Phylogenetic relationships and genome divergence among the AA- genome species of the genus Oryza as revealed by 53 nuclear genes and 16 intergenic regions.

Rapid radiations have long been regarded as the most challenging issue for elucidating poorly resolved phylogenies in evolutionary biology. The eight diploid AA- genome species in the genus Oryza represent a typical example of a closely spaced series of recent speciation events in plants. However, questions regarding when and how they diversified have long been an issue of extensive interest but remain a mystery. Here, a data set comprising >60 kb of 53 singleton fragments and 16 intergenic regions is used to perform phylogenomic analyses of all eight AA- genome species plus four diploid Oryza species with BB-, CC-, EE- and GG- genomes. We fully reconstruct phylogenetic relationships of AA- genome species with confidence. Oryza meridionalis, native to Australia, is found to be the earliest divergent lineage around 2.93 mya, whereas O. punctata, a BB- genome species, serves as the best outgroup to distinguish their phylogenetic relationships. They separated from O. punctata approximately 9.11 mya during the Miocene epoch, and subsequently radiated to generate the entire AA- genome lineage diversity. The success in resolving the phylogeny of AA- genome species highlights the potential of phylogenomics to determine their divergence and evolutionary histories.

Biochemical and physiological characterization of a tau class glutathione transferase from rice (Oryza sativa).

The classical phase II detoxification glutathione transferases (GSTs) are key metabolic enzymes that catalyze the conjugation of glutathione to various electrophilic compounds. A tau class GST gene (OsGSTU17) was cloned from rice, which encodes a protein of 223 amino acid residues with a calculated molecular mass of 25.18 kDa. The recombinant OsGSTU17 formed a homodimer protein and showed GSH-conjugating activity with various xenobiotics. Kinetic analysis with respect to NBD-Cl as substrate revealed a K(m) of 0.324 mM and V(max) of 0.219 micromol/min per mg of protein. The enzyme had a maximum activity at pH 7.5, and a high thermal stability with 81% of its initial activity at 55 degrees C for 15 min. Site-directed mutagenesis revealed that Ser15 in the N-terminal domain is a critical catalytic residue, responsible for stabilisation of the thiolate anion of enzyme-bound glutathione. OsGSTU17 mRNA was expressed in different tissues of rice, both above and below ground. The relative transcript levels of OsGSTU17 mRNA varied significantly among the tissues in response to CDNB, hydrogen peroxide and atrazine treatments, indicating the gene has diverse regulation mechanisms in response to abiotic stresses.