Halophytes


Introduction

Over a century ago, halophytes were defined simply but usefully as species adapted to perpetually saline conditions. These occur naturally throughout the terrestrial earth. Sometimes, they are only transient, as following coastal flooding. In other cases, they are essentially perpetual, as in salt marshes or salt deserts. For the most part, the “salt” in “saline” is sodium chloride. Many soils, however, are affected by sodium carbonates or bicarbonates. Plants endemic to these “sodic” conditions may or may not also be able to grow in saline condition. Some, for example rice, may be much more adapted to sodic soils than saline ones.

More recently, for purposes of physiological and molecular studies, more precise operational definitions have been useful, i.e. as plants capable of completing their life cycles on salt levels ranging from ca. 80 to 300 mM NaCl, depending on the author. There is, however, no consensus regarding other nutrient levels or ratios in such studies. Most studies have been based on plants growing under eutrophic conditions, with high N, P and micronutrients, and Ca:Mg ratios of ca. 5:1. The most common term used to describe these is “Hoagland’s”.  Fewer studies have been based on sea water dilutions (very low N and P, and Ca:Mg ratios of 1:5).

Overall estimates of halophyte numbers now range from ~350 to 2600 species.  They are, however, not an evolutionarily coherent group. Halophytes occur in 37 of the 65 orders of plants. [1] No families are strictly halophytic although some have disproportionately high numbers (e.g. the Chenopodiaceae, now included in the Amaranthaceae). Both halophytes and non-halophytes frequently co-occur in the same genus, e.g. in Aster, Glycine, Plantago, Solanum and Oryza.

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Evolution

How do halophytes evolve?

Large, unpredictable disruptions such as glaciers and volcanic eruptions, by creating severe habitat disequilibria, serve as the critical drivers in the evolution of halophytes and of edaphic endemics in general. [2] The severity of stress that follows disruption, particularly in contrast to neighboring habitats, the extent of the area affected, and proximity to existing similarly stressed sites determines the distinctiveness of the plant community that follows. The severity of the stress, for example, determines the number of marginally tolerant individuals suitable for colonizing the disturbed areas, while the other two factors determine the potential for migration of species among sites. Stress tolerant species colonizing ecological islands do not generally arise de novo. Rather, the founding individuals must be pre-adapted to the stresses, having previously evolved these capacities fortuitously in non-stressful habitats.

Following successful colonization, stabilization of the colonizers and their subsequent evolution as species involves both phenological and genomic changes. Both, critically, prevent – or at least greatly reduce – back-crossing with the original, unfit, progenitors; i.e. they lead to reproductive isolation.

L. maritima (yellow) and Romanzoffia tracyi (white) off the California coast. Photo courtesy of GD Carr.

The divergence of the halophyte, Lasthenia maritima (Asteraceae)from its progenitor, L. minor exemplifies the speed with which this can occur. [3] With the last retreat of the North American glaciers 10,000 to 15,000 years ago and concomitant rising sea levels, L. maritima evolved from a pre-adapted population of L. minor that was left stranded on guano encrusted and salt sprayed islands off the coast of California.

Clearly, what can evolve in a newly developing habitat depends on what is already present in the vicinity. This goes a long way to explaining the polyphyletic distributions of halophytes: orders, families or genera not present at such targets of opportunity do not include halophytes.

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Models

New experimental models for the evolution of halophytes 

There are numerous established and potential model species having populations which differ significantly in their ability to colonize and survive in saline conditions. These may well prove useful in clarifying the development of halophily and the evolution of halophytes. Atriplex canescens, for example, occurs in populations with different ploidy levels and extremely different levels of sodium accumulation even over small geographic areas. [4] Alternately, Lasthenia californica [5] occurs as at least two different races, one restricted to wet sites with high levels of Na+ and Mg2+ salts, and another which occurs at sites with much lower salinity, but also much less water. Under controlled conditions, plants of the first race accumulated 20-fold more Na+ than the second. Even Arabidopsis thaliana, across its wide distribution, may well have sufficient genetic variability that localized genotypes pre-adapted to salinity and to other stresses exist. [6]

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Genomics/evolution

Genomics and the evolution of halophytes [2]

The pre-adaptation of a mesophyte to salt stress – the first requirement to evolution of a halophyte – requires a set of “starter” genes. Conversion to halophytism also requires post-colonization changes at the genome level. The archetypal pre-angiosperm ancestral genome likely had ca.12000 protein-coding genes. Around 320 MYA, there was a genome triplication contemporaneous with the origin of angiosperms, and at 190 MYA, a duplication. Both events are reflected in most of today’s angiosperm genomes. Even taking into account subsequent general and lineage-specific gene loss, the history of angiosperms provides a rich source of genes and their variants to serve as the starter set.

Genome duplications were, of course, not limited to these two events, and hybridization and autopolyploidy continue to occur. All these are thought to confer increased chances of survival in new or novel habitats.This may have been critical, for example, at the KT boundary 66 MYA which cleared many niches and opened many new ones. It has been postulated that the persistence and subsequent expansion of angiosperms across the boundary was associated with a large number of independent genome duplication events.

Not all halophytes are polyploids nor are all polyploids stress tolerant, reflecting gene loss that begins soon after duplication. In some cases, ploidy levels make little difference in salt tolerance, but in other cases, autopolyploids are more salt tolerant than diploids. The continued study of stable polyploids in relation to genome duplication events is clearly warranted. In this case, Atriplex canescens might be a good subject, having stable 2x, 4x, 6x, 8x,10x, 12x, 14x, and 20x chromosome races at least three of which (2x, 4x and 6x) may co-occur at individual sites.

Tandem duplications, gene translocations and transposable element (TE) insertions can also increase copy numbers of genes useful in stress tolerance. Their rate of occurrence may be accelerated by environmental challenges even in a few generations. The potential for deciphering and understanding their importance to halophyte evolution has been increased by the genome sequencing of Schrenkiella parvula and Eutrema salsugineum. Thus far, for example, because the two lineages diverged only about 12 MYA, macro-synteny still characterize the S. parvula and A. thaliana genomes, but with the S. parvula genome interrupted extensively by duplications, translocations and insertions absent in A. thaliana. The contributions of TEs and small RNAs to genome and transcriptome divergence has also been studied by comparing the chasmophyte, Arabidopsis lyrata, with A. thaliana.

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eHALOPH

eHALOPH – an Important halophyte resource

In the last several years, Flowers and co-workers [7] have undertaken to create and curate an electronic database of halophytes, called eHALOPH. This database dates to 1989 when James Aronson published a print list of 1,560 halophytes, ‘compiled for anyone growing or planning to grow halophytes’. Aronson’s list, included information on life form, plant type, distribution, maximum salinity tolerated, photosynthetic pathway and economic uses, and required that the presumed tolerance be manifested over a significant portion of the plant’s life cycle. Aronson’s original data have been supplemented – when possible – with links to literature on the “usual suspects”, i.e. antioxidants, secondary metabolites, compatible solutes, germination responses, molecular data, microbial interactions and mycorrhizal status. Geographical distribution information has been enriched in eHALOPH by providing distribution maps.

Although eHALOPH is already a very important and useful compendium, and its authors have invited users to help update it by supplying the information for their favorite species. At this point, they have listed more than 1500 species, and 265 entries have been fully curated. Their list of links to books and special journal issues devoted to halophytes is also a valuable contribution.

This database has numerous areas of application, including biosaline agriculture, phytoremediation and ecological rehabilitation of secondarily salinized areas, and ecological restoration of desert and coastal ecosystems that are naturally saline.

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Practical applications

Halophytes, the concept of salt tolerance and its practical application 

Technically, it is possible to contrast halophytes with “most” plants because the number of halophytes is a small fraction of the total number of angiosperms (there are apparently no halophytic gymnosperms). Commonly, the distinction is phrased in terms of “salt tolerance” (although halophytes can actually seem to be simply “salt indifferent”).

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Acknowledgements

Acknowledgements – The developers thank Tim Flowers (University of Sussex) for his assistance and valuable input in developing the halophyte pages.

References

Bibliography for the backstory

Bibliography for the backstory

1.
Flowers TJ, Galal HK, Bromham L. Evolution of halophytes: multiple origins of salt tolerance in land plants [Internet]. Vol. 37, Functional Plant Biology. CSIRO Publishing; 2010. p. 604. Available from: http://dx.doi.org/10.1071/FP09269 [Source]
2.
Cheeseman JM. The evolution of halophytes, glycophytes and crops, and its implications for food security under saline conditions [Internet]. Vol. 206, New Phytologist. Wiley-Blackwell; 2014. p. 557–70. Available from: http://dx.doi.org/10.1111/nph.13217
3.
Rajakaruna N. The Edaphic Factor in the Origin of Plant Species [Internet]. Vol. 46, International Geology Review. Informa UK Limited; 2004. p. 471–8. Available from: http://dx.doi.org/10.2747/0020-6814.46.5.471
4.
Glenn E, Pfister R, Brown JJ, Thompson TL, O’Leary J. Na and K Accumulation and Salt Tolerance of Atriplex canescens (Chenopodiaceae) Genotypes [Internet]. Vol. 83, American Journal of Botany. Botanical Society of America; 1996. p. 997. Available from: http://dx.doi.org/10.2307/2445988
5.
Rajakaruna N, Siddiqi MY, Whitton J, Bohm BA, Glass ADM. Differential responses to Na+/K+ and Ca2+/Mg2+ in two edaphic races of the Lasthenia californica (Asteraceae) complex: A case for parallel evolution of physiological traits [Internet]. Vol. 157, New Phytologist. Wiley-Blackwell; 2003. p. 93–103. Available from: http://dx.doi.org/10.1046/j.1469-8137.2003.00648.x
6.
DeRose-Wilson L, Gaut BS. Mapping Salinity Tolerance during Arabidopsis thaliana Germination and Seedling Growth [Internet]. Polymenis M, editor. Vol. 6, PLoS ONE. Public Library of Science (PLoS); 2011. p. e22832. Available from: http://dx.doi.org/10.1371/journal.pone.0022832
7.
Santos J, Al-Azzawi M, Aronson J, Flowers T. eHALOPH a Database of Salt-Tolerant Plants: Helping put Halophytes to Work. Plant Cell Physiol. 2016 Jan 1;57(1):e10. [PubMed]