Accumulating candidate genes for broad-spectrum resistance to rice blast in a drought-tolerant rice cultivar

Abstract

Biotic stresses, including diseases, severely affect rice production, compromising producers' ability to meet increasing global consumption. Understanding quantitative responses for resistance to diverse pathogens can guide development of reliable molecular markers, which, combined with advanced backcross populations, can accelerate the production of more resistant varieties. A candidate gene (CG) approach was used to accumulate different disease QTL from Moroberekan, a blast-resistant rice variety, into Vandana, a drought-tolerant variety. The advanced backcross progeny were evaluated for resistance to blast and tolerance to drought at five sites in India and the Philippines. Gene-based markers were designed to determine introgression of Moroberekan alleles for 11 CGs into the progeny. Six CGs, coding for chitinase, HSP90, oxalate oxidase, germin-like proteins, peroxidase and thaumatin-like protein, and 21 SSR markers were significantly associated with resistance to blast across screening sites. Multiple lines with different combinations, classes and numbers of CGs were associated with significant levels of race non-specific resistance to rice blast and sheath blight. Overall, the level of resistance effective in multiple locations was proportional to the number of CG alleles accumulated in advanced breeding lines. These disease resistant lines maintained tolerance to drought stress at the reproductive stage under blast disease pressure.

Figure 1

Breeding scheme describing the development of intermated lines from BC₃F₄ Vandana/Moroberekan families generated by Wu et al..

Figure 2

Performance of F₄ intermated lines carrying different number of candidate genes measured by area under the disease progressive curve (AUDPC) in Almora, India.

Figure 3

Neighbor-joining tree generated by calculating Gower distances between genotypes of F₆ intermated lines based on the similarity to the population’s parents (left panel). Genotyping of intermated lines using 60 SSR markers spanning five chromosomes (see Fig. S3 for reference) and ten CG markers (black box highlight) shows greater similarity to Vandana (1—yellow) than Moroberekan (2—blue); some regions were heterozygous (3—light green), while others did not fit the above classes (4—dark green) (center panel). Lines accumulating Moroberekan regions performed better under seedling blast conditions in Almora (right panel). The same lines showed higher yields under heavy blast conditions. Performance under drought at IRRI is consistent with the parent Vandana. Chi (Chitinase), OXO (Oxalate Oxidase), ar (Aldose reductase), thau7 (Thaumatin like), pox (Peroxidase), oxlp (Germin-like protein), HSP90 (Heat shock protein 90), PR1 and PR10 (Pathogenesis-related genes).

Figure 4

(a) Performance of F₆ intermated lines with different number and candidate gene combinations (highlighted in red boxes) in Almora and Hazaribag, India. DLA disease leaf area, DAS days after sowing. (b) Leaf blast field phenotypes in Almora. Left: IR78221-19-6-56 (R); center: IR78222-20-1A-7 (S); right: IR78224-22-2-114 (HS). (R) Resistant; (S) Susceptible; (HS) Highly susceptible. Red arrowhead = blast lesions.

Figure 5

Performance of F₆ intermated lines with different CG genes after inoculation with R. solani strain RM01401 in greenhouse tests. Introgressed lines with multiple CGs, particularly those with OXO, are more resistant than lines with few CGs. Line number (CG content): 19-6-7-B-B (OXO, HSP90, POX); 19-6-56-B-B (Chitinase, OXO, POX, GLP); 19-6-99-B-B (OXO); 19-3-196-B-B (Chitinase).

Figure 6

Yield during DS drought conditions at IRRI against seedling blast infection recorded at Cavinti and Almora of selected 60 F₅ progenies from intermated BC₃F₄ lines. Highlighted in the graphic, filled data points correspond to lines (from left to right): IR78221-19-6-7-B-B (green); IR78221-19-6-3-B-B (yellow); IR78221-19-6-33-B-B (red); IR78222-20-7-128-B-B (gray); IR78221-19-6-90-B-B (orange); IR78221-19-6-99-B-B (blue); IR78221-19-6-56-B-B (purple); black (Vandana).