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Facilitated transport of Mn2+ in sycamore (Acer pseudoplatanus) cells and excised maize root tips. A comparative 31P n.m.r. study in vivo.

ABSTRACT: Movement of paramagnetic Mn2+ into sycamore (Acer pseudoplatanus) cells has been indirectly examined by observing the line broadening exhibited in its 31P n.m.r. spectra. Mn2+ was observed to pass into the vacuole, while exhibiting a very minor accumulation in the cytoplasm. With time, gradual leakage of phosphate from the vacuole to the cytoplasm was observed along with an increase in glucose-6-phosphate. Anoxia did not appear to affect the relative distribution of Mn2+ in the cytoplasm and vacuole. Under hypoxic conditions restriction of almost all movement of Mn2+ across the plasmalemma as well as the tonoplast was observed. In contrast, maize root tips showed entry and complete complexation of nucleotide triphosphate by Mn2+ during hypoxia. The rate of passage of Mn2+ across the tonoplast in both sycamore and maize root cells is approximately the same. However, the rates of facilitated movement across the respective plasma membranes appear to differ. More rapid movement of Mn2+ across the plasmalemma in maize root tip cells allows a gradual build-up of metal ion in the cytoplasm prior to its diffusion across the tonoplast. Sycamore cells undergo a slower uptake of Mn2+ into their cytoplasms (comparable with the rate of diffusion through the tonoplast), so little or no observable accumulation of Mn2+ is observed in this compartment.

SUBMITTER: Roby C

PROVIDER: S-EPMC1149159 | biostudies-other | 1988 Jun

REPOSITORIES: biostudies-other

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Project description:The availability of methods to fractionate non-green plastids and to prepare their limiting envelope membranes [Alban, Joyard & Douce (1988) Plant Physiol. 88, 709-717] allowed a detailed analysis of the biosynthesis of lysophosphatidic acid, phosphatidic acid, diacylglycerol and monogalactosyl-diacylglycerol (MGDG) in two different types of non-green starch-containing plastids: plastids isolated from cauliflower buds and amyloplasts isolated from sycamore cells. An enzyme [acyl-ACP (acyl carrier protein):sn-glycerol 3-phosphate acyltransferase) recovered in the soluble fraction of non-green plastids transfers oleic acid from oleoyl-ACP to the sn-1 position of sn-glycerol 3-phosphate to form lysophosphatidic acid. Then a membrane-bound enzyme (acyl-ACP:monoacyl-sn-glycerol 3-phosphate acyltransferase), localized in the envelope membrane, catalyses the acylation of the available sn-2 position of 1-oleoyl-sn-glycerol 3-phosphate by palmitic acid from palmitoyl-ACP. Therefore both the soluble phase and the envelope membranes are necessary for acylation of sn-glycerol 3-phosphate. The major difference between cauliflower (Brassica oleracea) and sycamore (Acer pseudoplatanus) membranes is the very low level of phosphatidate phosphatase activity in sycamore envelope membrane. Therefore, very little diacylglycerol is available for MGDG synthesis in sycamore, compared with cauliflower. These findings are consistent with the similarities and differences described in lipid metabolism of mature chloroplasts from 'C18:3' and 'C16:3' plants (those with MGDG containing C18:3 and C16:3 fatty acids). Sycamore contains only C18 fatty acids in MGDG, and the envelope membranes from sycamore amyloplasts have a low phosphatidate phosphatase activity and therefore the enzymes of the Kornberg-Pricer pathway have a low efficiency of incorporation of sn-glycerol 3-phosphate into MGDG. By contrast, cauliflower contains MGDG with C16:3 fatty acid, and the incorporation of sn-glycerol 3-phosphate into MGDG by the enzymes associated with envelope membranes is not limited by the phosphatidate phosphatase. These results demonstrate that: (1) non-green plastids employ the same biosynthetic pathway as that previously established for chloroplasts (the formation of glycerolipids is a general property of all plastids, chloroplasts as well as non-green plastids), (2) the envelope membranes are the major structure responsible for the biosynthesis of phosphatidic acid, diacylglycerol and MGDG, and (3) the enzymes of the envelope Kornberg-Pricer pathway have the same properties in non-green starch-containing plastids as in mature chloroplasts from C16:3 and C18:3 plants.

Project description:1. The patterns of incorporation of radioactivity from d-[6-(3)H]-, d-[1-(14)C]-, d-[U-(14)C]- and d-[6-(14)C]-glucose and [U-(14)C]myoinositol into the neutral sugars and uronic acids of the polysaccharides synthesized in different regions of the root-tip of maize were determined. 2. The root-cap tissue synthesized a slime in which a polysaccharide that contained a high proportion of fucose (32%) and galactose (21%) was found. This polysaccharide is synthesized only by the root-cap cells, and very little polysaccharide containing fucose is synthesized in adjacent tissues. Part of the meristematic tissue of the root is surrounded by the cap cells. A section of the root that contains both these tissues can be analysed, and the polysaccharide synthesized by the meristematic region can be obtained since the contribution of the root-cap cells can be found by the amount of fucose formed. 3. It was shown that there is very little difference in the polysaccharide synthesis of the meristematic region from that of the cells immediately behind it. In the more mature cells, however, the amount of xylose synthesized relative to that of arabinose is increased, and the proportion of xylose and arabinose formed in the matrix polysaccharides is increased whereas that of galactose is decreased. 4. The effect of 2,4-dichlorophenoxyacetic acid (2,4-D) on polysaccharide synthesis was to bring about a decrease in the relative amount of galactose synthesized in the matrix polysaccharides of cells immediately adjacent to the meristematic region and also in the more mature tissue. The growth factor also increased the amount of xylose synthesized relative to that of arabinose in the more mature tissue. These metabolic effects were related to a very obvious change in the morphological appearance of the root-tips. 5. Radioactivity from [U-(14)C]myoinositol was incorporated mainly into xylose, arabinose and galacturonic acid rather than into the hexoses, although small amounts of these sugars were formed.

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Facilitated transport of Mn2+ in sycamore (Acer pseudoplatanus) cells and excised maize root tips. A comparative 31P n.m.r. study in vivo.

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