Objectives
 To address current gaps in our knowledge of phosphate homeostasis in regulating metal biology and stress resistance in a major human fungal pathogen C. albicans. In particular, to investigate the importance of Pi mobilisation from the Pi storage molecule, polyphosphate (polyP), in stress resistance and virulence.

Methods
 Strains were constructed lacking either or both of the putative polyphosphatases Ppn1 and Ppx1 which are predicted to regulate the mobilisation of vacuolar polyP stores. Confirmation of the role of Ppn1 and Ppx1 in polyP mobilisation was sought using a combination of cell staining and gel electrophoresis approaches. ICP-MS analysis was used to test the impact of inhibiting polyP mobilisation on intracellular phosphate and metal levels. Finally extensive phenotypic characterisation of ppn1, ppx1 and ppn1/ ppx1 strains was undertaken.

Results
 PolyP analysis in ppn1, ppx1 strains revealed a level of functional redundancy between these genes, as only ppn1/ ppx1 cells contained higher polyP levels compared to wild-type cells.  Significantly, polyP mobilisation in response to phosphate starvation was dependent on both Ppx1 and Ppn1. Cells lacking these polyphosphatases displayed impaired stress resistance to metal cations, alkaline pH and superoxide stress in Pi limiting conditions- similar to that seen with cells lacking the Pho4 phosphate responsive transcription factor. Intriguingly, cells lacking both Ppn1 and Ppx1 also displayed notable morphological defects.

Conclusion
 In Pi limiting environments, C. albicans cells rely on Pi mobilisation from polyP to promote stress resistance. Strikingly, Pi mobilisation from polyP also appears to be a pre-requisite for normal cell morphology even in phosphate replete conditions.

Poly-P Mobilisation in the Pathobiology of Candida albicans