br Results br Discussion Cell responds
Discussion Cell responds to fluctuating environmental factors by activating a set of compensatory mechanisms including changes in the lipid profile. Compensatory reactions occurring in membranes in response to phosphate deprivation include replacement of phospholipids by phosphorus-free lipids such as galactolipids (Härtel et al., 2000), sulfolipids (Benning et al., 1993, Essigmann et al., 1998) or betaine lipids (Benning et al., 1995, Riekhof et al., 2014). The present study increases our understanding of this biochemical strategy aimed at maintaining Pi homeostasis. This is the first report of Pi deficiency-induced DGTS synthesis in basidial fungi. Another feature to be shown for basidiomycete F. velutipes grown under phosphate starvation is elevated level of PS. PS is synthesized from CDP-DAG by CHO1-encoded PS synthase and is regulated by genetic and biochemical mechanisms. The elevated expression of the S. cerevisiae CHO1 gene results in an increase in CHO1 mRNA abundance, PS synthase protein, and its activity (Bailis et al., 1987, Iwanyshyn et al., 2004). Another mechanism by which PS synthase activity is regulated is control of mRNA stability, namely its rate of decay (Choi et al., 2004). Finally, phosphatidylserine synthase from S. cerevisiae is inhibited by phosphorylation (Kinney and Carman, 1988, Choi et al., 2010). The latter mechanism can clarify interaction between Pi deficiency and elevating level of PS. However, the effect of Pi starvation on PS can be more complicated. There is evidence that the concentrations of most PS species with very long chain fatty acids were higher in phosphorus-starved leaves of Arabidopsis than in normally grown ones whereas the concentrations of other major molecular species in phospholipid NADP/NADPH Quantitation Colorimetric Kit were lower in phosphorus-starved leaves (Li et al., 2006). Authors suggest that higher concentrations of individual PS species may serve as signaling molecules to modulate the survival of stressed cells. The ability of fungi to regulate the synthesis of betaine lipids depending on nutrient availability can provide an explanation for the heterogeneous distribution of DGTS in the fruiting bodies of some groups of Basidiomycetes (Dembitsky, 1996, Vaskovsky et al., 1998). It seems unlikely that the absence of DGTS in natural occurring fruiting bodies can be a permanent chemotaxonomic marker and is instead due to the transient downregulation of BTA1 gene expression. Patterns in DGTS distribution in certain fungal orders, such as Boletales and Russulales, may be evidence of differences in regulatory mechanisms of DGTS synthesis or ecological association of these taxons with ecotopes with low or high Pi availability. BLAST analysis of BTA1 genes from C. reinhardtii and K. lactis show that databases contain sequences of BTA1 orthologs from more than 30 species of basidiomycetes, all from fungal whole-genome projects. None of them has been functionally characterized. We demonstrated that BTA1 gene of basidiomycete F. velutipes is inducible by Pi deficiency. Unfortunately, the regulatory mechanism of BTA1 induction in basidiomycetes has not yet been studied. It is known that DGTS synthesis during Pi limitation in ascomycete fungi is under the control of the PHO regulon, mediated by the transcription factor referred to as Pho4p in S. cerevisiae and NUC-1 in N. crassa (Riekhof et al., 2014). While the PHO pathway is widespread among ascomycete fungi (Tomar and Sinha, 2014), no information is available about the function of this pathway in basidiomycetes. Based on similarity searches, we could not identify clear orthologs of Pho4 and Pho2 proteins in complete genomes of basidiomycetes but found putative orthologs of other components of the PHO pathway: cyclin Pho80, cyclin-dependent kinase Pho85 and cyclin-dependent kinase inhibitor Pho81. However, the role of these proteins in phosphate starvation response in basidiomycete fungi has not been elucidated. Additionally, there is evidence that in some ascomycete fungi, orthologs of the S. cerevisiae PHO pathway components can have another functions, for example, the cyclin-CDK complex SpPHO80-SpPHO85 in S. pombe (Henry et al., 2011). Thus, while the basidiomycete F. velutipes and the ascomycetes N. crassa and K. lactis demonstrate a similar phosphate starvation response by the replacement of phospholipid PC with non-phosphorus DGTS, it is unclear how the Pi starvation response is regulated in fungi.