Widespread occurrence of a covalent linkage between xyloglucan and acidic polysaccharides in suspension-cultured angiosperm cells

* BACKGROUND AND AIMS: Covalent linkages between xyloglucan and rhamnogalacturonan-I (RG-I) have been reported in the primary cell walls of cultured Rosa cells and may contribute to wall architecture. This study investigated whether this chemical feature is general to angiosperms or whether Rosa is unusual. * METHODS: Xyloglucan was alkali-extracted from the walls of l-[1-3H]arabinose-fed suspension-cultured cells of Arabidopsis, sycamore, rose, tomato, spinach, maize and barley. The polysaccharide was precipitated with 50 % ethanol and subjected to anion-exchange chromatography in 8 m urea. Eluted fractions were Driselase-digested, yielding [3H]isoprimeverose (diagnostic of [3H]xyloglucan). The Arabidopsis cells were also fed [6-14C]glucuronic acid, and radiolabelled pectins were extracted with ammonium oxalate. * KEY RESULTS: [3H]Xyloglucan was detected in acidic (galacturonate-containing) as well as non-anionic polysaccharide fractions. The proportion of the [3H]isoprimeverose units that were in anionic fractions was: Arabidopsis, 45 %; sycamore, 60 %; rose, 44 %; tomato, 75 %; spinach, 70 %; maize, 50 %; barley, 70 %. In Arabidopsis cultures fed d-[6-(14)C]glucuronate, 20 % of the (galacturonate-14C)-labelled pectins were found to hydrogen-bond to cellulose, a characteristic normally restricted to hemicelluloses such as xyloglucan. * CONCLUSIONS: Alkali-stable, anionic complexes of xyloglucan (reported in the case of Rosa to be xyloglucan-RG-I covalent complexes) are widespread in the cell walls of angiosperms, including gramineous monocots.     

Developmental genetics of leaf morphogenesis in dicotyledonous plants

A full understanding of the leaf is essential for a full understanding of plant morphology. However, leaf morphogenesis is still poorly understood, in particular in dicotyledonous plants, because of the complex nature of the development of leaves. Mutational analysis seems to be the most suitable strategy for investigations of such processes, and should allow us to dissect the developmental pathways into genetically programmed unit processes. The techniques of developmental genetics have been applied to the study of leaf morphogenesis in model plants, such asArabidopsis thaliana, and several key processes in leaf morphogenesis have been identified. The fundamental processes in leaf morphogenesis include the identification of leaf organs, determination of leaf primordia (occurrence of marginal meristem), and the polar or non-polar elongation of leaf cells. This review will focus on the genes that are essential for these processes and have been identified in mutational analyses. Mutational analyses of the photomorphogenesis is also briefly summarized from the perspective of the plasticity of leaf morphogenesis.     

Molecular evolution of nitrate reductase genes

To understand the evolutionary mechanisms and relationships of nitrate reductases (NRs), the nucleotide sequences encoding 19 nitrate reductase (NR) genes from 16 species of fungi, algae, and higher plants were analyzed. The NR genes examined show substantial sequence similarity, particularly within functional domains, and large variations in GC content at the third codon position and intron number. The intron positions were different between the fungi and plants, but conserved within these groups. The overall and nonsynonymous substitution rates among fungi, algae, and higher plants were estimated to be 4.33 × 10⁻¹⁰ and 3.29 × 10⁻¹⁰ substitutions per site per year. The three functional domains of NR genes evolved at about one-third of the rate of the N-terminal and the two hinge regions connecting the functional domains. Relative rate tests suggested that the nonsynonymous substitution rates were constant among different lineages, while the overall nucleotide substitution rates varied between some lineages. The phylogenetic trees based on NR genes correspond well with the phylogeny of the organisms determined from systematics and other molecular studies. Based on the nonsynonymous substitution rate, the divergence time of monocots and dicots was estimated to be about 340 Myr when the fungi–plant or algae–higher plant divergence times were used as reference points and 191 Myr when the rice–barley divergence time was used as a reference point. These two estimates are consistent with other estimates of divergence times based on these reference points. The lack of consistency between these two values appears to be due to the uncertainty of the reference times. Received: 10 April 1995 / Accepted: 10 September 1995     

Spatial patterns of dicot diversity in Argentina

In this paper, we analyzed the taxonomic diversity of the Argentine dicots to evaluate their relationships with area, latitude, and longitude. We also evaluated species diversity and higher taxa diversity relationships. The families, genera and species diversity in Argentine dicots was not explained by the area of each province but it varied through latitudinal and longitudinal gradients. The taxonomic diversity of these plants increased from high to low latitudes and west–east longitudes. These patterns would explain why the main diversity centers are located in the North region of this country. As we expected the species diversity and higher taxa diversity showed a positive relationship. At this scale, higher taxa diversity could be use as surrogate for species diversity.     

Structural and Evolutionary Relationships Among Chitinases of Flowering Plants

The analysis of nuclear-encoded chitinase sequences from various angiosperms has allowed the categorization of the chitinases into discrete classes. Nucleotide sequences of their catalytic domains were compared in this study to investigate the evolutionary relationships between chitinase classes. The functionally distinct class III chitinases appear to be more closely related to fungal enzymes involved in morphogenesis than to other plant chitinases. The ordering of other plant chitinases into additional classes mainly relied on the presence of auxiliary domains—namely, a chitin-binding domain and a carboxy-terminal extension—flanking the main catalytic domain. The results of our phylogenetic analyses showed that classes I and IV form discrete and well-supported monophyletic groups derived from a common ancestral sequence that predates the divergence of dicots and monocots. In contrast, other sequences included in classes I* and II, lacking one or both types of auxiliary domains, were nested within class I sequences, indicating that they have a polyphyletic origin. According to phylogenetic analyses and the calculation of evolutionary rates, these chitinases probably arose from different class I lineages by relatively recent deletion events. The occurrence of such evolutionary trends in cultivated plants and their potential involvement in host–pathogen interactions are discussed. Received: 5 July 1996 / Accepted: 9 January 1997