Understanding the nature of blast resistance in combined bacterial leaf blight and blast gene pyramided lines of rice variety tellahamsa.

BACKGROUND: Rice blast and bacterial leaf blight (BLB) are the most limiting factors for rice production in the world which cause yield losses typically ranging from 20 to 30% and can be as high as 50% in some areas of Asia especially India under severe infection conditions. METHODS AND RESULTS: An improved line of Tellahamsa, TH-625-491 having two BLB resistance genes (xa13 and Xa21) and two blast resistance genes (Pi54 and Pi1) with 95% Tellahamsa genome was used in the present study. TH-625-491 was validated for all four target genes and was used for backcrossing with Tellahamsa. Seventeen IBC1F1 plants heterozygous for all four target genes, 19 IBC1F2 plants homozygous for four, three and two gene combinations and 19 IBC1F2:3 plants also homozygous for four, three and two gene combinations were observed. Among seventeen IBC1F1 plants, IBC1F1-62 plant recorded highest recurrent parent genome (97.5%) covering 75 polymorphic markers. Out of the total of 920 IBC1F2 plants screened, 19 homozygous plants were homozygous for four, three and two target genes along with bacterial blight resistance. Background analysis was done in all 19 homozygous IBC1F2 plants possessing BLB resistance (possessing xa13, Xa21, Pi54 and Pi1 in different combinations) with five parental polymorphic SSR markers. IBC1F2-62-515 recovered 98.5% recurrent parent genome. The four, three and two gene pyramided lines of Tellahamsa exhibited varying resistance to blast. CONCLUSIONS: Results show that there might be presence of antagonistic effect between bacterial blight and blast resistance genes since the lines with Pi54 and Pi1 combination are showing better resistance than the combinations with both bacterial blight and blast resistance genes.

Reclassification of Rhodospirillum photometricum Molisch 1907, Rhodospirillum sulfurexigens Anil Kumar et al. 2008 and Rhodospirillum oryzae Lakshmi et al. 2013 in a new genus, Pararhodospirillum gen. nov., as Pararhodospirillum photometricum comb. nov., Pararhodospirillum sulfurexigens comb. nov. and Pararhodospirillum oryzae comb. nov., respectively, and emended description of the genus Rhodospirillum.

The genus Rhodospirillum is represented by four species, with three of them showing phylogenetic divergence compared to the type species, Rhodospirillum rubrum. Differences in the major diagnostic properties such as internal photosynthetic membranes, quinones, fatty acids, carotenoid composition and a few other phenotypic properties warrant the reclassification of members of this genus. Resultantly, a new genus, Pararhodospirillum gen. nov., is proposed based on the analysis of nine strains to accommodate Rhodospirillum photometricum, Rhodospirillum sulfurexigens and Rhodospirillum oryzae as Pararhodospirillum photometricum comb. nov., Pararhodospirillum sulfurexigens comb. nov. and Pararhodospirillum oryzae comb. nov., respectively. The type species of the genus is Pararhodospirillum photometricum comb. nov. An emended description of the genus Rhodospirillum is also proposed.

Rhodopseudomonas pentothenatexigens sp. nov. and Rhodopseudomonas thermotolerans sp. nov., isolated from paddy soils.

Two strains (JA575(T) and JA576(T)) of orange- to pink-pigmented, rod-shaped, motile and budding phototrophic bacteria were isolated from paddy soils. Both strains contained bacteriochlorophyll a and carotenoids of spirilloxanthin series. Both strains had C(18 : 1)ω7c as the major cellular fatty acid, ubiquinone-10 (Q(10)) as the main quinone, and diphosphatidylglycerol, phosphatidylglycerol, phosphatidylethanolamine and phosphatidylcholine as polar lipids. Phylogenetic analysis on the basis of 16S rRNA gene sequences showed that both strains clustered with species of the genus Rhodopseudomonas in the class Alphaproteobacteria. Strains JA575(T) and JA576(T) were genotypically (<35 % DNA-DNA relatedness) and phenotypically distinct from each other. Further, both strains showed less than 48 % DNA-DNA relatedness with the type strains of all recognized species of the genus Rhodopseudomonas. The molecular evidence is supported by phenotypic evidence. It is proposed that strains JA575(T) and JA576(T) be classified as representing two novel species of the genus Rhodopseudomonas with the species names Rhodopseudomonas pentothenatexigens sp. nov. and Rhodopseudomonas thermotolerans sp. nov., respectively. The type strains of the proposed novel species are JA575(T) (= NBRC 108862(T) = KCTC15143(T)) and JA576(T) (= NBRC 108863(T) = KCTC 15144(T)), respectively.

Blastochloris gulmargensis sp. nov., isolated from an epilithic phototrophic biofilm.

A novel Gram-negative, motile, bacteriochlorophyll b-containing purple non-sulfur bacterium, strain JA248(T), was isolated from phototrophic enrichments of a yellow-green epilithic biofilm sample collected from Gulmarg, India. The genomic DNA G+C content of strain JA248(T) was 63.8 mol%. A phylogenetic tree based on 16S rRNA gene sequence analysis showed that strain JA248(T) had highest similarity to members of the genus Blastochloris and was closely related to Blastochloris sulfoviridis DSM 729(T) (98.5 % sequence similarity) and Blastochloris viridis DSM 133(T) (98.4 %) of the class Alphaproteobacteria. Strain JA248(T) was characterized based on polyphasic taxonomy, and distinct phenotypic and molecular differences based on DNA-DNA hybridization (relatedness of <46.5 % with the two species of the genus Blastochloris), multilocus sequence analysis, and phenotypic and chemotaxonomic evidence separated strain JA248(T) from other species of the genus Blastochloris. Strain JA248(T) therefore represents a novel species in the genus Blastochloris, for which the name Blastochloris gulmargensis sp. nov. is proposed. The type strain is JA248(T) ( = JCM 14795(T)  = DSM 19786(T)).

Marker Assisted Gene Pyramiding (MAGP) for bacterial blight and blast resistance into mega rice variety "Tellahamsa".

Bacterial blight (BB) and fungal blast diseases are the major biotic constraints that limit rice productivity. To sustain yield improvement in rice, it is necessary to developed yield potential of the rice varieties by incorporation of biotic stress resistance genes. Tellahamsa is a well-adapted popular high yielding rice variety in Telangana state, India. However, the variety is highly susceptible to BB and blast. In this study, simultaneous stepwise transfer of genes through marker-assisted backcross breeding (MABB) strategy was used to introgress two major BB (Xa21 and xa13) and two major blast resistance genes (Pi54 and Pi1) into Tellahamsa. In each generation (from F1 to ICF3) foreground selection was done using gene-specific markers viz., pTA248 (Xa21), xa13prom (xa13), Pi54MAS (Pi54) and RM224 (Pi1). Two independent BC2F1 lines of Tellahamsa/ISM (Cross-I) and Tellahamsa/NLR145 (Cross-II) possessing 92% and 94% recurrent parent genome (RPG) respectively were intercrossed to develop ICF1-ICF3 generations. These gene pyramided lines were evaluated for key agro-morphological traits, quality, and resistance against blast at three different hotspot locations as well as BB at two locations. Two ICF3 gene pyramided lines viz., TH-625-159 and TH-625-491 possessing four genes exhibited a high level of resistance to BB and blast. In the future, these improved Tellahamsa lines could be developed as mega varieties for different agro-climatic zones and also as potential donors for different pre-breeding rice research.