Effect Of Phosphate Starvation Research Articles

Differential responses of oat cultivars to phosphate deprivation: plant growth and acid phosphatase activities

The effect of phosphate starvation on growth and acid phosphatases (APases) localization and activity in oat tissues was investigated. Oat cultivars (Avena sativa L.—Arab, Polar, Szakal) were grown for 1–3 weeks in complete nutrient medium (+P) and without phosphate (−P). Pi concentration in plant tissues decreased strongly after culturing on −P medium. Pi deficit reduced shoot growth, stimulated root elongation and increased ratio of root/shoot in all oat cultivars. Pi deficit had a greater impact on growth of oat cv. Polar than other varieties. A decrease in the internal Pi status led to an increase of acid phosphatase activities in extracts from shoots and roots, and in root exudates. The highest activity of secreted APases was observed for oat cv. Arab, during the third week of growth under Pi-deficient conditions. The activity of extracellular APase was high in young, growing zones of roots of −P plants. Histochemical visualization indicated high activity of APases in the epidermis and vascular tissues of −P plants. Pi deficiency increased intracellular APase activity in shoot mainly in oat cv. Polar, whereas APase activity in roots was the highest in oat cv. Szakal. Protein extracts from roots and shoots were run on native discontinuous PAGE to determine which isoform(s) may be affected by Pi deficiency. Three major APase isoforms were detected in all oat plants; one was strongly induced by Pi deficit. The studied oat cultivars differed in terms of acclimation to deficiency of phosphate—used various pools of APases to acquire Pi from external or internal sources.

The effect of phosphate starvation on the lipid and fatty acid composition of the fresh water eustigmatophyte Monodus subterraneus

Phosphate limitation caused significant changes in the fatty acid and lipid composition of Monodus subterraneus. With decreasing phosphate availability from 175 to 52.5, 17.5 and 0 μM (K 2HPO 4), the proportion of the major VLC-PUFA, eicosapentaenoic acid (EPA), gradually decreased from 28.2 to 20.8, 19.4 and 15.5 mol% (of total fatty acids), respectively. The cellular total lipid content of starved cells increased, mainly due to the dramatic increase in triacylglycerols (TAG) levels. Among polar lipids, cellular contents of digalactosyldiacylglycerol (DGDG) and diacylglyceroltrimethylhomoserine (DGTS) increased sharply from 0.29 and 0.19 to 0.60 and 0.38 fg cell −1, respectively, while that of monogalactosyldiacylglycerol (MGDG) was not significantly changed. In the absence of phosphate, the proportion of phospholipids was significantly reduced from 8.3% to 1.4% of total lipids, and the proportion of triacylglycerols (TAG) increased from 6.5% up to 39.3% of total lipids. The share of MGDG was substantially reduced, from 35.7% to 13.3%, while that of DGDG and DGTS reduced less from 18.3% to 15.1%, and 12.2% to 8.6%, respectively. The most distinctive change in the fatty acid composition was noted in that of DGDG, where the proportion of EPA, located exclusively at the sn-1 position, increased from 11.3% to 21.5% at the expense of 16:0, 16:1 and 18:1. In MGDG, however, the proportion of EPA did not change appreciably. In contrast to higher plants, DGDG accumulated under P-deprivation in M. subterraneus, did not resemble PC and the positional distribution of its fatty acids was not altered, preserving the C20/C16 structure of its molecular species. We suggest that under phosphate starvation DGTS is a likely source of C 20 acyl groups that can be exported to the sn-1 position of DGDG and can partially compensate for the decrease in PE, the apparent source of C 20 acyl-containing diacylglycerols in this alga. Moreover, accumulation of non-esterified 18:0 indicates that no polar lipid can replace PC, which appears to be the only lipid capable of C 18 desaturation in this alga.

Evidence for an aminoendopeptidase localized near the cell surface of Escherichia coli. Regulation of synthesis by inorganic phosphate.

An enzyme capable of hydrolyzing the substrate L-alanine p-nitroanilide has been found in the various Escherichia coli strains tested. This enzyme has been called aminoendopeptidase since it shows both activities (see accompanying paper). It is released from the cells by osmotic shock and by lysozyme -- EDTA spheroplasting treatment, and 50% of the total activity is directly detectable with suspensions of intact cells. However, the release by osmotic shock or spheroplasting is not as efficient as it is for alkaline phosphatase. This periplasmic aminoendopeptidase is constitutively produced but the differential rate of synthesis is increased 4-fold when the cell growth is limited by Pi. The occurrence of this 'derepression' is simultaneous with that of alkaline phosphatase. Increasing the concentration of inorganic phosphate in the medium has no effect on the constitutive aminoendopeptidase synthesis. The effect of phosphate starvation is specific since starvation for neither nitrogen nor carbon and energy source are effective in derepressing aminoendopeptidase.