Interaction of cytosolic and plastidic nitrogen metabolism in plants. (49/132)

In angiosperms, the assimilation of ammonia resulting from nitrate reduction and from photorespiration depends on the operation of the plastidic GS/GOGAT cycle. The precursor for ammonia assimilation, 2-oxoglutarate, is synthesized in the mitochondria and in the cytosol. It is imported into the plastid by a 2-oxoglutarate/malate translocator (DiT1). In turn, the product of ammonia assimilation, glutamate, is exported from the plastids by a glutamate/malate translocator (DiT2). These transport processes link plastidic and cytosolic nitrogen metabolism and are essential for plant metabolism. DiT1 was purified to homogeneity from spinach chloroplast envelope membranes and identified as a protein with an apparent molecular mass of 45 kDa. Peptide sequences were obtained from the protein and the corresponding cDNA was cloned. The function of the DiT1 protein and its substrate specificity were confirmed by expression of the cDNA in yeast cells and functional reconstitution of the recombinant protein into liposomes. Recent advances in the molecular cloning of DiT2 and in the analysis of the in vivo function of DiT1 by antisense repression in transgenic tobacco plants will be discussed. In non-green tissues, the reducing equivalents required for glutamate formation by NADH-GOGAT are supplied by the oxidative pentose phosphate pathway. Glucose 6-phosphate, the immediate precursor of the oxidative pentose phosphate pathway is generated in the cytosol and imported into the plastids by the plastidic glucose 6-phosphate/phosphate translocator.  (+info)

Molecular and enzymatic analysis of ammonium assimilation in woody plants. (50/132)

Ammonium is assimilated into amino acids through the sequential action of glutamine synthetase (GS) and glutamate synthase (GOGAT) enzymes. This metabolic pathway is driven by energy, reducing power and requires the net supply of 2-oxoglutarate that can be provided by the reaction catalysed by isocitrate dehydrogenase (IDH). Most studies on the biochemistry and molecular biology of N-assimilating enzymes have been carried out on annual plant species and the available information on woody models is far more limited. This is in spite of their economic and ecological importance and the fact that nitrogen is a common limiting factor for tree growth. GS, GOGAT and IDH enzymes have been purified from several woody species and their kinetic and molecular properties determined. A number of cDNA clones have also been isolated and characterized. Although the enzymes are remarkably well conserved along the evolutionary scale, major differences have been found in their compartmentation within the cell between angiosperms and conifers, suggesting possible adaptations to specific functional roles. The analysis of the gene expression patterns in a variety of biological situations such as changes in N nutrition, development, biotic or abiotic stresses and senescence, suggest that cytosolic GS plays a central and pivotal role in ammonium assimilation and metabolism in woody plants. The modification of N assimilation efficiency has been recently approached in trees by overexpression of a cytosolic pine GS in poplar. The results obtained, suggest that an increase in cytosolic GS might lead to a global effect on the synthesis of nitrogenous compounds in the leaves, with enhanced vegetative growth of transgenic trees. All these data suggest that manipulation of cytosolic GS may have consequences for plant growth and biomass production.  (+info)

Evolution of class B floral homeotic proteins: obligate heterodimerization originated from homodimerization. (51/132)

The class B floral homeotic genes from the higher eudicot model systems Arabidopsis and Antirrhinum are involved in specifying the identity of petals and stamens during flower development. These genes exist in two different types termed DEF- and GLO-like genes. The proteins encoded by the class B genes are stable and functional in the cell only as heterodimeric complexes of a DEF- and a GLO-like protein. In line with this, heterodimerization is obligate for DNA binding in vitro. The genes whose products have to heterodimerize to be stable and functional are each other's closest relatives within their genomes. This suggests that the respective genes originated by gene duplication, and that heterodimerization is of relative recent origin and evolved from homodimerization. To test this hypothesis we have investigated the dimerization behavior of putative B proteins from phylogenetic informative taxa, employing electrophoretic mobility shift assays and the yeast two-hybrid system. We find that an ancestral B protein from the gymnosperm Gnetum gnemon binds DNA in a sequence-specific manner as a homodimer. Of the two types of B proteins from the monocot Lilium regale, the GLO-like protein is still able to homodimerize, whereas the DEF-like protein binds to DNA only as a heterodimeric complex with the GLO-like protein. These data suggest that heterodimerization evolved in two steps after a gene duplication that gave rise to DEF- and GLO-like genes. Heterodimerization may have originated after the gymnosperm-angiosperm split about 300 MYA but before the monocot-eudicot split 140-200 MYA. Heterodimerization may have become obligate for both types of flowering plant B proteins in the eudicot lineage after the monocot-eudicot split.  (+info)

Male-driven evolution of mitochondrial and chloroplastidial DNA sequences in plants. (52/132)

Although there is substantial evidence that, in animals, male-inherited neutral DNA evolves at a higher rate than female-inherited DNA, the relative evolutionary rate of male- versus female-inherited DNA has not been investigated in plants. We compared the substitution rates at neutral sites of maternally and paternally inherited organellar DNA in gymnosperms. The analysis provided substantial support for the presence of a higher evolutionary rate in both the mitochondrial and chloroplastidial DNA when the organelle was inherited paternally than when inherited maternally. These results suggest that, compared with eggs, sperm tend to carry a greater number of mutations in mitochondrial and chloroplastidial DNA. The existence of a male mutation bias in plants is remarkable because, unlike animals, the germ-lines are not separated from the somatic cells throughout an individual's lifetime. The data therefore suggest that even a brief period of male and female germ-line separation can cause gender-specific mutation rates. These results are the first to show that, at least in some species, germ-lines influence the number of mutations carried in the gametes. Possible causes of male mutation bias in plants are discussed.  (+info)

Stilbene derivatives from two species of Gnetaceae. (53/132)

Five new stilbene oligomers (gnemonols A, B and C, gnemonoside E and gnetal) were isolated together with 2b-hydroxyampelopsin F and gnetin E from Gnetum gnemon and G. gnemonoides. The structures of the compounds were elucidated on the basis of spectral evidence.  (+info)

Homologies in leaf form inferred from KNOXI gene expression during development. (54/132)

KNOTTEDI-like homeobox (KNOXI) genes regulate development of the leaf from the shoot apical meristem (SAM) and may regulate leaf form. We examined KNOXI expression in SAMs of various vascular plants and found that KNOXI expression correlated with complex leaf primordia. However, complex primordia may mature into simple leaves. Therefore, not all simple leaves develop similarly, and final leaf morphology may not be an adequate predictor of homology.  (+info)

Molecular cytogenetic analysis of Podocarpus and comparison with other gymnosperm species. (55/132)

DNA sequences have been mapped to the chromosomes of Podocarpus species from New Zealand and Australia by fluorescent in situ hybridization. Unlike other conifers, these species show only one pair of major sites of 45S rDNA genes, and two additional minor sites were seen in the Australian P. lawrencei. Unusually, 45S sequences collocalize to the same chromosomal region as the 5S rDNA. The telomere probe (TTTAGGG)n hybridizes to the ends of all chromosomes as well as to a large number of small sites distributed along the length of all chromosomes. Two other simple sequence repeats, (AAC)5 and (GATA)4, show a diffuse pattern of hybridization sites distributed along chromosomes. Southern blots using a variety of probes obtained from the reverse transcriptase of retroelements (gypsy, copia and LINE) from P. totara, P. nivalis and Dacrycarpus dacrydioides show that these retroelements are abundant and widespread in Podocarpaceae and also in others conifers. Some retroelements such as copia pPonty3 and gypsy pPot1li are more abundant in the genome of Picea abies and Ginkgo biloba than in the species from which they were amplified.  (+info)

Wettable and unsinkable: the hydrodynamics of saccate pollen grains in relation to the pollination mechanism in the two New Zealand species of Prumnopitys Phil. (Podocarpaceae). (56/132)

The pollination mechanism of most genera of the Podocarpaceae involves inverted ovules, a pollination drop and bisaccate pollen grains. Saccate grains have sometimes been referred to as 'non-wettable' due to their buoyant properties, while non-saccate pollen grains have been described as 'wettable'. The hydrodynamic properties of saccate pollen grains of seven podocarp species in five genera, Dacrydium Sol. ex G. Forst., Dacrycarpus (Endl.) de Laub., Manoao Molloy, Podocarpus L'Her. ex Pers. and Prumnopitys Phil. have been tested in water, together with saccate and non-saccate pollen of four other conifer genera, Cedrus Trew (Pinaceae), Cephalotaxus Siebold & Zucc. ex Endl. (Cephalotaxaceae), Cupressus L. (Cupressaceae) and Phyllocladus Rich. ex Mirb. (Phyllocladaceae), and spores of three fern species and one lycopod species. All four spore types studied were non-wettable, whereas the bisaccate and trisaccate pollen types, like all other conifer pollen types, were wettable, enabling the grains to cross the surface tension barrier of water. Once past this barrier, grain behaviour was governed by presence or absence of sacci. Non-saccate and vestigially saccate grains sank, whereas saccate grains behaved like air bubbles, floating up to the highest point. In addition, the grains were observed to float in water with sacci uppermost, consistent with the suggestion that distally placed sacci serve to orientate the germinal furrow of the pollen grain towards the nucellus of an inverted ovule. Observations of pollen grains in the pollen chambers of naturally pollinated Prumnopitvs ovules confirmed this. The combination of buoyancy and wettability in saccate pollen has implications for the efficiency of the typical podocarp pollination mechanism.  (+info)