Acrostichum spp.

The genus Acrostichum, with three species, is the only terrestrial fern growing in the mangrove ecosystem.  A. aureum is also the only mangrove found in both the Indo West-Pacific (IWP) and American East-Pacific (AEP) areas ¹. In Central America, the Caribbean, and south Florida, A danaeifolium is found in sunny, “somewhat saline” to freshwater swamps, but not generally in coastal or mangrove areas.  In the same region, A. aureum is associated with the coastal, shaded saline and mangrove habitats although it, too, can grow happily and rapidly in freshwater lake and canal margins.²  Indeed, a supply of freshwater, even in the mangrove areas, is critical, especially for sexual reproduction; the gametophytic generation appears to be much less salt tolerant than the sporophytic.³

In the IWP, A. speciosum is a rapidly growing species found in the intermediate estuarine and high intertidal zones, and in freshwater. On the Daintree River (Queensland), its range extends from nearly to the mouth, upriver as far as 12 km.¹ It may be more tolerant of saltwater inundation than A. aureum.  In all, these characteristics contribute to it being considered opportunistic and a colonizer of disturbed areas.

In Puerto Rico, both salinity and sun exposure are important factors determining A. aureum size and density.³ Under low salinity conditions, plants are larger in full sun, while at higher salinity, they grow better in the shade of tree mangroves, possibly because of lower evaporative demand.³

While knowledge of physiology is certainly not a prerequisite for deciding to sequence the genome or transcriptome of a new species, in the case of eXtreme plants, it can be a powerful force in deciding what to do with the results.  This is certainly the case for mangroves in general, and for Acrostichum in particular, the Puerto Rico study suggests a number of worthwhile projects.  For example, as leaf osmotic potentials decrease with increasing salinity, the major solutes responsible are Na⁺, Mg²⁺ and sucrose. However, Na⁺ levels clearly indicate one of the better exclusionary capacities for a mangrove; the leaf Cl^–/Na⁺ ratios are consistently higher in A. aureum (2.5) compared to the trees, Laguncularia racemosa (1.2) and Rhizophora mangle (1.5). Na⁺/K⁺ ratios are also lower in the fern (0.4) compared to the trees (greater than 2). These results indicate well-developed mechanisms for ion selectivity and water filtration at the root level, the molecular basis for which is unclear in any species.

Comparing leaves in saline (>30‰) and oligosaline (<5‰) environments, Na⁺, Mg²⁺ and sucrose concentrations increased 1.3, 2.8 and 1.8 fold. However, the putative “compatible osmoticum”, cyclitol D-1-0-methyl-muco-inositol, showed the greatest relative increase, 4.9 fold; under saline conditions, its concentration reached >25% of the sucrose level.  All of these changes are significant enough that there must be an accompanying complex but decipherable network of transcriptome level changes.

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Genomic and transcriptomic resources

To date, just as there have been very few physiological studies including Acrosticum, there has been only one transcriptome project ⁴ and there are fewer than 150 ESTs currently in the GenBank repository. The transcriptome included the two IWP species, A. aureum and A. speciosum, which diverged only ~2.2 MYA. Although both are diploid (with chromosome numbers of 2n = 60), like other ferns, they have very large genomes, ~23.8 Gb, which precludes straightforward genome sequencing.

Using the ratio of the nonsynonymous substitution rate to the synonymous substitution rate (Ka/Ks), the authors were able to identify 27-31 putative positively selected genes in the mangrove ferns. Although they posited them to be involved in “metabolic processes, RNA or DNA binding and specific enzymatic reactions” (and thus that they play a role in responses to light and salt stresses), this seems really to say very little; the data on fern genomes is so far less than that of spermatophytes that only about 25% of the unigenes could be annotated.

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Resources from the literature

The following list was generated by searching PubMed.  As there are many journals not indexed there, this list may be incomplete. The list should be automatically updated as additional publications are indexed.

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Concluding thoughts

It seems unlikely that there will ever be as much attention to physiological or molecular considerations of any fern as to spermatophytes, just as there will never be as much attention paid to slow-growing trees from eXtreme environments as there will be to small, insignificant plants that can be grown in Petri dishes.  Nevertheless, studies sorting out the fundamental mechanisms of adaptation in eXtremophytes should profit from inclusion of Acrostichum aureum.

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