Type III Secretion System of Bradyrhizobium sp. SUTN9-2 Obstructs Symbiosis with Lotus spp.

The rhizobial type III secretion system secretes effector proteins into host plant cells, which may either promote or inhibit symbiosis with legumes. We herein demonstrated that the type III secretion system of Bradyrhizobium sp. SUTN9-2 obstructed symbiosis with Lotus japonicus Miyakojima, L. japonicus Gifu, and Lotus burttii. A mutant of SUTN9-2 that is unable to secrete effector proteins showed better nodulation and plant growth promotion than wild-type SUTN9-2 when paired with these Lotus spp. We propose that SUTN9-2 is a useful strain for understanding the mechanisms by which effector proteins obstruct symbiosis between Bradyrhizobium and Lotus spp.

Homocitrate Synthase Genes of Two Wide-Host-Range Bradyrhizobium Strains are Differently Required for Symbiosis Depending on Host Plants.

The nifV gene encodes homocitrate synthase, the enzyme that catalyzes the formation of homocitrate, which is essential for arranging the FeMo-cofactor in the catalytic center of nitrogenase. Some host plants, such as Lotus japonicus, supply homocitrate to their symbionts, in this case, Mesorhizobium loti, which lacks nifV. In contrast, Bradyrhizobium ORS285, a symbiont of Aeschynomene cross-inoculation (CI) groups 2 and 3, requires nifV for symbiosis with Aeschynomene species that belong to CI group 3, and some species belonging to CI group 2. However, it currently remains unclear whether rhizobial nifV is required for symbiosis with Aeschynomene species belonging to CI group 1 or with other legumes. We generated nifV-disruption (ΔnifV) mutants of two wide-host-range rhizobia, Bradyrhizobium SUTN9-2 and DOA9, to investigate whether they require nifV for symbiosis. Both ΔnifV mutant strains showed significantly less nitrogenase activity in a free-living state than the respective wild-type strains. The symbiotic phenotypes of SUTN9-2, DOA9, and their ΔnifV mutants were examined with four legumes, Aeschynomene americana, Stylosanthes hamata, Indigofera tinctoria, and Desmodium tortuosum. nifV was required for the efficient symbiosis of SUTN9-2 with A. americana (CI group 1), but not for that of DOA9. SUTN9-2 established symbiosis with all three other legumes; nifV was required for symbiosis with I. tinctoria and D. tortuosum. These results suggest that, in addition to Aeschynomene CI groups 2 and 3, CI group 1 and several other legumes require the rhizobial nifV for symbiosis.

Indigenous actinorhizal plants of Australia.

Indigenous species of actinorhizal plants of Casuarinaceae, Elaeagnaceae and Rhamnaceae are found in specific regions of Australia. Most of these plants belong to Casuarinaceae, the dominant actinorhizal family in Australia. Many of them have significant environmental and economical value. The other two families with their indigenous actinorhizal plants have only a minor presence in Australia. Most Australian actinorhizal plants have their native range only in Australia, whereas two of these plants are also found indigenously elsewhere. The nitrogen-fixing ability of these plants varies between species. This ability needs to be investigated in some of these plants. Casuarinas form a distinctive but declining part of the Australian landscape. Their potential has rarely been applied in forestry in Australia despite their well-known uses, which are being judiciously exploited elsewhere. To remedy this oversight, a programme has been proposed for increasing and improving casuarinas that would aid in greening more regions of Australia, increasing the soil fertility and the area of wild life habitat (including endangered species). Whether these improved clones would be productive with local strains of Frankia or they need an external inoculum of Frankia should be determined and the influence of mycorrhizal fungi on these clones also should be investigated.

Diversity of culturable ericoid mycorrhizal fungi of Rhododendron decorum in Yunnan, China.

The diversity of ericoid mycorrhizal fungi isolated from Rhododendron decorum Franch. in Yunnan, southwestern China, was examined for the first time. In total 300 hair-root samples were collected from 13 R. decorum individuals in two adjacent wild population sites and one cultivated population site. Two hundred eighteen slow-growing isolates were obtained; the ability of some to form ericoid mycorrhiza was tested in vitro. One hundred twenty-five isolates formed hyphal structures morphologically corresponding to ericoid mycorrhiza, and these were determined by morphological and molecular means to belong to 12 fungal species. There were hardly any differences in species among the three sampled populations. The sequences of several isolates were similar to those of Oidiodendron maius and ericoid mycorrhizal fungi from Helotiales, accounting respectively for 18.4% and 24.8% of the total culturable ericoid mycorrhizal fungi assemblage. Dark septate endophytes were detected in the sampled hair roots by microscopy.

Comparative phylogenetic analysis of small GTP-binding genes of model legume plants and assessment of their roles in root nodules.

Small GTP-binding genes play an essential regulatory role in a multitude of cellular processes such as vesicle-mediated intracellular trafficking, signal transduction, cytoskeletal organization, and cell division in plants and animals. Medicago truncatula and Lotus japonicus are important model plants for studying legume-specific biological processes such as nodulation. The publicly available online resources for these plants from websites such as http://www.ncbi.nih.gov, http://www.medicago.org, http://www.tigr.org, and related sites were searched to collect nucleotide sequences that encode GTP-binding protein homologues. A total of 460 small GTPase sequences from several legume species including Medicago and Lotus, Arabidopsis, human, and yeast were phyletically analysed to shed light on the evolution and functional characteristics of legume-specific homologues. One of the main emphases of this study was the elucidation of the possible involvement of some members of small GTPase homologues in the establishment and maintenance of symbiotic associations in root nodules of legumes. A high frequency of vesicle-mediated trafficking in nodules led to the idea of a probable subfunctionalization of some members of this family in legumes. As a result of the analyses, a group of 10 small GTPases that are likely to be mainly expressed in nodules was determined. The sequences determined as a result of this study could be used in more detailed molecular genetic analyses such as creation of RNA interference silencing mutants for further clarification of the role of GTPases in nodulation. This study will also assist in furthering our understanding of the evolutionary history of small GTPases in legume species.