Draft Genome Sequence of Rhizobium tropici SARCC-755, a Free-Living Rhizobium That Nodulated and Promoted Growth in Pigeonpea [Cajanus cajan (L.) Millsp.].

Rhizobium tropici SARCC-755 is a free-living soil bacterium that formed nodules on pigeonpea roots in the present study. However, the draft genome sequence reveals that this Rhizobium species contains the nolR gene but lacks the common nodulation (nodABC) genes and probably uses other pathways to induce nodules on the legume plant.

Near-isogenic lines of Triticum aestivum with distinct modes of resistance exhibit dissimilar transcriptional regulation during Diuraphis noxia feeding.

Russian wheat aphid (Diuraphis noxia, Kurdjumov) feeding on susceptible Triticum aestivum L. leads to leaf rolling, chlorosis and plant death - symptoms not present in resistant lines. Although the effects of several D. noxia (Dn) resistance genes are known, none have been isolated or characterized. Wheat varieties expressing different Dn genes exhibit distinct modes of D. noxia resistance, such as antibiosis (Dn1), tolerance (Dn2), and antixenosis (Dn5). However, the mechanism whereby feeding aphids are perceived, and how subsequent transcriptional responses are partitioned into resistance categories, remains unclear. Here we report on downstream events in near-isogenic wheat lines containing different Dn genes after D. noxia biotype SA1 feeding. Transcripts involved in stress, signal transduction, photosynthesis, metabolism and gene regulation were differentially regulated during D. noxia feeding. Expression analyses using RT-qPCR and RNA hybridization, as well as enzyme activity profiling, provide evidence that the timing and intensity of pathways induced are critical in the development of particular modes of resistance. Pathways involved include the generation of kinase signalling cascades that lead to a sustained oxidative burst, and a hypersensitive response that is active during antibiosis. Tolerance is a passive resistance mechanism that acts through repair or de novo synthesis of photosystem proteins. Results further suggest that ethylene-mediated pathways are possibly involved in generating volatile compounds and cell wall fortification during the antixenosic response.

Transcript profiling of wheat genes expressed during feeding by two different biotypes of Diuraphis noxia.

Diuraphis noxia (Kurdjumov)(Russian wheat aphid) has severe economic impacts on wheat and barley production in the United States. The interaction between the Russian wheat aphid and its cereal hosts is poorly understood. However, the recent appearance of new biotypes in the United States showed that specific interactions exist between wheat resistance loci and Russian wheat aphid biotypes. At present, Dn7 is the only known gene in hexaploid wheat that confers resistance against all U.S. Russian wheat aphid biotypes. This study was conducted to investigate the molecular mechanism of Dn7-mediated resistance against two U.S. Russian wheat aphid biotypes (Russian wheat aphid 1 and Russian wheat aphid 2). Using GeneChip Wheat Genome Arrays, we compared transcript profiles of resistant and susceptible lines infested with either Russian wheat aphid 1 or Russian wheat aphid 2 using two time intervals (5 and 48 h after infestation). Russian wheat aphid feeding on hexaploid wheat led to the induction of groups of genes functioning in oxidative and general stress, photosynthesis, cell respiration and energy production, signal transduction, calcium-dependent signaling, pathogenesis related (PR) responses, and defense compound synthesis. The number of differentially expressed genes was higher in plants infested with Russian wheat aphid 1 compared with those infested with Russian wheat aphid 2. Although most genes involved in basic cellular functions were shared, unique genes were also obtained. This finding may indicate subtle differences in genes induced in response to different virulence proteins.