Adaptation to high salt concentrations in halotolerant/halophilic fungi: a molecular perspective

Abstract

Molecular studies of salt tolerance of eukaryotic microorganisms have until recently been limited to the baker's yeast Saccharomyces cerevisiae and a few other moderately halotolerant yeast. Discovery of the extremely halotolerant and adaptable fungus Hortaea werneckii and the obligate halophile Wallemia ichthyophaga introduced two new model organisms into studies on the mechanisms of salt tolerance in eukaryotes. H. werneckii is unique in its adaptability to fluctuations in salt concentrations, as it can grow without NaCl as well as in the presence of up to 5 M NaCl. On the other hand, W. ichthyophaga requires at least 1.5 M NaCl for growth, but also grows in up to 5 M NaCl. Our studies have revealed the novel and intricate molecular mechanisms used by these fungi to combat high salt concentrations, which differ in many aspects between the extremely halotolerant H. werneckii and the halophilic W. ichthyophaga. Specifically, the high osmolarity glycerol signaling pathway that is important for sensing and responding to increased salt concentrations is here compared between H. werneckii and W. ichthyophaga. In both of these fungi, the key signaling components are conserved, but there are structural and regulation differences between these pathways in H. werneckii and W. ichthyophaga. We also address differences that have been revealed from analysis of their newly sequenced genomes. The most striking characteristics associated with H. werneckii are the large genetic redundancy, the expansion of genes encoding metal cation transporters, and a relatively recent whole genome duplication. In contrast, the genome of W. ichthyophaga is very compact, as only 4884 protein-coding genes are predicted, which cover almost three quarters of the sequence. Importantly, there has been a significant increase in their hydrophobins, cell-wall proteins that have multiple cellular functions.

Impact of a natural soil salinity gradient on fungal endophytes in wild barley (Hordeum maritimum With.)

“…As far as we known, among these genera only Alternaria was previously identified in natural hypersaline environments (GundeCimerman et al 2009;Wang et al 2013;Gunde-Cimerman and Zalar 2014). These variations in the salt tolerance may be attributed to the different mechanisms of tolerance exhibited by different isolates (Plemenitaš et al 2014). Tolerance to NaCl also decreased with incubation time, which suggested that toxicity of NaCl increased with incubation time.…”

Aspergillus glaucus Aquaglyceroporin Gene glpF Confers High Osmosis Tolerance in Heterologous Organisms

“…The levels of glycerol are increased with increasing salinity and decreased after hy-poosmotic shock (51,52). Several genes known to be involved in glycerol production were found in the genomes of H. werneckii and W. ichthyophaga (53). Among these genes, there are three aquaglyceroporin homologues corresponding to AgglpF.…”

Osmosensing and osmoregulation in unicellular eukaryotes

“…In S. pombe the putative sensors are cytoplasmic and have PAS domains, identified in oxygen and redox sensing in bacteria (Gacto et al 2003;Bahn 2008). Cytoplasmic osmosensors have also been recently identified in the highly halotolerant yeasts Debaryomyces hansenii (DhNik1) and Hortaea werneckii (Meena et al 2010;Plemenitas et al 2014). D. hansenii's osmosensor is a group III HK, characterized by having HAMP domain repeats with no transmembrane domains; deletion analysis has shown that interaction among the HAMP repeats could function as an on-off switch during the osmotic stress response (Meena et al 2010).…”

“…Since there are excellent reviews for yeast osmosensing and regulation (Saito 2001;Santos and Shiozaki 2001;O'Rourke et al 2002;Gacto et al 2003;Westfall et al 2004;Hohmann et al 2007;Bahn 2008;Plemenitas et al 2014) we will cover this group for comparison.…”