The Asian rice gall midge, Orseolia oryzae (Wood‐Mason) (Diptera: Cecidomyiidae), is a major pest of rice [Oryza sativa L. (Poaceae)] in India. Breeding resistant varieties and their cultivation has been the main approach to manage this pest. However, the breakdown of resistance conferred by the major genes, deployed one at a time, through evolution of virulent biotypes has become a major setback to this approach. Development of polymerase chain reaction‐based molecular markers for eight of the 10 resistance genes and their possible use in marker‐assisted selection has enabled breeders to pyramid resistance genes for achieving durable resistance. However, the choice of resistance genes needs to be made with a better understanding of the virulence composition of the pest populations in the target area and the genetics of plant resistance and insect virulence, as the rice–gall midge interaction is a gene‐for‐gene one. We adopted a single‐female test and coupled it with a modified F2 screen test to note the virulence composition of gall midge populations and estimated the frequency of virulence alleles for adaptation at three pest endemic locations in India, namely, Warangal, Ragolu, and Raipur. Results on biotype composition showed heterogeneous pest populations in all the tests and at all the locations. Tests at Warangal repeated after 8 years showed a rapid increase in frequency of the virulence allele conferring adaptation to the plant resistance gene Gm2 as compared to that of the allele for adaptation to the resistance gene Gm1. This is probably the first direct measurement of a durability parameter of plant genes conferring insect resistance. Results supported earlier observations that sex‐linked virulence against Gm2 makes it less durable. The sex ratio did not deviate from the expected 1:1 ratio at Warangal, but at Ragolu females outnumbered males.
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 welladapted 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 F 1 to ICF 3) foreground selection was done using gene-specific markers viz., pTA248 (Xa21), xa13prom (xa13), Pi54MAS (Pi54) and RM224 (Pi1). Two independent BC 2 F 1 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-ICF 3 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 ICF 3 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.
Improved-Samba-Mahsuri (ISM), a high-yielding, popular bacterial blight resistant (possessing Xa21, xa13, and xa5), fine-grain type, low glycemic index rice variety is highly sensitive to low soil phosphorus (P). We have deployed marker-assisted backcross breeding (MABB) approach for targeted transfer of Pup1, a major QTL associated with low soil P tolerance, using Swarna as a donor. A new co-dominant marker, K20-1-1, which is specific for Pup1 was designed and used for foreground selection along with functional markers specific for the bacterial blight resistance genes, Xa21, xa13, and xa5. A set of 66 polymorphic SSR marker were used for the background selection along with a pair of flanking markers for the recombination selection in backcross derived progenies and in BC2F2 generation, 12 plants, which are homozygous for Pup1, all the three bacterial blight resistance genes and possessing agro-morphological traits equivalent to or better than ISM were selected and selfed to produce BC2F3s. They were evaluated in plots with low soil P and normal soil P at ICAR-IIRR, Hyderabad for their low soil P tolerance, and bacterial blight resistance and superior lines were advanced to BC2F6. One of the lines, when tested at multiple locations in India was found promising under both normal as well as low soil P conditions.
Large genetic variation exists for grain iron and zinc in rice germplasm including wild species and deep water rices. Conventional breeding is an easy and acceptable approach to biofortify rice. Analysis of 126 rice accessions using atomic absorption spectrophotometry showed that Fe concentration in brown rice ranged from 6 ppm in Athira to 72 ppm in O. nivara and Zn concentration from 27 ppm in Jyothi to 67 ppm in O. rufipogon. Quantitative trait loci (QTL) for high Fe and Zn concentration in grains were mapped from 3 wild progenitors and 2 deep water rices with the aim of gene discovery and also to develop iron/zinc‐rich lines of two widely grown, popular rice varieties Swarna (MTU7029) and Samba Mahsuri (BPT5204) through conventional breeding approaches. Three advanced backcross mapping populations were developed using 3 wild accessions: Swarna x O.nivara IRGC81832, Swarna x O.nivara IRGC81848 and Samba Mahsuri x O.rufipogon WR119. In addition, F7 recombinant inbred lines (RILs) were developed from two crosses: Madhukar x Swarna and Jalmagna x Swarna. Madhukar and Jalmagna are deep‐water rice varieties with high grain iron and zinc. Overall, Fe concentration ranged from 0.2 to 224 ppm and Zn concentration from 0.4 to 104 ppm in the 5 mapping populations. QTLs for Fe and Zn concentration in polished rice were mapped from two Swarna x O. nivara BC2F3 mapping populations using accession IRGC81848‐ 227 families, 100 SSR markers and accession IRGC81832‐ 245 families, 75 SSR markers. qFe2.1, qFe3.1, qFe8.1 and qFe8.2 coincided in the two populations. QTLs for Fe and Zn concentration coincided on chromosomes 2, 3, 8 and 12. Five QTLs for Fe and 3 QTLs for Zn each explained more than 15% phenotypic variance. QTLs were also mapped for Fe and Zn concentration in brown rice from Madhukar x Swarna F7 RILs using 110 SSR markers including 9 gene specific markers. Seven QTLs for Fe and six QTLs for Zn were identified each explaining >30% phenotypic variance. QTLs for Fe and Zn concentration coincided on chromosomes 7 and 12. Madhukar allele increased Fe in qFe7.1 and qFe7.2 and Swarna allele increased Zn in qZn12.1 and qZn12.2. Only two QTL flanking markers RM243 for qFe1.1 and RM517 for qZn3.1 were common in the 3 mapping populations. Genotyping of 2 populations is in progress. Candidate genes OsYSL1, OsNAC, OsYSL16, OsZIP4, OsYSL17 and OsNAAT1 underlie the Fe or Zn QTLs mapped in the Swarna x O.nivara mapping population. Likewise, OsYSL1, OsMTP1, OsNAS1, OsNAS3, OsNRAMP1, heavy metal ion transporter, OsAPRT underly QTLs for Fe and Zn mapped in the Madhukar x Swarna mapping population. In all, 20 elite lines with >80 ppm iron and >50ppm zinc in brown rice were identified from 5 mapping populations. These non‐transgenic rice lines with high iron, high zinc or both are a useful resource for functional genomics and biofortification programmes.
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