Recent development and new insight of diversification and symbiosis specificity of legume rhizobia: mechanism and application.

Currently, symbiotic rhizobia (sl., rhizobium) refer to the soil bacteria in α- and ß-Proteobacteria that can induce root and/or stem nodules on some legumes and a few of nonlegumes. In the nodules, rhizobia convert the inert dinitrogen gas (N2 ) into ammonia (NH3 ) and supply them as nitrogen nutrient to the host plant. In general, this symbiotic association presents specificity between rhizobial and leguminous species, and most of the rhizobia use lipochitooligosaccharides, so called Nod factor (NF), for cooperating with their host plant to initiate the formation of nodule primordium and to inhibit the plant immunity. Besides NF, effectors secreted by type III secretion system (T3SS), exopolysaccharides and many microbe-associated molecular patterns in the rhizobia also play important roles in nodulation and immunity response between rhizobia and legumes. However, the promiscuous hosts like Glycine max and Sophora flavescens can nodulate with various rhizobial species harbouring diverse symbiosis genes in different soils, meaning that the nodulation specificity/efficiency might be mainly determined by the host plants and regulated by the soil conditions in a certain cases. Based on previous studies on rhizobial application, we propose a '1+n-N' model to promote the function of symbiotic nitrogen fixation (SNF) in agricultural practice, where '1' refers to appreciate rhizobium; '+n' means the addition of multiple trace elements and PGPR bacteria; and '-N' implies the reduction of chemical nitrogen fertilizer. Finally, open questions in the SNF field are raised to future think deeply and researches.

Mapping quantitative trait loci for sheath blight disease resistance in Yangdao 4 rice.

Rice sheath blight (ShB), which is caused by Rhizoctonia solani, has become the most serious rice disease in China. Yangdao 4, a cultivar with partial resistance to ShB, was crossed with Lemont, a susceptible cultivar, to develop mapping populations that were used to analyze quantitative trait loci (QTL) that confer resistance to ShB. QTL analysis were performed in 3 environments (E1-E3) using 2 F2 and 1 F2:3 populations, respectively. Three traits were recorded to evaluate ShB resistance, including disease rating (DR), lesion height (LH), and percentage of lesion height (PLH). Based on field evaluation of ShB resistance and the 2 genetic maps constructed, we identified a total of 8 QTLs for DR (4 in E1, 4 in E2, and 3 in E3), 6 QTLs for LH (1 in E1, 3 in E2, and 2 in E3), and 7 QTLs for PLH (1 in E1, 4 in E2, and 2 in E3). Sixteen of the ShB-QTLs co-localized as 6 clusters on chromosomes 3, 7, 11, and 12. Four of the 6 clusters contained ShB-QTLs that were detected in 2 environments, while the other 2 clusters with ShB-QTLs were detected in 1 environment. Three ShB-QTLs (qSBD-3-2, qSBL-3-1, and qSBPL-3-1) were delimited to a 581-kb region flanked by markers D333B and D334 on chromosome 3. The resistance alleles of Yangdao 4 at the qSBD-3-2 locus decreased DR by 0.68 and 0.79 in E2 and E3, respectively.

Development of 1047 insertion-deletion markers for rice genetic studies and breeding.

In this study, a total of 1047 insertion-deletion (InDel) primer pairs distributed across the rice genome were developed and experimentally validated. The primer pairs were designed based on the InDel length polymorphisms between 93-11 (Oryza sativa ssp indica cv.) and Nipponbare (Oryza sativa ssp japonica cv.), aiming for utilization between indica and japonica rice, or between other inter-subspecific rice cultivars. The 1047 primer pairs were dispersed across all 12 of the rice chromosomes, with one InDel marker found every 371.3 kb on average. The InDel length of the markers varied from 3 to 39 bp: 88.2% of the markers contained 6 to 25 bp, only 6.2% of markers were ≤ 5 bp, and 5.6% were ≥ 26 bp. Six hundred and twenty-three (59.5%) of the 1047 InDel markers were shown to amplify well and were polymorphic between Taichung65 and IR8, and 476 (45.5%) markers were polymorphic between Lemont and Yangdao4, while 398 (38.0%) were polymorphic in both combinations. These results demonstrated that the polymerase chain reaction-based InDel markers developed in this study could be of immediate use for rice genetic studies and breeding programs.