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Seed development culminates in programmed cell death (PCD) and hardening of organs enclosing the embryo (e.g., pericarp, seed coat) providing essentially a physical shield for protection during storage in the soil. We examined the proposal that dead organs enclosing embryos are unique entities that store and release upon hydration active proteins that might increase seed persistence in soil, germination and seedling establishment. Proteome analyses of dead seed coats of Brassicaceae species revealed hundreds of proteins being stored in the seed coat and released upon hydration, many are stress-associated proteins such as nucleases, proteases and chitinases. Functional analysis revealed that dead seed coats function as long-term storage for multiple active hydrolytic enzymes (e.g., nucleases) that can persist in active forms for decades. Substances released from the dead seed coat of the annual desert plant Anastatica hierochuntica displayed strong antimicrobial activity. Our data highlighted a previously unrecognized feature of dead organs enclosing embryos (e.g., seed coat) functioning not only as a physical shield for embryo protection but also as a long-term storage for active proteins and other substances that are released upon hydration to the "seedsphere" and could contribute to seed persistence in the soil, germination and seedling establishment.

Background: Seed of Shismus arabicus, a desert annual, display a seasonal tolerance to dehydration. The occurrence of a metabolic seasonal rhythm and its relation with the fluctuations in seed dehydration tolerance was investigated., Results: Dry seeds metabolism was the least affected by the season, while the metabolism of germinated and dehydrated seeds exhibit distinct seasonal patterns. Negative associations exist between amino acids, sugars and TCA cycle intermediates and seed survival, while positive relations exist with seed germination. In contrast, associations between the level of secondary metabolites identified in the dehydrated seeds and survival percentage were evenly distributed in positive and negative values, suggesting a functional role of these metabolites in the establishment of seed dehydration tolerance., Conclusion: Our results indicate the occurrence of metabolic biorhythms in germinating and dehydrating seeds associated with seasonal changes in germination and, more pronouncedly, in seed dehydration tolerance. Increased biosynthesis of protective compounds (polyphenols) in dehydrating seeds during the winter season at the expenses of central metabolites likely contributes to the respective enhanced dehydration tolerance monitored.

Background and Aims: Repair of damage to DNA of seed embryos sustained during long periods of quiescence under dry desert conditions is important for subsequent germination. The possibility that repair of embryo DNA can be facilitated by small amounts of water derived from dew temporarily captured at night by pectinaceous surface pellicles was tested. These pellicles are secreted during early seed development and form mucilage when hydrated., Methods: Seeds of Artemisia sphaerocephala and Artemisia ordosica were collected from a sandy desert. Their embryos were damaged by gamma radiation to induce a standard level of DNA damage. The treated seeds were then exposed to nocturnal dew deposition on the surface of soil in the Negev desert highlands. The pellicles were removed from some seeds and left intact on others to test the ability of mucilage to support repair of the damaged DNA when night-time humidity and temperature favoured dew formation. Repair was assessed from fragmentation patterns of extracted DNA on agarose gels., Key Results: For A. sphaerocephala, which has thick seed pellicles, DNA repair occurred in seeds with intact pellicles after 50 min of cumulative night dew formation, but not in seeds from which the pellicles had been removed. For A. ordosica, which has thin seed pellicles, DNA repair took at least 510 min of cumulative night dewing to achieve partial recovery of DNA integrity. The mucilage has the ability to rehydrate after daytime dehydration., Conclusions: The ability of seeds to develop a mucilaginous layer when wetted by night-time dew, and to repair their DNA under these conditions, appear to be mechanisms that help maintain seed viability under harsh desert conditions.

The more extreme the desert and the smaller and more unpredictable the annual precipitation and its distribution during the rainy season, the more important the sets of complementary adaptations and survival strategies during the different stages of the life cycle of winter annuals. The history of each seed, from seed development and maturation to seed wetting and germination, may affect the phenotypic plasticity of seed germination. This may be affected by environmental factors such as day length during seed development and maturation. Also involved are maternal influences and the influence of the position of the seeds in the plant inflorescences or capsules, even 30 years after seed maturation. Post-maturation primary or secondary seed dormancy mechanisms and strategies may also be involved. In addition, environmental factors during seed wetting, such as the amount of rain, relative humidity, temperature, light, soil salinity, and germination inhibitors, may play a role in the regulation of the percentage of seeds that are ready to germinate after a particular rainfall. Adaptations and survival strategies in desert annual plant species have also evolved during the stages of seedling development, flowering, and seed dispersal. These include escape or protection mechanisms and strategies of seed dispersal that may decrease seed collection and contribute to the large and long-lasting living aerial or soil seed banks of the species. Many of the common plant species occurring in the Negev, where the average annual rainfall gradient ranges from 25 to 200 mm, produce large quantities of tiny, dust-like seeds. These plant species have developed escape strategies of seed dispersal as well as opportunistic strategies of seed germination. In contrast, other plant species that produce relatively larger seeds in much smaller quantities, have protection strategies of seed dispersal as well as cautious germination strategies. All these adaptations of mechanisms and strategies may affect the chances of these plant species to survive in their desert locations and habitats. The greater the fitness of a plant genotype to its habitat, the greater the chance for the seeds that germinate to complete their life cycles by producing new mature seeds and retaining a large and long-lasting seed bank in such arid areas and extreme deserts. [ABSTRACT FROM AUTHOR]