Bacterial community structure along the Changjiang River (which is more than 2,500 km long) was studied by using denaturing gradient gel electrophoresis (DGGE) and clone library analysis of PCR-amplified 16S ribosomal DNA (rDNA) with universal bacterial primer sets. DGGE profiles and principal-component analysis (PCA) demonstrated that the bacterial community gradually changed from upstream to downstream in both 1998 and 1999. Bacterial diversity, as determined by the Shannon index (H), gradually decreased from upstream to downstream. The PCA plots revealed that the differences in the bacterial communities among riverine stations were not appreciable compared with the differences in two adjacent lakes, Lake Dongting and Lake Poyang. The relative stability of the bacterial communities at the riverine stations was probably due to the buffering action of the large amount of water flowing down the river. Clone library analysis of 16S rDNA revealed that the dominant bacterial groups changed from ␤-proteobacteria and the Cytophaga-FlexibacterBacteroides group upstream to high-G؉C-content gram-positive bacteria downstream and also that the bacterial community structure differed among the stations in the river and the lakes. The results obtained in this study should provide a reference for future changes caused by construction of the Three Gorges Dam.

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Sucrose synthase is a key enzyme in sucrose metabolism in plant cells, and it is involved in the synthesis of cell wall cellulose. Although the sucrose synthase gene (SUS) family in the model plants Arabidopsis thaliana has been characterized, little is known about this gene family in trees. This study reports the identification of two novel SUS genes in the economically important poplar tree. These genes were expressed predominantly in mature xylem. Using molecular cloning and bioinformatics analysis of the Populus genome, we demonstrated that SUS is a multigene family with seven members that each exhibit distinct but partially overlapping expression patterns. Of particular interest, three SUS genes were preferentially expressed in the stem xylem, suggesting that poplar SUSs are involved in the formation of the secondary cell wall. Gene structural and phylogenetic analyses revealed that the Populus SUS family is composed of four main subgroups that arose before the separation of monocots and dicots. Phylogenetic analyses associated with the tissue-and organ-specific expression patterns. The high intraspecific nucleotide diversity of two SUS genes was detected in the natural population, and the π nonsyn /π syn ratio was significantly less than 1; therefore, SUS genes appear to be evolving in Populus, primarily under purifying selection. This is the first comprehensive study of the SUS gene family in woody plants; the analysis includes genome organization, gene structure, and phylogeny across land plant lineages, as well as expression profiling in Populus.

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SummaryChinese white poplar (Populus tomentosa), an important commercial tree species for timber and pulp production in northern China, has been used to examine the individual genes and allelic diversity responsible for complex traits controlling growth and lignocellulosic biosynthesis. Taking advantage of the low degree of linkage disequilibrium (LD) within P. tomentosa association populations, we examined associations between 15 cellulose synthase (PtoCesA) genes and traits including growth and wood properties.Thirty-six novel simple sequence repeat (SSR) markers within PtoCesA genes were detected by re-sequencing and genotyped in an association population (460 individuals). Single-marker and haplotype-based LD approaches were used to identify significant marker-trait associations. Family-based linkage studies and real-time PCR testing were conducted to validate the functional significance of SSR variation.Fifteen single-marker associations from seven PtoCesA genes and nine haplotype-based associations within six genes were identified in the association population (false discovery rate Q < 0.05). Next, five SSR marker-trait associations (Q < 0.05) from four PtoCesA genes were successfully validated in a linkage mapping population (1200 individuals).The results imply a functional role for these genes in mediating wood properties, demonstrating the potential of combining single-marker and haplotype-based LD approaches to detect functional allelic variation underlying quantitative traits in a low-LD population.