The development of Brassica nigra seedlings over 20 d of growth was disrupted by the fungicide phosphonate (Phi) i n a manner inversely correlated with nutritional inorganic phosphate (Pi) levels.The growth of Pi-sufficient (1.25 mM Pi) seedlings was suppressed when 10, but not 5, mM Phi was added to the nutrient medium. I n contrast, the fresh weights and rookshoot ratios of Pi-limited (0.1 5 mM) seedlings were significantly reduced at 1.5 mM Phi, and they progressively declined to about 40% of control values as medium Phi concentration was increased to 10 mM. lntracellular Pi levels generally decreased in Phi-treated seedlings, and Phi accumulated in leaves and roots t o levels up t o 6-and 16-fold that of Pi i n Pi-sufficient and Pi-limited plants, respectively. Extractable activities of the Pi-starvation-inducible enzymes phosphoenolpyruvate phosphatase and inorganic pyrophosphate-dependent phosphofructokinase were unaltered in Pi-sufficient seedlings grown on 5 or 10 mM Phi. However, when Pi-limited seedlings were grown on 1.5 to 10 mM Phi (a) the induction of phosphoenolpyruvate phosphatase and inorganic pyrophosphate-dependent phosphofructokinase activities by Pi limitation was reduced by 40 to 90%, whereas (b) soluble protein concentrations and the activities of the ATP-dependent phosphofructokinase and pyruvate kinase were unaffected. It is concluded that Phi specifically interrupts processes involved i n regulation of the Pi-starvation response i n B. nigra. ~ -The Phi anion (HP03-) is an isostere of the Pi anion in which hydrogen replaces one of the oxygens bound to the P atom. In spite of the apparent similarity, most enzymes that catalyze the transfer of Pi groups can distinguish between the two anions. Phi was traditionally regarded as inert with respect to plant and animal metabolism and was once used as a buffer in systems in which Pi was unsuitable (Robertson and Boyer, 1956). Evidence suggesting that Phi is not biologically inert began to accumulate following the discovery that foliar applications or trunk injections of Fosetyl-A1 (aluminum tris-O-ethyl Phi) effectively suppress severa1 soil-borne plant diseases caused by pseudofungi belonging to the order Oomycetes, particularly Phytophthora sp. (Williams et al

The high concentrations of short-chain soluble polyphosphates, previously reported in Phytophthora palmiwora, have been found in six other species of Phytophthora, and appear to be characteristic of this genus. The distribution of phosphorus among the various acid-soluble pools was similar in all species except P. infestans, which accumulated less polyphosphate than other species when grown in high phosphate medium, and contained several unique and as yet unidentified organic phosphorus compounds. When grown in the presence of potassium phosphonate sufficient to reduce growth b y 50%, all species showed an increased accumulation of phosphorus in both pyrophosphate and polyphosphate, without parallel increases in sugar phosphate or nucleotide phosphorus pools. The internal concentration of phosphonate required to produce 50% inhibition varied with species, ranging from 4-6 to 52 pmol (g dry wt-'. Assimilation of orthophosphate was only slightly reduced in the presence of phosphonate, resulting in an increased concentration of phosphorus per unit mass. Metalaxyl, which also inhibits growth of Phytophthora spp. did not cause accumulation of either pyro-or polyphosphate. Phosphonate treatment also resulted in the formation of a compound identified as isohypophosphate, a metabolite of phosphonate not previously reported in Phytophthora. Taken together, these observations suggest that the primary site of phosphonate inhibition in Phytophthora spp. lies in the metabolism of pyrophosphate.

The influence of phosphite (H2PO3-) on the response of Saccharomyces cerevisiae to orthophosphate (HPO4(2-); Pi) starvation was assessed. Phosphate-repressible acid phosphatase (rAPase) derepression and cell development were abolished when phosphate-sufficient (+Pi) yeast were subcultured into phosphate-deficient (-Pi) media containing 0.1 mM phosphite. By contrast, treatment with 0.1 mM phosphite exerted no influence on rAPase activity or growth of +Pi cells. 31P NMR spectroscopy revealed that phosphite is assimilated and concentrated by yeast cultured with 0.1 mM phosphite, and that the levels of sugar phosphates, pyrophosphate, and particularly polyphosphate were significantly reduced in the phosphite-treated -Pi cells. Examination of phosphite's effects on two PHO regulon mutants that constitutively express rAPase indicated that (i) a potential target for phosphite's action in -Pi yeast is Pho84 (plasmalemma high-affinity Pi transporter and component of a putative phosphate sensor-complex), and that (ii) an additional mechanism exists to control rAPase expression that is independent of Pho85 (cyclin-dependent protein kinase). Marked accumulation of polyphosphate in the delta pho85 mutant suggested that Pho85 contributes to the control of polyphosphate metabolism. Results are consistent with the hypothesis that phosphite obstructs the signaling pathway by which S. cerevisiae perceives and responds to phosphate deprivation at the molecular level.

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The effects of ultrasound application on skim milk (10% w/w total solids at natural pH 6.7 or alkali-adjusted to pH 8.0) prior to the renneting of milk at pH 6.7 were examined. Skim milk, made by reconstituting skim milk powder, was sonicated at 20kHz and 30°C (dissipated power density 286kJkg(-1)) in an ultrasonic reactor. The rennet gelation time, curd firming rate, curd firmness, and the connectivity of the rennet gel network were improved significantly in rennet gels made from milk ultrasonicated at pH 8.0 and re-adjusted back to pH 6.7 compared to those made from milk sonicated at pH 6.7. These renneting properties were also improved in milk sonicated at pH 6.7 compared to those of the non-sonicated control milk. The improvements in renneting behavior were related to ultrasound-induced changes to the proteins in the milk. This study showed that ultrasonication has potential to be used as an intervention to manipulate the renneting properties of milk for more efficient manufacturing of cheese.