay sensitivity, thereby providing a large window of AA detection. We established that AA in dormant seeds depends on the genotype. It ranged from very strong AA to complete absence of AA, even after prolonged imbibition. AA depends also on the stages of germination and seedling emergence. Exudates from germinated seeds and seedlings showed very strong AA, while those from dormant seeds exhibited less activity for the same imbibition time. The exudate AA did not impact the growth of a pathogenic fungus host of tomato, Alternaria alternata, illustrating the adaptation of this fungus to its host.

CONCLUSIONS

We demonstrate that our nephelometry method is a simple yet powerful bioassay to quantify AA in seed exudates. Different developmental stages from dormant seed to seedlings show different levels of AA in the exudate that vary between genotypes, highlighting a genetic diversity x developmental stage interaction in defense. These findings will be important to identify molecules in the exudates conferring antifungal properties and obtain a better understanding of the regulatory and biosynthetic pathways through the lifecycle of seeds, from dormant seeds until seedling emergence.

Assessing the weed infestation potential of dredged streambed sediments targeted for reuse in agricultural fields

One of the problematic outcomes of soil erosion is sedimentation in stream channels adjacent to agricultural areas. Excess sediments routinely dredged are subsequently dumped in the riparian zone, where the prolonged presence of dredged sediment piles threatens the eco-hydrological balance of the agricultural-riverine environments. Reusing dredged sediments as an amendment for adjacent agricultural fields may serve as an alternative solution. However, farmers are hesitant to use this material in their fields due to the potential for weed infestation, with the associated costs incurred by crop losses. Here, we investigate the potential for weed infestation associated with reusing dredged sediments in agriculture. The research findings validate farmers' concerns regarding the undesirable proliferation of weeds following soil amelioration with dredged sediments. We present a comprehensive protocol for assessing the necessity for weed management intervention, based on infestation potential of weeds, while specifically targeting the reduction of adverse effects caused by agricultural weeds.

The orchid seed coat: a developmental and functional perspective

Orchid seeds are 'dust-like.' The seed coat is usually thin, with only one to a few cell layers. It originates from the integuments formed during ovule development. In orchids, the outer integument is primarily responsible for forming a mature seed coat. The inner integument usually fails to develop after fertilization, becomes compressed, and collapses over the expanding embryo. Hence, the seed coat is formed from the funiculus, chalaza, and outer integumentary cells. The outermost layer of the seed coat, the testa, is lignified, usually at the radial and inner tangential walls. The subepidermal thin-walled layer(s), the tegmen, subsequently cold, resulting in seeds having only a single layer of seed coat cells. In some species, cells of the inner integument remain alive with the ability to synthesize and accumulate lipidic and or phenolic compounds in their walls covering the embryo. This cover is called the 'carapace,' a protective shield contributing to the embryo's added protection. A developmental and functional perspective of the integuments and seed coat during seed development and germination is presented in this review.

The loss of polyphenol oxidase function is associated with hilum pigmentation and has been selected during pea domestication

Seed coats serve as protective tissue to the enclosed embryo. As well as mechanical there are also chemical defence functions. During domestication, the property of the seed coat was altered including the removal of the seed dormancy. We used a range of genetic, transcriptomic, proteomic and metabolomic approaches to determine the function of the pea seed polyphenol oxidase (PPO) gene. Sequencing analysis revealed one nucleotide insertion or deletion in the PPO gene, with the functional PPO allele found in all wild pea samples, while most cultivated peas have one of the three nonfunctional ppo alleles. PPO functionality cosegregates with hilum pigmentation. PPO gene and protein expression, as well as enzymatic activity, was downregulated in the seed coats of cultivated peas. The functionality of the PPO gene relates to the oxidation and polymerisation of gallocatechin in the seed coat. Additionally, imaging mass spectrometry supports the hypothesis that hilum pigmentation is conditioned by the presence of both phenolic precursors and sufficient PPO activity. Taken together these results indicate that the nonfunctional polyphenol oxidase gene has been selected during pea domestication, possibly due to better seed palatability or seed coat visual appearance.

Proteome-Wide Response of Dormant Caryopses of the Weed, Avena fatua, After Colonization by a Seed-Decay Isolate of Fusarium avenaceum

Promoting seed decay is an ecological approach to reducing weed persistence in the soil seedbank. Previous work demonstrated that Fusarium avenaceum F.a.1 decays dormant Avena fatua (wild oat) caryopses and induces several defense enzyme activities in vitro. The objectives of this study were to obtain a global perspective of proteins expressed after F.a.1-caryopsis colonization by conducting proteomic evaluations on (i) leachates, soluble extrinsic (seed-surface) proteins released upon washing caryopses in buffer and (ii) proteins extracted from whole caryopses; interactions with aluminum (Al) were also evaluated in the latter study because soil acidification and associated metal toxicity are growing problems. Of the 119 leachate proteins classified as defense/stress, 80 were induced or repressed. Defense/stress proteins were far more abundant in A. fatua (35%) than in F.a.1 (12%). Avena defense/stress proteins were also the most highly regulated category, with 30% induced and 35% repressed by F.a.1. Antifungal proteins represented 36% of Avena defense proteins and were the most highly regulated, with 36% induced and 37% repressed by F.a.1. These results implicate selective regulation of Avena defense proteins by F.a.1. Fusarium proteins were also highly abundant in the leachates, with 10% related to pathogenicity, 45% of which were associated with host cell wall degradation. In whole caryopsis extracts, fungal colonization generally resulted in induction of a similar set of Avena proteins in the presence and absence of Al. Results advance the hypothesis that seed decay pathogens elicit intricate and dynamic biochemical responses in dormant seeds.

What Worth the Garlic Peel

Plants have two types of reproduction: sexual, resulting in embryo production, and asexual, resulting in vegetative bodies commonly derived from stems and roots (e.g., bulb, tuber). Dead organs enclosing embryos (DOEEs, such as seed coat and pericarp) are emerging as central components of the dispersal unit acting to nurture the embryo and ensure its survival in the habitat. Here we wanted to investigate the properties of dead organs enclosing plant asexual reproductive bodies, focusing on the garlic (Allium sativum) bulb. We investigated the biochemical and biological properties of the outer peel enclosing the bulb and the inner peel enclosing the clove using various methodologies, including bioassays, proteomics, and metabolomics. The garlic peels differentially affected germination and post-germination growth, with the outer peel demonstrating a strong negative effect on seed germination of Sinapis alba and on post-germination growth of Brassica juncea. Proteome analysis showed that dead garlic peels possess 67 proteins, including chitinases and proteases, which retained their enzymatic activity. Among primary metabolites identified in garlic peels, the outer peel accumulated multiple sugars, including rhamnose, mannitol, sorbitol, and trehalose, as well as the modified amino acid 5-hydroxylysine, known as a major component of collagen, at a higher level compared to the clove and the inner peel. Growth of Escherichia coli and Staphylococcus aureus was promoted by garlic peel extracts but inhibited by clove extract. All extracts strongly inhibited spore germination of Fusarium oxysporum f.sp. melonis. Thus, the garlic peels not only provide physical protection to vegetative offspring but also appear to function as a refined arsenal of proteins and metabolites for enhancing growth and development, combating potential pathogens, and conferring tolerance to abiotic stresses.

Single and Combined Salinity and Heat Stresses Impact Yield and Dead Pericarp Priming Activity

In the face of climate change and the predicted increase in the frequency and severity of abiotic stresses (e.g., hot spell, salinity), we sought to investigate the effect of salinity (S), short episodes of high temperature (HS) and combination of salinity and high temperature (SHS), at the reproductive phase, on yield with a special focus on the properties of dead pericarps of Brassica juncea. Three interval exposures to HS resulted in massive seed abortion, and seeds from salt-treated plants germinated poorly. Germination rate and final germination of B. juncea seeds were slightly reduced in the presence of salt and SHS pericarp extracts. All pericarp extracts completely inhibited seed germination of tomato and Arabidopsis, but removal of pericarp extracts almost fully restored seed germination. Heat and salinity profoundly affected the accumulation of phytohormones in dead pericarps. Combined stresses highly reduced IAA and ABA levels compared with salt, and enhanced the accumulation of GA1, but abolished the positive effect of salt on the accumulation of GA4, JA and SA. Interestingly, pericarp extracts displayed priming activity and significantly affected seedling performance in a manner dependent on the species and on the origin of the pericarp. While control pericarps improved and reduced the seedlings’ performance of autologous and heterologous species, respectively, pericarps from salt-treated plants were harmless or improved heterologous seedling performance. Thus, the strategy employed by the germinating seed for securing resources is set up, at least partly, by the mother plant in conjunction with the maternal environment whose components are stored in the dead maternal organs enclosing the embryo.

Differential Response to Single and Combined Salt and Heat Stresses: Impact on Accumulation of Proteins and Metabolites in Dead Pericarps of Brassica juncea

Dead organs enclosing embryos, such as seed coats and pericarps, are emerging as important maternally-derived components of the dispersal unit that affect seed performance and fate. In the face of climate change and increased incidents of heatwaves, we sought to investigate the effect of salinity (S), short episodes of high temperature (HS), and combination of S + HS (SHS), at the reproductive phase, on the properties of dead pericarps of Brassica juncea. Proteome and metabolome analyses revealed multiple proteins and metabolites stored in dead pericarps whose levels and composition were altered under single and combined stress conditions. The protein profile of SHS showed a higher correlation with salt than with HS indicating the dominant effect of salt over heat stress. On the other hand, the analysis of metabolites showed that the profile of SHS has better correlation with HS than with salt. The integration of metabolic and proteomic data showed that changes in TCA cycle intermediates and certain amino acids (e.g., proline) under salt treatments (S and SHS) are highly correlated with changes in proteins involved in their biosynthetic pathways. Thus, accumulation of proteins and metabolites in dead pericarps is differently affected by single and combination of salt and heat stresses. Salinity appears to dominate plant response to combined stresses at the protein level, while heat appears to be the major factor affecting metabolite accumulation in dead pericarps.

Interspecific competition in germination of bird-dispersed seeds in a habitat with sparse tree vegetation in South Africa

BACKGROUND

By transporting and scarifying the seeds during ingestion, avian frugivores reduce the competition with siblings, and may improve the germination which is critical for dispersal effectiveness and population recruitment. However, generally, there is limited knowledge on how deposited seeds interact/compete in the new microsite. We tested the hypothesis that the bird-dispersed seeds benefit from improved germination after their passage through the bird's gut; and we investigated the potential impact of seed density on competition at the microsites by determining whether seed density and species diversity influence germination in the Free State Province, South Africa.

RESULTS

Overall, the results partly supported the hypothesis. Germination trials with defecated seeds of five plant species compared with the manually depulped seeds showed that only Searsia lancea had significantly higher seed germination success and improved germination speed after passage through the bird gut while Ziziphus mucronata only benefited rapid germination. There was a significant correlation between seed size and the germination of bird-ingested seeds except in Olea subsp. africana possibly due to possession of extremely hard protective seed cover. Seed competition experiments pointed to Z. mucronata and O. subsp. africana having significant germination performance that was positively correlated to seed density and seed size while Ehretia rigida did not germinate at all. Seed species diversity in the germination trays did not have a significant impact since the seeds of two former plant species consistently displayed significantly higher germination across the competition levels.

CONCLUSIONS

We conclude that different plant species respond differently to seed ingestion by birds, and that further long-term tests for germination physiological responses of the seeds' samples used in this study are required since poor germination observed in other tree/shrub species cannot be attributed to competition solely.

Functional packaging of seeds

The encapsulation of seeds in hard coats and fruit walls (pericarp layers) fulfils protective and dispersal functions in many plant families. In angiosperms, packaging structures possess a remarkable range of different morphologies and functionalities, as illustrated by thermo and hygro-responsive seed pods and appendages, as well as mechanically strong and water-impermeable shells. Key to these different functionalities are characteristic structural arrangements and chemical modifications of the underlying sclerenchymatous tissues. Although many ecological aspects of hard seed encapsulation have been well documented, a detailed understanding of the relationship between tissue structure and function only recently started to emerge, especially in the context of environmentally driven fruit opening and seed dispersal (responsive encapsulations) and the outstanding durability of some seed coats and indehiscent fruits (static encapsulations). In this review, we focus on the tissue properties of these two systems, with particular consideration of water interactions, mechanical resistance, and force generation. Common principles, as well as unique adaptations, are discussed in different plant species. Understanding how plants integrate a broad range of functions and properties for seed protection during storage and dispersal plays a central role for seed conservation, population dynamics, and plant-based material developments.

Anatomy and Histochemistry of Seed Coat Development of Wild (Pisum sativum subsp. elatius (M. Bieb.) Asch. et Graebn. and Domesticated Pea (Pisum sativum subsp. sativum L.)

In angiosperms, the mature seed consists of embryo, endosperm, and a maternal plant-derived seed coat (SC). The SC plays a role in seed filling, protects the embryo, mediates dormancy and germination, and facilitates the dispersal of seeds. SC properties have been modified during the domestication process, resulting in the removal of dormancy, mediated by SC impermeability. This study compares the SC anatomy and histochemistry of two wild (JI64 and JI1794) and two domesticated (cv. Cameor and JI92) pea genotypes. Histochemical staining of five developmental stages: 13, 21, 27, 30 days after anthesis (DAA), and mature dry seeds revealed clear differences between both pea types. SC thickness is established early in the development (13 DAA) and is primarily governed by macrosclereid cells. Polyanionic staining by Ruthenium Red indicated non homogeneity of the SC, with a strong signal in the hilum, the micropyle, and the upper parts of the macrosclereids. High peroxidase activity was detected in both wild and cultivated genotypes and increased over the development peaking prior to desiccation. The detailed knowledge of SC anatomy is important for any molecular or biochemical studies, including gene expression and proteomic analysis, especially when comparing different genotypes and treatments. Analysis is useful for other crop-to-wild-progenitor comparisons of economically important legume crops.

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.

Extending Plant Defense Theory to Seeds

Plant defense theory explores how plants invest in defenses against natural enemies but has focused primarily on the traits expressed by juvenile and mature plants. Here we describe the diverse ways in which seeds are chemically and physically defended. We suggest that through associations with other traits, seeds are likely to exhibit defense syndromes that reflect constraints or trade-offs imposed by selection to attract dispersers, enable effective dispersal, ensure appropriate timing of seed germination, and enhance seedling performance. We draw attention to seed and reproductive traits that are analogous to defense traits in mature plants and describe how the effectiveness of defenses is likely to differ at pre- and postdispersal stages. We also highlight recent insights into the mutualistic and antagonistic interactions between seeds and microbial communities, including fungi and endohyphal bacteria, that can influence seed survival in the soil and subsequent seedling vigor.

Dead but Not Dead End: Multifunctional Role of Dead Organs Enclosing Embryos in Seed Biology

Dry fruits consist of two types, dehiscent and indehiscent, whereby the fruit is splitting open or remains closed at maturity, respectively. The seed, the dispersal unit (DU) of dehiscent fruits, is composed of three major parts, the embryo and the food reserve, encapsulated by the maternally-derived organ, the seed coat. Indehiscent fruit constitutes the DU in which the embryo is covered by two protective layers (PLs), the seed coat and the fruit coat. In grasses, the caryopsis, a one-seeded fruit, can be further enclosed by the floral bracts to generate two types of DUs, florets and spikelets. All protective layers enclosing the embryo undergo programmed cell death (PCD) at maturation and are thought to provide mainly a physical shield for embryo protection and a means for dispersal. In this review article, I wish to highlight the elaborate function of these dead organs enclosing the embryo as unique storage structures for beneficial substances and discuss their potential role in seed biology and ecology.

Extreme drought alters progeny dispersal unit properties of winter wild oat (Avena sterilis L.)

The dead husk is a vital component of the dispersal unit whose biochemical properties can be modified following exposure to drought. This might affect seed performance and fate, soil properties and consequently plant biodiversity. We investigated the effects of extreme drought on the dispersal unit (DU) properties of winter wild oat (Avena sterilis L.) in the Mediterranean ecosystems focusing on a commonly ignored component of the DU, namely the dead floral bracts (husk). DUs were collected from a climate change experimental research station in the Judean Hills, Israel, simulating extreme drought and from two additional sites differing in the rainfall amounts. Our results showed that drought conditions significantly affected A. sterilis reproductive traits displaying reduced DUs and caryopses weights. The husk contributes profoundly to seed performance showing that germination from the intact DUs or the intact florets 1 was higher, faster and more homogenous compared to naked caryopses; no effect of drought on germination properties was observed. The husk stored hundreds of proteins that retain enzymatic activity and multiple metabolites including phytohormones. Changes in rainfall amounts affected the composition and levels of proteins and other metabolites accumulated in the husk, with a notable effect on abscisic acid (ABA). The husk of both control and drought plants released upon hydration substances that selectively inhibited other species seed germination as well as substances that promoted microbial growth. Our data showed that the dead husk represents a functional component of the DU that have been evolved to nurture the embryo and to ensure its success in its unique habitat. Furthermore, drought conditions can modify husk biochemical properties, which in turn might affect seed performance and fate, soil microbiota and soil fertility and consequently plant species diversity.

Maternal environment alters dead pericarp biochemical properties of the desert annual plant Anastatica hierochuntica L

The dead organs enclosing embryos (DOEEs) emerge as central components of the dispersal unit (DU) capable for long-term storage of active proteins and other substances that affect seed performance and fate. We studied the effect of maternal environment (salt and salt+heat) on progeny DU (dry indehiscent fruit) focusing on pericarp properties of Anastatica hierochuntica. Stressed plants displayed increased seed abortion and low level and rate of germination. Hydrated pericarps released antimicrobial factors and allelopathic substances that inhibit germination of heterologous species. Proteome analysis of dead pericarps revealed hundreds of proteins, among them nucleases, chitinases and proteins involved in reactive oxygen species detoxification and cell wall modification. Salt treatment altered the composition and level of proteins stored in the pericarp. We observed changes in protein profile released from seeds of salt-treated plants with a notable increase in a small anti-fungal protein, defensin. The levels of phytohormones including IAA, ABA and salicylic acid were reduced in dead pericarps of stressed plants. The data presented here highlighted the predominant effects of maternal environment on progeny DUs of the desert plant A. hierochuntica, particularly on pericarp properties, which in turn might affect seed performance and fate, soil fertility and consequently plant biodiversity.

Genomic Characterization and Expressional Profiles of Autophagy-Related Genes (ATGs) in Oilseed Crop Castor Bean (Ricinus communis L.)

Cellular autophagy is a widely-occurring conserved process for turning over damaged organelles or recycling cytoplasmic contents in cells. Although autophagy-related genes (ATGs) have been broadly identified from many plants, little is known about the potential function of autophagy in mediating plant growth and development, particularly in recycling cytoplasmic contents during seed development and germination. Castor bean (Ricinus communis) is one of the most important inedible oilseed crops. Its mature seed has a persistent and large endosperm with a hard and lignified seed coat, and is considered a model system for studying seed biology. Here, a total of 34 RcATG genes were identified in the castor bean genome and their sequence structures were characterized. The expressional profiles of these RcATGs were examined using RNA-seq and real-time PCR in a variety of tissues. In particular, we found that most RcATGs were significantly up-regulated in the later stage of seed coat development, tightly associated with the lignification of cell wall tissues. During seed germination, the expression patterns of most RcATGs were associated with the decomposition of storage oils. Furthermore, we observed by electron microscopy that the lipid droplets were directly swallowed by the vacuoles, suggesting that autophagy directly participates in mediating the decomposition of lipid droplets via the microlipophagy pathway in germinating castor bean seeds. This study provides novel insights into understanding the potential function of autophagy in mediating seed development and germination.

The role of seed appendage in improving the adaptation of a species in definite seasons: a case study of Atriplex centralasiatica

BACKGROUND

As a common accompanying dispersal structure, specialized seed appendages play a critical role in the successful germination and dispersal of many plants, and are regarded as an adaptation character for plants survival in diverse environments. However, little is known about how the appendages modulate the linkage between germination and environmental factors. Here, we tested the responses of germination to seasonal environmental signals (temperature and humidity) via seed appendages using Atriplex centralasiatica, which is widely distributed in salt marshlands with dry-cold winter in northern China. Three types of heteromorphic diaspores that differ in morphology of persistent bracteole and dormancy levels are produced in an individual plant of A. centralasiatica.

RESULTS

Except for the nondormant diaspore (type A, with a brown seed enclosed in a persistent bracteole), bracteoles regulated inner seed dormancy of the other two dormant diaspore types, i.e., type B (flat diaspore with a black inner seed) and type C (globular diaspore with a black inner seed). For types B and C, germination of bracteole-free seeds was higher than that of intact diaspores, and was limited severely when incubated in the bracteole-soaking solution. Dormancy was released at a low temperature (< 10 °C) and suitable humidity (5-15%) condition. Oppositely, high temperature and unfit humidity induced secondary dormancy via inhibitors released by bracteoles. Type C with deeper dormancy needed more stringent conditions for dormancy release and was easier for dormancy inducement than type B. The germination windows were broadened and the time needed for dormancy release decreased after the bracteole flushing for the two dormant types in the field condition.

CONCLUSIONS

Bracteoles determine the germination adaptation by bridging seeds and environmental signals and promising seedlings establishment only in proper seasons, which may also restrict species geographical distribution and shift species distributing ranges under the global climate change scenarios.

Programmed cell death in soybean seed coats

Seed coat is the tissue which establishes an interface between the seed inner tissues and external environment. Our group has shown that cowpea seed coat undergoes coordinated events of programmed cell death (PCD) during development. In relation to germinating seed coats, little is known on PCD events. The goal here was to investigate the biochemical aspects of germinating soybean seed coat, focusing on proteolytic activities related to PCD. In gel and in solution activity profiles of quiescent and germinating seed coat extracts revealed a complex pattern of caspase- and metacaspase-like cysteine protease activities. Trypsin inhibitor and reserve proteins were revealed as potential substrates for these proteases. A pancaspase inhibitor (z-VAD-CHO) affected the radicle length of seeds germinated under its presence. Ultrastructural analysis showed the absence of cell organelles in all seed coat layers after imbibition, while oligonucleosome fragments peaked at 72 h after imbibition (HAI). Altogether, the data suggest the presence of biochemical PCD hallmarks in germinating soybean seed coat and point to the involvement of the detected protease activities in processes such as reserve protein mobilization and weakening of seed coat.

Roles of Hardened Husks and Membranes Surrounding Brachypodium hybridum Grains on Germination and Seedling Growth

Several studies have assessed the function and significance of the presence of dead, hardened husks on germination and seedling growth in several grass species and reached to inconsistent results. Here, we assess the roles of husks (dead lemma and palea) and an inner membrane surrounding the grains on germination behaviour and seedling growth of Brachypodium hybridum, one of three species of the genetic model B. distachyon complex, in an arid mountain of Arabia. The interactive effects between temperature and the incubation light were assessed on germination of husked and dehusked-demembraned grains. Germination and seedling growth were assessed for different combinations of grain treatments (soaked and non-soaked husked, dehusked-membraned and dehusked-demembraned). Dehusked-demembraned grains were also germinated in different dormancy regulating compounds (DRCs) and light qualities (light, dark and different red: far red [R: FR] ratios). The results indicated an insignificant difference between husked and dehusked-membraned grains on final germination and the germination rate index (GRI), with the former producing significantly bigger seedlings. Removal of the inner-membrane resulted in a significant reduction in all traits. Soaking grains in water resulted in significant enhancements in germination and seedling growth of only husked grains. Husked-membraned and demembraned grains germinated more significantly and faster at lower rather than higher temperatures. None of different concentrations of several DRCs succeeded in enhancing final germination of dehusked-demembraned grains. Red-rich light significantly enhanced germination of dehusked-membraned grains in comparison to other light qualities. It could be concluded that the role of husks is to mainly enhance seedling growth, while the major role of the membrane is to increase final germination. The ability of red-rich light in enhancing the germination of dehusked-membraned but not dehusked-demembraned grains suggest a role for the inner membrane in regulating dormancy through differential filtering of light properties.

Cadmium and Plant Development: An Agony from Seed to Seed

Anthropogenic pollution of agricultural soils with cadmium (Cd) should receive adequate attention as Cd accumulation in crops endangers human health. When Cd is present in the soil, plants are exposed to it throughout their entire life cycle. As it is a non-essential element, no specific Cd uptake mechanisms are present. Therefore, Cd enters the plant through transporters for essential elements and consequently disturbs plant growth and development. In this review, we will focus on the effects of Cd on the most important events of a plant's life cycle covering seed germination, the vegetative phase and the reproduction phase. Within the vegetative phase, the disturbance of the cell cycle by Cd is highlighted with special emphasis on endoreduplication, DNA damage and its relation to cell death. Furthermore, we will discuss the cell wall as an important structure in retaining Cd and the ability of plants to actively modify the cell wall to increase Cd tolerance. As Cd is known to affect concentrations of reactive oxygen species (ROS) and phytohormones, special emphasis is put on the involvement of these compounds in plant developmental processes. Lastly, possible future research areas are put forward and a general conclusion is drawn, revealing that Cd is agonizing for all stages of plant development.

Characterization of Soaking Process' Impact in Common Beans Phenolic Composition: Contribute from the Unexplored Portuguese Germplasm

Despite the common beans' nutritional and phytochemical value, in Portugal its consumption decreased more than 50% in the last decade. The present study aimed to characterize phenolic composition of the Portuguese traditional varieties and corresponding soaked seed fractions (including soaking water). With such purpose, the phenolic composition (total content of soluble phenolics, flavonoids, and proanthocyanidins) and in vitro antioxidant activity were evaluated in the raw whole flour of 31 Portuguese common bean varieties. The phenolic composition of the soaked fractions was respectively compared to the raw flour. Phenolic compounds' identification and relative quantification were achieved by UPLC-TripleTOF-MS for one representative variety and their fractions. The highest phenolic content was found in colored varieties and the brown market class highlighted as the richest one. The loss of phenolic compounds to the soaking water was highly dependent on variety. The predominant phenolic compounds' classes were flavan-3-ols (soaking water and coats), flavonols (coats), and phenolic acids (cotyledons). This characterization study showed the diversity on the phenolic composition of Portuguese varieties and the need to adjust the soaking and peeling processes to the variety (considering the possible loss of potential health promoter compounds, e.g., phenolic compounds).

Plant proteases during developmental programmed cell death

Proteases are among the key regulators of most forms of programmed cell death (PCD) in animals. Many PCD processes have also been associated with protease expression or activation in plants, However, functional evidence for the roles and actual modes of action of plant proteases in PCD remains surprisingly limited. In this review, we provide an update on protease involvement in the context of developmentally regulated plant PCD. To illustrate the diversity of protease functions, we focus on several prominent developmental PCD processes, including xylem and tapetum maturation, suspensor elimination, endosperm degradation, and seed coat formation, as well as plant senescence processes. Despite the substantial advances in the field, protease functions are often only correlatively linked to developmental PCD, and the specific molecular roles of proteases in many developmental PCD processes remain to be elucidated.

Variation in wild pea (Pisum sativum subsp. elatius) seed dormancy and its relationship to the environment and seed coat traits

BACKGROUND

Seed germination is one of the earliest key events in the plant life cycle. The timing of transition from seed to seedling is an important developmental stage determining the survival of individuals that influences the status of populations and species. Because of wide geographical distribution and occurrence in diverse habitats, wild pea (Pisum sativum subsp. elatius) offers an excellent model to study physical type of seed dormancy in an ecological context. This study addresses the gap in knowledge of association between the seed dormancy, seed properties and environmental factors, experimentally testing oscillating temperature as dormancy release clue.

METHODS

Seeds of 97 pea accessions were subjected to two germination treatments (oscillating temperatures of 25/15 °C and 35/15 °C) over 28 days. Germination pattern was described using B-spline coefficients that aggregate both final germination and germination speed. Relationships between germination pattern and environmental conditions at the site of origin (soil and bioclimatic variables extracted from WorldClim 2.0 and SoilGrids databases) were studied using principal component analysis, redundancy analysis and ecological niche modelling. Seeds were analyzed for the seed coat thickness, seed morphology, weight and content of proanthocyanidins (PA).

RESULTS

Seed total germination ranged from 0% to 100%. Cluster analysis of germination patterns of seeds under two temperature treatments differentiated the accessions into three groups: (1) non-dormant (28 accessions, mean germination of 92%), (2) dormant at both treatments (29 acc., 15%) and (3) responsive to increasing temperature range (41 acc., with germination change from 15 to 80%). Seed coat thickness differed between groups with dormant and responsive accessions having thicker testa (median 138 and 140 µm) than non-dormant ones (median 84 mm). The total PA content showed to be higher in the seed coat of dormant (mean 2.18 mg g-1) than those of non-dormant (mean 1.77 mg g-1) and responsive accessions (mean 1.87 mg g-1). Each soil and bioclimatic variable and also germination responsivity (representing synthetic variable characterizing germination pattern of seeds) was spatially clustered. However, only one environmental variable (BIO7, i.e., annual temperature range) was significantly related to germination responsivity. Non-dormant and responsive accessions covered almost whole range of BIO7 while dormant accessions are found in the environment with higher annual temperature, smaller temperature variation, seasonality and milder winter. Ecological niche modelling showed a more localized potential distribution of dormant group. Seed dormancy in the wild pea might be part of a bet-hedging mechanism for areas of the Mediterranean basin with more unpredictable water availability in an otherwise seasonal environment. This study provides the framework for analysis of environmental aspects of physical seed dormancy.

Regulation of Root-Knot Nematode Behavior by Seed-Coat Mucilage-Derived Attractants

Seed exudates influence the behavior of soil organisms, but how this occurs remains unclear, particularly for multicellular animals. Here we show that compounds associated with Arabidopsis seed-coat mucilage regulate the behavior of soil-borne animals, specifically root-knot nematodes (RKNs). Infective RKN J2 larvae actively travel toward Arabidopsis seeds through chemotaxis. Analysis of Arabidopsis mucilage mutants demonstrated that the attraction of RKNs to Arabidopsis seeds requires the synthesis and extrusion of seed-coat mucilage. Extracted mucilage alone is not sufficient to attract RKNs, but seed-surface carbohydrates and proteins are required for this process. These findings suggest that the RKN chemoattractant is synthesized de novo upon mucilage extrusion but may be highly unstable. RKNs attracted by this mucilage-dependent mechanism can infect the emerging seedling. However, the attraction signal from seedling roots likely acts independently of the seed-coat signal and may mask the attraction to seed-coat mucilage after germination. Multiple RKN species are attracted by Arabidopsis seeds, suggesting that this mechanism is conserved in RKNs. These findings indicate that seed exudate can regulate the behavior of multicellular animals and highlight the potential roles of seed-coat mucilage in biotic interactions with soil microorganisms.

A cell-wall protein SRPP provides physiological integrity to the Arabidopsis seed

Seed and root hair protective protein (SRPP) is expressed in seeds and root hairs, localized in the cell wall, and involved in cell wall integrity. We analyzed a loss-of-function mutant of SRPP, focusing on siliques and seeds. The srpp-1 plants generated dark brown shrunken seeds at a high rate. The germination rate of these defect seeds of srpp-1 was less than 6%, although apparently normal srpp-1 seeds germinated at a rate of 83%. The production ratio of severe phenotypic seeds was dependent on the growth conditions. When the srpp-1 plants were cultivated at low humidity, the defect ratio was 73%, which was significantly higher than that at normal humidity. Defects of the silique and seeds could be detected on day 7 after pollination and the apical region of the siliques displayed a severe phenotype at a high frequency. Complementation with an SRPP gene under the control of promoters specific to the embryo, seed coat, or valve (carpel) partially rescued the phenotype, and complementation using the SRPP promoter fully rescued the phenotype. Furthermore, overexpression of SRPP enhanced the thermotolerance. After the treatment of seeds at 50 °C for 2 h, the germination rate of the seeds from overexpression with the 35S promoter increased to levels twice that of the wild-type seeds. Under the same conditions, no srpp-1 seeds germinated. These results indicate that SRPP is essential for the production of normal viable seeds in siliques under stress conditions. It is possible that modification of the SRPP gene improves seed integrity.

Variation in wild pea (Pisum sativum subsp. elatius) seed dormancy and its relationship to the environment and seed coat traits

BACKGROUND

Seed germination is one of the earliest key events in the plant life cycle. The timing of transition from seed to seedling is an important developmental stage determining the survival of individuals that influences the status of populations and species. Because of wide geographical distribution and occurrence in diverse habitats, wild pea (Pisum sativum subsp. elatius) offers an excellent model to study physical type of seed dormancy in an ecological context. This study addresses the gap in knowledge of association between the seed dormancy, seed properties and environmental factors, experimentally testing oscillating temperature as dormancy release clue.

METHODS

Seeds of 97 pea accessions were subjected to two germination treatments (oscillating temperatures of 25/15 °C and 35/15 °C) over 28 days. Germination pattern was described using B-spline coefficients that aggregate both final germination and germination speed. Relationships between germination pattern and environmental conditions at the site of origin (soil and bioclimatic variables extracted from WorldClim 2.0 and SoilGrids databases) were studied using principal component analysis, redundancy analysis and ecological niche modelling. Seeds were analyzed for the seed coat thickness, seed morphology, weight and content of proanthocyanidins (PA).

RESULTS

Seed total germination ranged from 0% to 100%. Cluster analysis of germination patterns of seeds under two temperature treatments differentiated the accessions into three groups: (1) non-dormant (28 accessions, mean germination of 92%), (2) dormant at both treatments (29 acc., 15%) and (3) responsive to increasing temperature range (41 acc., with germination change from 15 to 80%). Seed coat thickness differed between groups with dormant and responsive accessions having thicker testa (median 138 and 140 µm) than non-dormant ones (median 84 mm). The total PA content showed to be higher in the seed coat of dormant (mean 2.18 mg g^-1) than those of non-dormant (mean 1.77 mg g^-1) and responsive accessions (mean 1.87 mg g^-1). Each soil and bioclimatic variable and also germination responsivity (representing synthetic variable characterizing germination pattern of seeds) was spatially clustered. However, only one environmental variable (BIO7, i.e., annual temperature range) was significantly related to germination responsivity. Non-dormant and responsive accessions covered almost whole range of BIO7 while dormant accessions are found in the environment with higher annual temperature, smaller temperature variation, seasonality and milder winter. Ecological niche modelling showed a more localized potential distribution of dormant group. Seed dormancy in the wild pea might be part of a bet-hedging mechanism for areas of the Mediterranean basin with more unpredictable water availability in an otherwise seasonal environment. This study provides the framework for analysis of environmental aspects of physical seed dormancy.

Seeds-An evolutionary innovation underlying reproductive success in flowering plants

"Seeds nourish, seeds unite, seeds endure, seeds defend, seeds travel," explains the science writer Thor Hanson in his book The Triumph of Seeds (2015). The seed is an ultimate product of land plant evolution. The nursing and protective organization of the seed enable a unique parental care of the progeny that has fueled seed plant radiation. Seeds promote dispersal and optimize offspring production and thus reproductive fitness through biological adaptations that integrate environmental and developmental cues. The composite structure of seeds, uniting tissues that originate from three distinct organisms, enables the partitioning of tasks during development, maturation, and storage, while a sophisticated interplay between the compartments allows the fine-tuning of embryonic growth, as well as seed maturation, dormancy, and germination. In this review, we will highlight peculiarities in the development and evolution of the different seed compartments and focus on the molecular mechanisms underlying the interactions between them.

Seed coats as an alternative molecular factory: thinking outside the box

Seed coats as commodities. Seed coats play important roles in the protection of the embryo from biological attack and physical damage by the environment as well as dispersion strategies. A significant part of the energy devoted by the mother plant to seed production is channeled into the production of the cell layers and metabolites that surround the embryo. Nevertheless, in crop species these are often discarded post-harvest and are a wasted resource that could be processed to yield co-products. The production of novel compounds from existing metabolites is also a possibility. A number of macromolecules are already accumulated in these maternal layers that could be exploited in industrial applications either directly or via green chemistry, notably flavonoids, lignin, lignan, polysaccharides, lipid polyesters and waxes. Here, we summarize our knowledge of the in planta biosynthesis pathways of these macromolecules and their molecular regulation as well as potential applications. We also outline recent work aimed at providing further tools for increasing yields of existing molecules or the development of novel biotech approaches, as well as trial studies aimed at exploiting this underused resource.

The Dead Can Nurture: Novel Insights into the Function of Dead Organs Enclosing Embryos

Plants have evolved a variety of dispersal units whereby the embryo is enclosed by various dead protective layers derived from maternal organs of the reproductive system including seed coats (integuments), pericarps (ovary wall, e.g., indehiscent dry fruits) as well as floral bracts (e.g., glumes) in grasses. Commonly, dead organs enclosing embryos (DOEEs) are assumed to provide a physical shield for embryo protection and means for dispersal in the ecosystem. In this review article, we highlight recent studies showing that DOEEs of various species across families also have the capability for long-term storage of various substances including active proteins (hydrolases and ROS detoxifying enzymes), nutrients and metabolites that have the potential to support the embryo during storage in the soil and assist in germination and seedling establishment. We discuss a possible role for DOEEs as natural coatings capable of "engineering" the seed microenvironment for the benefit of the embryo, the seedling and the growing plant.

Dead Pericarps of Dry Fruits Function as Long-Term Storage for Active Hydrolytic Enzymes and Other Substances That Affect Germination and Microbial Growth

It is commonly assumed that dead pericarps of dry indehiscent fruits have evolved to provide an additional physical layer for embryo protection and as a means for long distance dispersal. The pericarps of dry fruits undergo programmed cell death (PCD) during maturation whereby most macromolecules such DNA, RNA, and proteins are thought to be degraded and their constituents remobilized to filial tissues such as embryo and endosperm. We wanted to test the hypothesis that the dead pericarp represents an elaborated layer that is capable of storing active proteins and other substances for increasing survival rate of germinating seeds. Using in gel assays we found that dead pericarps of both dehiscent and indehiscent dry fruits of various plant species including Arabidopsis thaliana and Sinapis alba release upon hydration multiple active hydrolytic enzymes that can persist in an active form for decades, including nucleases, proteases, and chitinases. Proteomic analysis of indehiscent pericarp of S. alba revealed multiple proteins released upon hydration, among them proteases and chitinases, as well as proteins involved in reactive oxygen species (ROS) detoxification and cell wall modification. Pericarps appear to function also as a nutritional element-rich storage for nitrate, potassium, phosphorus, sulfur, and others. Sinapis alba dehiscent and indehiscent pericarps possess germination inhibitory substances as well as substances that promote microbial growth. Collectively, our study explored previously unknown features of the dead pericarp acting also as a reservoir of biological active proteins, and other substances capable of “engineering” the microenvironment for the benefit of the embryo.