Inorganic polyphosphate in Dictyostelium discoideum: influence on development, sporulation, and predation

Abstract

Dictyostelium discoideum, a social slime mold that forms fruiting bodies with spores, depends on inorganic polyphosphate (poly P) for its cycles of development and for nutritional predation on bacteria. The synthesis of poly P, a polymer of tens or hundreds of phosphate residues linked by high energy, ATP-like bonds, is catalyzed in most bacteria by poly P kinase (PPK1). The eukaryote D. discoideum possesses a homolog of PPK1. We report here that mutants of D. discoideum PPK1 (DdPPK1) have reduced levels of poly P and are deficient in development. Fruiting bodies are smaller and produce fewer spores, which appear to germinate like the wild type (WT). The DdPPK1 mutant formed smaller plaques on bacterial lawns compared with those of the WT. Predation by D. discoideum, assessed by uptake and digestion of Klebsiella aerogenes, showed that fewer bacteria were taken up by the DdPPK1 mutant compared with the WT and were killed less rapidly, indicating a role of poly P and/or DdPPK1 in phagocytosis. On Pseudomonas aeruginosa lawns, cleared plaques were observed with the bacterial PPK1 mutant but not with the WT P. aeruginosa. Thus, poly P is important in predation both for the predator and prey.

Fig. 1.

Growth of D. discoideum in HL5 medium.

Fig. 2.

Growth of D. discoideum on a K. aerogenes lawn. (A) D. discoideum cells (1 × 10³) were mixed with K. aerogenes and plated on SM5 plate. Plaques were formed by WT (Left) and mutant (Right) cells on lawns after 2 days. (B) D. discoideum cells (20–50) were mixed with K. aerogenes and plated on SM5 agar. Plaque sizes were measured over a 7-day period.

Fig. 3.

Development of D. discoideum on K. aerogenes lawns. D. discoideum cells (1 × 10³) were mixed with K. aerogenes and plated on a SM5 plate. Fruiting bodies formed on lawns after 4 days for WT (Left) and mutant + Ddppk1 (Right) and 7 days for the mutant (Center).

Fig. 4.

A comparison of spore germination and poly P levels.

Fig. 5.

Uptake and digestion of K. aerogenes by D. discoideum cells. WT and mutant cells (1 × 10⁶ per ml) were placed in tissue culture wells and infected with K. aerogenes at a multiplicity of ≈100:1. Cultures were incubated at 22°C for 30 min, at which time gentamicin (Gm) was added to kill extracellular K. aerogenes. D. discoideum were collected at indicated time points, lysed, and plated on nutrient agar plates to determine the colony-forming units (cfu)/ml D. discoideum cell lysates.

Fig. 6.

Plaque formation of D. discoideum cells on P. aeruginosa lawns. WT cells were mixed with P. aeruginosa WT (Left) and mutant (Right) and plated on SM5 agar and incubated at 22° for 5 days.