Identification of QTLs for Blast, Bacterial Blight, and Planthopper Resistance Using SNP-Based Linkage Maps from Two Recombinant Inbred Rice Lines

Rice is the most significant global food security. Several biotic factors limit rice production, breeding biotic-resistant rice has, therefore, become an increasingly important goal. Two elite rice lines, IR71033-121-15 (IR71033) and IR57514-PMI-5-B-1-2 (IR57514), provide potential genes for biotic stress resistance traits. In this study, genotyping by sequencing (GBS) for single nucleotide polymorphism (SNP)-based linkage map construction was used to detect quantitative trait loci (QTLs) for blast (BL), bacterial blight (BB), whitebacked planthopper (WBPH), and brown planthopper (BPH) resistance. IR71033 was derived from Oryza minuta and carried BL, BB, WBPH, and BPH resistance QTLs. IR57514 is a well-adapted rainfed lowland line that carries BL and BB resistance QTLs. Two sets of recombinant inbred line (RIL) populations derived from crosses of KDML105 × IR71033 and KDML105 × IR57514 were used to dissect the genetic basis of disease and insect pest resistance. The RIL populations were evaluated for BL, BB, WBPH, and BPH resistance from 2016 to 2018 at four rice research centers in Thailand. From these, we identified a large number of SNPs through GBS and constructed high-resolution linkage maps. By combining phenotypic evaluation with the GBS data, a total of 24 QTLs on four chromosomes were detected that confered pest resistance and explained 7.3% - 61.4% of the phenotypic variance. These findings should facilitate identifying novel resistance genes and applying marker-assisted selection for resistance to the four major rice pests investigated here. These strategies will improve the resilience and reliability of rice varieties adapted to the low-yielding environment of rainfed lowland areas worldwide.

Loss of Resistance to <i>Nilaparvata lugens</i>May Be Due to the Low-Level Expression of <i>BPH32 </i>in Rice Panicles at the Heading Stage

The brown planthopper (BPH), Nilaparvata lugens (St&#229;l), is the most important insect pest of rice in Asia. Host plant resistance is one of the strategies currently used to control BPH. The resistant rice cultivar Rathu Heenati (RH) carrying the BPH3 gene (recently renamed as “BPH32”) remains effective despite more than 30 years of deployment. RH has been determined to be resistant against BPH at all growth stages. However, we observed that BPH could feed on panicles but not on the leaf sheaths of RH. The resistance gene BPH32 was introduced into KDML105 through marker-assisted selection, and the introgression line UBN03078 was developed. This rice line was used to observe the patterns of target gene’s regulation. A low-level expression of BPH32 on panicles has been hypothesized to cause susceptibility in UBN03078 at the heading stage. Findings from our gene expression analysis support the hypothesis that the resistance gene was down regulated in the uppermost internodes compared with the leaf sheaths of the heading rice plant. This phenomenon may allow BPH to feed on the panicles of the resistant plants, but this requires further investigation.

Chromosomal Location of a Recessive Red-Eye Mutant Gene in the Brown Planthopper <i>Nilaparvata lugens</i>(Stål) (Insecta: Hemiptera)

The color of compound eyes is an important biological characteristic of insects. A red eye color mutation is commonly found in the brown planthopper (BPH), Nilaparvata lugens (St&#229l) (Hemiptera: Delphacidae), a serious insect pest of rice in tropical and temperate Asia. The genetic inheritance and physiological effect of the eye color mutation in the BPH have been studied, but the location of a red gene controlling the red eye mutant phenotype on a chromosome has not been elucidated. In this study, simple sequence repeats (SSRs), together with bulked segregant analysis (BSA), was performed to identify and map the location of the red gene. A total of 387 SSR markers distributed throughout the BPH autosome were used to survey two bulked DNA samples. Samples were generated from 29 brown-eyed and 29 red-eyed individuals derived from an F2 generation of a cross between brown-eyed wild type and red-eyed mutant colonies. The SSR marker BM20 was shown to be associated with the red eye mutant phenotype. Ninety-five offspring of the F2 generation were then used to map the gene. The present study constitutes the discovery of the location of the red gene, which may lead to the acquisition of the genetic determinant of the compound eye color mutation in BPH.