Phosphate starvation

Ternan NG, JP Quinn (1998) Phosphate starvation-independent 2-aminoethylphosphonic acid biodegradation in a newly isolated strain of Pseudomonas putida, NG2. System Appl Microbiol 21 346-352. 

W. C. Plaxton, Metabolic aspects of phosphate starvation in plants. Phosphorus in Plant Biology Regulatory Roles in Molecular, Cellular, Organi.smic, and Eco.sy.s-lein Proces.ses (J. P. Lynch and J. Deikman eds.), American Society of Plant Physiologists, 1998, p. 229. 

Atlung, T., Knudsen, K., Heerfordt, L. and Broendsted, L. (1997) Effects of sigma(S) and the transcriptional activator AppY on induction of the Escherichia coli hya and cbdAB-appA operons in response to carbon and phosphate starvation./o r a/ of Bacteriology, 179,2141-6. 

Phosphate starvation inducible enzymes and proteins in higher plants... 

Many of the E. coli psi genes function to enhance Pi availability in, and uptake from, the external medium. For example, phosphate starvation induces pho A whose product is alkaline phosphatase, a hydrolytic enzyme that is excreted into the periplasmic space where it acts to cleave extracellular organic P to Pi. A second psi gene system, the phosphate-specific transport (Pst) operon uses energy to transport Pi across the E. coli membrane. The affinity of this four-gene transport system is much greater than that of the constitutive Pi shuttle. Many of these same molecular starvation rescue mechanisms have been characterised in yeast. 

The increased secretion of the epsi-APase is one of the earliest and most dramatic responses to phosphate starvation. The time frame and... 

Fig. 3. A northern blot of poly(A)+ RNA probed with a psi cDNA clone showed enhanced levels of psi messenger RNA as phosphate starvation became more severe. Poly(A)+ RNA was isolated from cultures at 3, 6 and 9 days after transfer to —Pi or +Pi medium (A. Danon et al., unpublished data in preparation). Equal amounts of RNA (1 pg per treatment) were separated on a denaturing formaldehyde/agarose gel and used for a northern blot. The filter was probed using a psi cDNA clone (identified by —/+ screening using standard methods) as the probe. Enhanced levels of mRNA for this clone are seen as early as 3 days after transfer to —Pi medium although cell growth equivalent to the unstressed control continued until day 8.

In the process of developing an in vitro system to select phosphate starvation resistant cell lines we simultaneously selected for a line that was constitutively induced for APase excretion. Tissue cultured tomato cells were plated onto solid medium containing starvation levels of phosphate. While most cells died, we identified isolated clumps of callus capable of near-normal rates of growth. Starvation-resistant cells were used to start suspension cultures that were kept under phosphate starva-... 

Fig. 5. A cell line selected for phosphate starvation resistance was con-stitutively induced for the excretion of APase into the medium. Three-day-old tomato cells selected for phosphate starvation resistance (PSR) and unselected cells (L. esculentum cv. VF36) were grown under Pi-sufficient conditions. Proteins excreted by the cells were separated by SDS-PAGE and immunoblotted with AP3 antiserum from which the Xylose-binding component had been removed via stem bromalin treatment (Goldstein, 1991). The selected cells showed constitutive excretion of high levels of APase protein based on the large signal obtained from the immunoblot. Measurement of enzyme activity gave a similar result (not shown).

When viewed as a whole, these data provide evidence for the proposal that phosphate starvation can regulate expression of epsi-RNase genes in tomato. However, based on the complete amino acid sequence of this enzyme and the isolation of genomic clones, additional experiments are currently under way that will allow us quickly to confirm or disprove this thesis. 

Glund, K. Goldstein, A.H. (1992). Regulation, synthesis and secretion of a phosphate starvation inducible RNase by cultured tomato cells. In Control of Plant Gene Expression, ed. D.P.S. Verma. Boca Raton CRC Press. 

Goldstein, A.H., Baertlein, D.A. McDaniel, R.G. (1988a). Phosphate starvation inducible metabolism in L. esculentum I. Plant Physiology 87, 711-15. 

Nurnberger, T., Abel, S., Jost, W. Glund, K. (1990). Induction of an extracellular ribonuclease in cultured tomato cells upon phosphate starvation. Plant Physiology 92, 970-6. 

In E. coli, the level of PolyP drops drastically under phosphate starvation, and the subsequent addition of orthophosphate to the medium restores the initial phosphate level (Nesmeyanova et al, 1973, 1974a,b Nesmeyanova, 2000). Some genetic manipulations increased the ability of E. coli to accumulate PolyP (Kato et al, 1993a Ilardoyoct al, 1994 Ohtake et al., 1994 Sharfstein et al., 1996). High levels of accumulation were achieved... 

In some culture conditions, extracellular PolyP was identified as a good source of phosphate (Curless et al, 1996). Using a typical medium in a high-cell-density fermentation of E. coli, 40 % higher cell density was obtained when using PolyP instead of Pi as a phosphate source (Curless et al, 1996). It is probable that the expression of specific porins allows PolyP transfer from the culture medium into the cells. The outer membrane porin PhoE of E. coli (Bauer et al, 1989) and the OprO porin of Pseudomonas aeruginosa (Siehnel et al, 1992 Hancock etal, 1992), induced by phosphate starvation, are examples of proteins which prefere PP and PolyP rather than P . 

The accumulation of Mn2+ in the vacuoles of Saccharomyces carlbergensis (Lichko et al., 1982) correlated well with the increase in PolyP content (Table 7.2). During the accumulation of Mn2+ by S. carlsbergensis, both of the PolyP and Mn2+ contents increased simultaneously. This accumulation took place even when the incubation medium contained no Pi and was accompanied by a simultaneous decrease of Pi content in the vacuoles. This complex-forming function of PolyP may be very important for the yeast cell, since under a short-term phosphate starvation in the presence of metal cations in the medium the vacuolar PolyP content slightly decreases (Table 7.3) (Lichko et al., 1982). A stable Pi content in the cytosol under the above conditions is maintained mainly due to a decrease in the vacuolar P pool but not in the vacuolar PolyP pool. It is probable that the ability of fungi to accumulate large amounts of heavy metals is connected with the PolyP pools, especially in vacuoles. 

Table 7.3 The contents (jimol of P per g of wet biomass) of P and PolyP in vacuoles of S. carlsbergensis under phosphate starvation and phosphate overplus (Lichko et al 1982). The cells were grown for 5 h.

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