Plant Science Letters最新文献

Optimal conditions for the protection against growth inhibition induced by methotrexate (MTX) have been investigated. MTX was found to be much more toxic to protoplast-derived tobacco cells grown on nitrate as the sole nitrogen source, than to cells supplied with reduced nitrogen. In a reduced nitrogen-containing medium the growth inhibition was prevented by a mixture of methionine, adenine and thymidine. A valine-resistant tobacco mutant, affected in amino acid uptake, was not able to grow under these conditions, which are permissive for the wild type.

The effect of MTX on nitrate assimilation was also studied. Partial inhibition of the nitrate reductase (NR) activity of cells cultured in the presence of the drug was observed. When cells were cultured in a medium containing ammonium succinate, intracellular concentrations of the molybdenum cofactor (Mo-cofactor) of NR increased linearly during the culture, and decreased rapidly when MTX was added to the culture medium. A significant and reproducible protection was induced by exogenous xanthine oxidase (XDH) but not by the cofactor extracted from this enzyme.

The subcellular localization of enzymes involved in ammonia assimilation and synthesis of asparagine in the host cells of the central, N₂-fixing zone of nodules of Lupinus albus L. has been examined using discontinuous and continuous sucrose density gradient separation of the soluble, plant organelle and bacteroid components of nodule extracts. Both glutamine synthetase (EC 6.3.1.2) and asparagine synthetase (EC 6.3.5.4) activities were cytosolic while significant NADH: glutamate synthase (EC 2.6.1.53) and aspartate aminotransferase (EC 2.6.1.1) activities were recovered together with starch-containing plastids. NAD: glutamate oxidoreductase (glutamate dehydrogenase; EC 1.4.1.2) was associated almost completely with intact mitochondria. The distribution of enzymes indicates complex subcellular organization of pathways of asparagine synthesis.

Carbohydrates soluble in 70% (v/v) ethanol were the major causes of the changes in the dry matter of the three internodes below the peduncle of a crop of winter wheat grown in Yorkshire, U.K. The concentration of soluble carbohydrates reached a maximum 24 days after anthesis when they were 42% of the dry matter and declined to 3% at grain maturity. Gel permeation chromatography was used to separate the polymers of fructose and there was little indication that the maximum degree of polymerisation was greater than 9 (DP9). Fructans of $\text{DP}\text{≧ 5}$ increased rapidly to a maximum concentration of 27% at 24 days while those of DP(3+4) reached a maximum concentration of 9% about 10 days after anthesis. Sucrose, fructose and glucose, in that order, were at lower concentrations in the internodes than the fructans. Polymerisation followed by depolymerisation was explained by the changing balance between net photosynthesis and rates of grain growth. Nitrogen nutrition of the crop had little effect on the quantitative and qualitative changes in the soluble carbohydrates of the internodes.

The aim of this work was to identify the major forms of organic sulphur transported from the leaf of mung bean, Vigna radiata (L.) Wilczek, cv. Katjang Idjoe, to a developing fruit. After introduction of [³⁵S]sulphate into a leaf by flap-feeding, the labelled metabolites were extracted from the peduncle, seeds and pericarp of a fruit at the same node, as well as the leaf lamina and petiole. Thiol groups were blocked by reduction and carboxymethylation, and the amino compounds examined as dansyl derivatives by thin-layer chromatography and autoradiography. In all tissues except the leaf lamina, the major labelled compound was homoglutathione (HGSH) (γ-glutamylcysteinyl-β-alanine); in the lamina its precursor γ-glutamylcysteine predominated. Minor labelling was also found in cysteine, methionine and glutathione. These results suggest a major physiological role for HGSH in the transport of reduced sulphur to the developing mung bean seed.