Association genetics of the parameters related to nitrogen use efficiency in Brassica juncea L.

Key message

Genome wide association studies allowed prediction of 17 candidate genes for association with nitrogen use efficiency. Novel information obtained may provide better understanding of genomic controls underlying germplasm variations for this trait in Indian mustard.

Abstract

Nitrogen use efficiency (NUE) of Indian mustard (Brassica juncea (L.) Czern & Coss.) is low and most breeding efforts to combine NUE with crop performance have not succeeded. Underlying genetics also remain unexplored. We tested 92 SNP-genotyped inbred lines for yield component traits, N uptake efficiency (NUPEFF), nitrogen utilization efficiency (NUTEFF), nitrogen harvest index (NHI) and NUE for two years at two nitrogen doses (N_o without added N and N₁₀₀ added @100 kg/ha). Genotypes IC-2489-88, M-633, MCP-632, HUJM 1080, GR-325 and DJ-65 recorded high NUE at low N. These also showed improved crop performance under high N. One determinate mustard genotype DJ-113 DT-3 revealed maximum NUTEFF. Genome wide association studies (GWAS) facilitated recognition of 17 quantitative trait loci (QTLs). Environment specificity was high. B-genome chromosomes (B02, B03, B05, B07 and B08) harbored many useful loci. We also used regional association mapping (RAM) to supplement results from GWAS. Annotation of the genomic regions around peak SNPs helped to predict several gene candidates for root architecture, N uptake, assimilation and remobilization. CAT9 (At1g05940) was consistently envisaged for both NUE and NUPEFF. Major N transporter genes, NRT1.8 and NRT3.1 were predicted for explaining variation for NUTEFF and NUPEFF, respectively. Most significant amino acid transporter gene, AAP1 appeared associated with NUE under limited N conditions. All these candidates were predicted in the regions of high linkage disequilibrium. Sequence information of the predicted candidate genes will permit development of molecular markers to aid breeding for high NUE.

     

Role of catalase,H2O2 and phenolics in resistance of pigeonpea towards Helicoverpa armigera (Hubner)

Rapid generation of superoxide radicals and accumulation of H₂O₂ is a characteristic early response of plants following perception of insect herbivory signals. Induction of oxidative burst on account of herbivory triggers various defense mechanisms in plants. Response of superoxide and H₂O₂-metabolizing enzymes and secondary metabolites in nine pigeonpea genotypes to Helicoverpa armigera feeding was investigated. Out of nine, four genotypes were found to be moderately resistant, three were intermediate and two were moderately susceptible. In general, H. armigera infestation resulted in increase in superoxide dismutase activity, H₂O₂ and phenolics content and decrease in catalase (CAT) activity in leaves, developing seeds and pod wall of pigeonpea genotypes. Peroxidase activity was found only in leaves. Among genotypes, the increase in phenolic constituents was found greater in moderately resistant genotypes than in moderately susceptible genotypes; this might determine their contribution in providing resistance to genotypes against H. armigera infestation. The capability of moderately resistant genotypes to maintain relatively lower H₂O₂ content and higher CAT activity in pod wall and developing seeds also appeared to determine resistance of genotypes towards H. armigera. Expression of resistance to H. armigera was found to be associated with a negative correlation of H₂O₂-metabolizing enzymes and phenolics with pod damage as well as with negative association between CAT activity and H₂O₂ content. A positive correlation found between H₂O₂ content and pod damage suggested the accumulation of H₂O₂ in response to pod borer attack. In addition, correlation analysis also revealed a positive association between CAT, phenolic compounds and DPPH radical scavenging activity following pod borer attack; this indicated their contribution in resistance mechanisms against H. armigera herbivory.