Utilisation of stored lipids during germination in dimorphic seeds of euhalophyte Suaeda salsa

Combined genome and transcriptome provides insight into the genetic evolution of an edible halophyte Suaeda salsa adaptation to high salinity

Suaeda salsa L. is a typical halophyte with high value as a vegetable. Here, we report a 447.98 Mb, chromosomal-level genome of S. salsa, assembled into nine pseudomolecules (contig N50 = 1.36 Mb) and annotated with 27,927 annotated protein-coding genes. Most of the assembled S. salsa genome, 58.03%, consists of transposable elements. Some gene families including HKT1, NHX, SOS and CASP related to salt resistance were significantly amplified. We also observed expansion of genes encoding protein that bind the trace elements Zn, Fe, Cu and Mn, and genes related to flavonoid and α-linolenic acid metabolism. Many expanded genes were significantly up-regulated under salinity, which might have contributed to the acquisition of salt tolerance in S. salsa. Transcriptomic data showed that high salinity markedly up-regulated salt-resistance related genes, compared to low salinity. Abundant metabolic pathways of secondary metabolites including flavonoid, unsaturated fatty acids and selenocompound were enriched, which indicates that the species is a nutrient-rich vegetable. Particularly worth mentioning is that there was no significant difference in the numbers of cis-elements in the promoters of salt-related and randomly selected genes in S. salsa when compared with Arabidopsis thaliana, which may affirm that plant salt tolerance is a quantitative rather than a qualitative trait in terms of promoter evolution. Our findings provide deep insight into the adaptation of halophytes to salinity from a genetic evolution perspective.

Utilization of halophytes in saline agriculture and restoration of contaminated salinized soils from genes to ecosystem: Suaeda salsa as an example

Halophytes can be used to screen genes for breeding salt-tolerant crops and are of great value in the restoration of salinized or contaminated soils. However, the potential of halophytes in improving saline soils remains limited. In this paper, based on the latest research progress, we use Suaeda salsa L. as an example to evaluate the value of halophytes in developing saline agriculture including: 1) some defined salt-resistance genes and high-affinity nitrate transporter genes in the species for breeding salt-tolerance and nitrogen efficiency crops; 2) the value of S. salsa and microorganisms from S. salsa in remediation of heavy metal contaminated and organic polluted saline soils; and 3) the capacity to remove salts from soils and the application of the species. In conclusion, S. salsa has high value as a candidate to explore the theoretical base and practical application for utilizing halophytes to improve salinized soils from genes to ecosystem.

Differences in seed characteristics, germination and seedling growth of Suaeda salsa grown in intertidal zone and on saline inland

The ecological restoration of saline land in the Yellow River Delta is essential for the sustainability of this region. Halophytic species, like Suaeda salsa, are critical for the restoration process. However, potential differences in traits of heteromorphic seeds collected from the intertidal zone and inland condition have been largely overlooked. The seeds were analyzed for hardness, nutrient elements, and secretions, while structural differences were observed under a stereomicroscope. Germination percentages of the different seed types and subsequent seedling growth were also recorded. Our study found that the black seeds from intertidal zone had the highest hardness when compared to the three other types of seeds. Nutrient analysis revealed that brown seeds had a higher iron (Fe) content than black seeds. Accordingly, brown seed embryos were greener compared to their black seed counterparts due to the iron's role in chlorophyll synthesis. Our results also revealed that brown seeds secreted greater amounts of exudates than black seeds. Finally, both the intertidal brown seeds and the inland-grown brown seeds had higher germination percentages and better early seedling growth than the corresponding black seeds. The differential characteristics between dimorphic seeds and seedlings may influence their environmental adaptation in different saline environments.

Positive effects of NaCl on the photoreaction and carbon assimilation efficiency in Suaeda salsa

Suaeda salsa L. is a typical euhalophyte and can be used as a model halophyte for research on salt tolerance. S. salsa seedlings were cultured with 0, 200 and 500 mM NaCl for 2 and 14 days. The results revealed that 200 mM NaCl promoted the shoot dry weight, net photosynthetic rate (Pn), chlorophyll content, electron transfer rate, NADPH level, activities of ferredoxin-NADP oxidoreductase, ribulose-1, 5-bisphosphate carboxylase (Rubisco) and Rubisco activase, particularly the Rubisco activity, chlorophyll content and Pn, when the seedlings were treated with 200 mM NaCl for 14 days. Moreover, 500 mM NaCl had no adverse effects on those parameters. In conclusion, NaCl improved the photoreaction and carbon assimilation efficiency of S. salsa. The increased Rubisco activity and chlorophyll content may play critical roles in the enhancement of the photosynthetic efficiency in S. salsa under saline conditions, which may explain why S. salsa can produce the highest biomass at approximately 200 mM NaCl.

Growth, Stoichiometry, and Palatability of Suaeda salsa From Different Habitats Are Demonstrated by Differentially Expressed Proteins and Their Enriched Pathways

Suaeda salsa (L.) Pall., a medicinal and edible plant, has green and red-violet ecotypes that exhibit different phenotypes, tastes, and growth characteristics. However, few studies have focused on these differences from the aspect of differentially expressed proteins under the conditions of different habitats in the field. In this study, two ecotypes of S. salsa from the intertidal (control) and supratidal (treatment) habitats of the Yellow River Delta were selected. A total of 30 individual leaves were mixed into six samples (three biological replicates for each) and subjected to protein extraction by using tandem mass tag-labeled quantitative proteomic technology. A total of 4771 proteins were quantitated. They included 317 differentially expressed proteins (2.0-fold change, p < 0.05), among which 143 were upregulated and the remaining 174 were downregulated. These differentially expressed proteins mainly participated in biological processes, such as response to stimulus, stress, and biotic stimulus; in molecular functions, such as methyltransferase activity, transferase activity, one-C group transfer, and tetrapyrrole binding; and in cell components, such as non-membrane-bound organelles, intracellular non-membrane-bound organelles, chromosomes, and photosystems. The differentially expressed proteins were mainly enriched in eight pathways, among which the ribosome, phenylpropanoid biosynthesis, and photosynthesis pathways had higher protein numbers than the other pathways. The upregulation of differentially expressed proteins related to the ribosome and photosynthesis increased the relative growth rate and reduced the N:P ratio of S. salsa from the supratidal habitat, thereby improving its palatability. By contrast, most of the differentially expressed proteins involved in phenylpropanoid biosynthesis were downregulated in S. salsa from the intertidal habitat. This result indicated that S. salsa from the intertidal habitat might accumulate flavonoids, lignin, and other secondary metabolites in its leaves that confer a bitter taste. However, these secondary metabolites might increase the medicinal value of S. salsa from the intertidal habitat. This work could provide a theoretical basis and data support for the sustainable and high-value utilization of medicinal and edible plants from coastal wetlands.

Ecological, (epi)genetic and physiological aspects of bet-hedging in angiosperms

KEY MESSAGE

Bet-hedging is a complex evolutionary strategy involving morphological, eco-physiological, (epi)genetic and population dynamics aspects. We review these aspects in flowering plants and propose further research needed for this topic. Bet-hedging is an evolutionary strategy that reduces the temporal variance in fitness at the expense of a lowered arithmetic mean fitness. It has evolved in organisms subjected to variable cues from the external environment, be they abiotic or biotic stresses such as irregular rainfall or predation. In flowering plants, bet-hedging is exhibited by hundreds of species and is mainly exerted by reproductive organs, in particular seeds but also embryos and fruits. The main example of bet-hedging in angiosperms is diaspore heteromorphism in which the same individual produces different seed/fruit morphs in terms of morphology, dormancy, eco-physiology and/or tolerance to biotic and abiotic stresses in order to 'hedge its bets' in unpredictable environments. The objective of this review is to provide a comprehensive overview of the ecological, genetic, epigenetic and physiological aspects involved in shaping bet-hedging strategies, and how these can affect population dynamics. We identify several open research questions about bet-hedging strategies in plants: 1) understanding ecological trade-offs among different traits; 2) producing more comprehensive phylogenetic analyses to understand the diffusion and evolutionary implications of this strategy; 3) clarifying epigenetic mechanisms related to bet-hedging and plant responses to environmental cues; and 4) applying multi-omics approaches to study bet-hedging at different levels of detail. Clarifying those aspects of bet-hedging will deepen our understanding of this fascinating evolutionary strategy.

Role of Suaeda salsa SsNRT2.1 in nitrate uptake under low nitrate and high saline conditions

The global annual loss in agricultural production resulting from soil salinization is significant. Although nitrate (NO₃^-) is known to play both nutritional and osmotic roles in the salt tolerance of halophytes, it remains unclear how halophytes such as Suaeda salsa L. take up NO₃^- under saline conditions. In the present study, the gene of nitrate transporter 2.1 (SsNRT2.1) was cloned from S. salsa and its function was identified in both S. salsa and Arabidopsis thaliana under salinity and low NO₃^--N (0.5 mM NO₃^-) conditions. The results revealed that SsNRT2.1 expression and NO₃^- concentration in the roots of S. salsa were higher at 200 mM NaCl, compared with that at 0 and 500 mM NaCl after 24 h treatment. The Arabidopsis overexpression lines showed a higher NO₃^- content compared to the WT lines at 0 and 50 mM NaCl. A similar trend was observed in the root length. In conclusion, salinity promoted the SsNRT2.1 expression in S. salsa, suggesting that this gene may contribute to the efficient NO₃^- uptake in S. salsa under low NO₃^- and high salinity conditions. This trait may explain why S. salsa can tolerate high salinity and produce the highest biomass at about 200 mM NaCl.

Heavy metal tolerance and potential for remediation of heavy metal-contaminated saline soils for the euhalophyte Suaeda salsa

Due to irrigation practices and industrial pollution, large areas of the lands in the world are simultaneously affected by salinity and heavy metal contamination. It has been considered that halophytes have adapted to salinity, and can be used to remediate heavy metal-contaminated saline soils. Suaeda salsa L. (S. salsa) is a high salt-resistance plant, which can efficiently absorb and accumulate salt and toxic metals from saline soils, suggesting that this may be potential plant species that can be used for the restoration of saline soils contaminated with heavy metals. The present brief review sheds light on the characteristics of S. salsa in the uptake and accumulation of high levels of heavy metals. Furthermore, the physiological and molecular mechanisms for heavy metal tolerance were highlighted. The potential values of S. salsa in the remediation of saline soils were also summarized.

Exposure to High Salinity During Seed Development Markedly Enhances Seedling Emergence and Fitness of the Progeny of the Extreme Halophyte Suaeda salsa

Irrigation with 200 mM NaCl significantly increases vegetative and reproductive growth of the extreme halophyte Suaeda salsa. However, little is known about how the progeny of S. salsa plants grown under a continuous NaCl supply behave in terms of growth and seed set parameters. We investigated various plant growth and reproductive parameters of the progeny that germinated from seeds harvested from mother plants grown under 0 or 200 mM NaCl over three generations. Seedling emergence, plant height, stem diameter, total branch length, flowering branch length, flowering branch ratio, and seed production were all significantly enhanced in the progeny produced by mother plants grown with 200 mM NaCl compared to progeny of mother plants grown on low salinity conditions. Therefore, irrigation with 200 mM of NaCl is beneficial to seed development in the halophyte S. salsa and possibly contributes to population establishment in high salinity environments. Likewise, the prolonged absence of NaCl in the growth environment inhibits seed development, results in lower seed quality, and thus limits seedling growth of the progeny, thereby restricting S. salsa to a high salinity ecological niche.

Transcriptomic analysis identifies novel genes and pathways for salt stress responses in Suaeda salsa leaves

Salinity is a critical abiotic stress, which significantly impacts the agricultural yield worldwide. Identification of the molecular mechanisms underlying the salt tolerance in euhalophyte Suaeda salsa is conducive to the development of salt-resistant crops. In the present study, high-throughput RNA sequencing was performed after S. salsa leaves were exposed to 300 mM NaCl for 7 days, and 7,753 unigenes were identified as differently expressed genes (DEGs) in S. salsa, including 3,638 increased and 4,115 decreased unigenes. Moreover, hundreds of pathways were predicted to participate in salt stress response in S. salsa by Gene Ontology (GO), MapMan and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses, including ion transport and sequestration as well as photoprotection of photosystem (PS) II. The GO enrichment analysis indicated that genes related to ion transport, reactive oxygen species (ROS) scavenging and transcriptional factors were highly expressed upon NaCl treatment. The excessive Na+ and Cl- ions were supposed to be absorbed into the vacuole for ion sequestration and balance adjustment by potassium transporters (such as KEA3) with high expressions. Moreover, we predicted that mutiple candidate genes associated with photosynthesis (such as PSB33 and ABA4), ROS (such as TAU9 and PHI8) and transcriptional regulation (HB-7 and MYB78) pathways could mitigate salt stress-caused damage in S. salsa.

Adaptation of euhalophyte Suaeda salsa to nitrogen starvation under salinity

Suaeda salsa L. (S. salsa) is an annual euhalophyte with high salt tolerance. The NO₃^- content in soils where S. salsa populations occur are very low, especially in intertidal habitat. However, it remains unclear how S. salsa populations adapt to low nitrogen environments. Plants of two S. salsa populations were pre-cultured with nitrate nitrogen (1 mM of NO₃^--N) for 30 days. Then, the seedlings were cultured with 1 mM of NO₃^--N and N-free solution (N starvation) at 200 mM of NaCl for an additional 14 days. The expression of two genes in S. salsa, nitrate transporter 1.7 (SsNRT1.7) and nitrate transporter 2.5 (SsNRT2.5) in old and mature leaves, was markedly upregulated during N starvation in the intertidal population, when compared to the inland population, but this was not the case in young leaves. After N starvation, the decrease in NO₃^- and chlorophyll content, net photosynthetic rate in young leaves, and shoot dry weight in the intertidal population were lower than those in the inland population. In conclusion, SsNRT1.7 and SsNRT2.5 may play a role in NO₃^- remobilization, especially in the intertidal population, during N starvation. This trait may benefit the intertidal population for adapting to low nitrogen environments.

Analysis of widely targeted metabolites of the euhalophyte Suaeda salsa under saline conditions provides new insights into salt tolerance and nutritional value in halophytic species

BACKGROUND

Suaeda salsa L. (S. salsa) is an annual euhalophyte with high salt tolerance and high value as an oil crop, traditional Chinese medicine and vegetable. However, there are few comprehensive studies on the metabolomics of S. salsa under saline conditions.

RESULTS

Seedlings of S. salsa were cultured with 0, 200 and 500 mM NaCl for two days. Then, widely targeted metabolites were detected with ultra performance liquid chromatography and tandem mass spectrometry. A total of 639 metabolites were annotated. Among these, 253 metabolites were differential metabolites. Salt treatment increased the content of certain metabolites, such as nucleotide and its derivates, organic acids, the content of amino acids, lipids such as α-linolenic acid, and certain antioxidants such as quercetin. These substances may be correlated to osmotic tolerance, increased antioxidant activity, and medical and nutritional value in the species.

CONCLUSION

This study comprehensively analyzed the metabolic response of S. salsa under salinity from the perspective of omics, and provides an important theoretical basis for understanding salt tolerance and evaluating nutritional value in the species.

Seed Heteromorphism: An Important Adaptation of Halophytes for Habitat Heterogeneity

Seed germination is a very critical and important step for seedling establishment under saline environments, as high level of salinity in the soil can prevent seed germination. However halophytes exhibit an interesting mechanism to cope with salt stress. Many halophytes produce heteromorphic seeds, which have different dormancy and germination behavior under saline conditions. This characteristic is related to the structural and physiological differences among heteromorphic seeds. It was unclear that how heteromorphic seeds differently accumulate organic and inorganic substances under saline conditions, and what are the physiological and molecular mechanisms involved in the production of heteromorphic seeds, and in the development of transgenerational plasticity in heteromorphic seeds. In the current brief review, dormancy and germination and the possible role of seed coat and storage compounds in this process of heteromorphic seeds development have been discussed. Moreover, the role of maternal effects on heteromorphic seeds production under saline environments and growth and reproduction capability of the descendants from them have been highlighted.

Analysis of storage compounds and inorganic ions in dimorphic seeds of euhalophyte Suaeda salsa

Suaeda salsa is an annual euhalophytic herb that produces dimorphic seeds, such as small black seeds and big brown seeds. In the present study, the fatty acid composition, content of total phenols, flavonoids, carotenoid and inorganic ions in dimorphic seeds of the species collected in the field were measured. There was no significant difference in total oil content between black and brown seeds. Seed total oil content was approximately 19% based on dry weight. The most abundant fatty acid was linoleic acid, and the content was 76.3 and 70.5% of total fatty acids in black and brown seeds, respectively. Furthermore, the contents of total phenols, flavonoids, carotenoids and inorganic ions in brown seeds were higher than those in black seeds, which might be the mechanism of higher salt tolerance of brown seeds than black seeds. The ecological, physiological and genetic mechanisms of the different abilities of nutrition accumulation in black and brown seeds of S. salsa are also discussed and worthy to be investigated in the future.