The Amaranthaceae is a family with a higher proportion of halophytes than any other family of flowering plants. Amongst these, the genus Suaeda with some 80 species has at least 41 species for which there is clear evidence that they are halophytes. Suaeda maritima, the annual or white seablite, is one of the species for which there is the most extensive published research – by early 2017, there were some 135 publications naming this species in the titles of papers listed in the Web of Science; some examples are listed in the bibliography. [Note for those who have been around too long or are who are trying to make sense of the literature on halophytes: until recently, many of the species in this family that are halophytes were assigned to the Chenopodiaceae.]
S. maritima is an annual species widely distributed across the world in coastal and inland saline soils. The species is tetraploid (2n=36) with variation that has prompted the nomination of a number of subspecies. According to The Plant List, there are six synonyms, five of which were formerly varieties and one a subspecies; there are two accepted subspecies, S. maritima subsp. pannonica and S. maritima subsp. salsa. For Suaeda maritima subsp. salsa, also known as Suaeda salsa, there is an even larger number of publications (358) than for S maritima and the (sub)species has been proposed as a model halophyte. [1]
The relationship between the phenology of the halophyte Suaedas and naturally occurring seasonal salinity trends may be reflected in seed germination responses. Germination is reportedly reduced under saline conditions [2] , but can be improved by treatment with gibberellic acid and following treatment with cold seawater. [2][3] However, there is still more to learn with respect to the interaction of storage/dormancy conditions, temperature and the ability to germinate in the presence of seawater. Once germinated, growth is stimulated by the presence of sodium chloride, being maximal at 200 to 300 mM NaCl. Carbon is fixed by the C3 pathway, and tolerance to flooding appears to involve the accumulation of considerable concentrations of lactate [4]; under such conditions the concentrations of some TCA-cycle intermediates, proline and glycine betaine also increase. [5] Under the hypoxic conditions typical of the salt marsh environment, iron toxicity may limit growth. [6]
The concept of the soil microbiome and its importance to plants has received considerable attention in recent years, and bacterial species have been characterized in association with the roots of S. maritima. [4]
In the future, molecular studies may throw more light on the adaptations of this intriguing species. At present, the primary molecular resources are those related to transcriptomics.
Transcriptomic resources for Suaeda maritima, S. glauca and S. fruticosa
1. Gharat et al. [7] analyzed the transcriptome in response to NaCl application (2% for 9 h) in order to “understand” the molecular basis of salt tolerance through the transcriptome profiling. Salt application resulted in ≥ 2-fold upregulation of 647 genes and downregulation of 735 genes. Of these, the majority were grouped into the “poorly characterized” category. Suaeda homologs were found to ~50% of the genes assigned to MapMan pathways, especially to transcription factors. The authors concluded that “processes related to salt tolerance might be highly complex”. The secondary assembly transcripts sequences have been deposited at NCBI as Bioproject PRJNA314928, SRA accession SRP071624.
2. A similar study was published by Jin et al. [8] using S. glauca, and 6 different NaCl treatments (up to 1M), applied to 1 month old seedlings and harvested after 11 d treatment. Again, the conclusions were as would be expected: more attention “should be paid to transcripts associated with signal transduction, transporters, the cell wall and growth, defense metabolism and transcription factors involved in salt tolerance.” The assembled transcripts were deposited NCBI as Bioproject PRJNA295637, with SRAs for each of the 6 treatments.
3. Diray-Arce et al. [9] characterized the transcriptome of S. fruticosa, an annual shrub found in coastal and inland regions of Pakistan and Mediterranean shores. The assembled transcripts were reportedly deposited at NCBI SRX973396 but the authors have not yet seen fit to share them – public access is still denied.
Acknowledgement – The bulk of this page was contributed by TJ Flowers from the University of Sussex, UK.