Improvement of Quality and Disease Resistance for a Heavy-Panicle Hybrid Restorer Line, R600, in Rice (Oryza sativa L.) by Gene Pyramiding Breeding

Abstract

The utilization of heavy-panicle hybrid rice exemplifies the successful integration of architectural enhancement and heterosis, which has been widely adopted in the southwest rice-producing area of China. Iterative improvement in disease resistance and grain quality of heavy-panicle hybrid rice varieties is crucial to promote their sustainable utilization. Here, we performed a molecular design breeding strategy to introgress beneficial alleles of broad-spectrum disease resistance and grain quality into a heavy-panicle hybrid backbone restorer line Shuhui 600 (R600). We successfully developed introgression lines through marker-assisted selection to pyramid major genes (Wx^b + ALK^A-GC + Pigm + Xa23) derived from three parents (Huanghuazhan, I135, I488), which significantly enhance grain quality and confer resistance to rice blast and bacterial blight (BB). The improved parental R600 line (iR600) exhibited superior grain quality and elevated disease resistance while maintaining the heavy-panicle architecture and high-yield capacity of R600. Moreover, the iR600 was crossed with male sterility line 608A to obtain a new heavy-panicle hybrid rice variety with excellent eating and cooking quality (ECQ) and high yield potential. This study presents an effective breeding strategy for rice breeders to expedite the improvement of grain quality and disease resistance in heavy-panicle hybrid rice.

Keywords: blast and bacterial blight resistance; grain quality; heavy panicle; iR600.

Figure 1

Co-improvement of high quality and disease resistance in R600. R600 was selected as a recurrent parent; I135, I488, and HHZ were chosen as donor parents. The schedule shows the breeding procedure for crossing and backcrossing. To pyramid four target genes in one iR600 line, several successive backcrossing processes were performed, and the backcrossing population was screened to select the desirable individuals. The three digits within the bracket indicate the screening parameters for each backcrossing population, representing the number of individuals evaluated (the first digit), the targeted gene numbers (the second digit), and the selected individuals for backcrossing (the third digit).

Figure 2

Molecular marker-assisted selection in this study. (a) Polymorphism analysis of Pigm among recurrent parent R600, donor parent I135, and improved line iR600. The double bands indicated the presence of the Pigm locus from the donor parent I135. (b) Polymorphism analysis of Xa23 among R600, I488, and iR600. The band genotype of Xa23 in I488 and iR600 was significantly smaller than that of R600. (c,d) KASP genotyping results of Wx and ALK. The blue circle represents the homozygous allele derived from the donor parent HHZ, while the purple circle signifies heterozygosity, and the red circle denotes alleles originating from R600.

Figure 3

Graphical genotype maps of iR600. The green bars represent the chromosome fragments derived from R600. The red lines represent chromosome fragments derived from donor parents. The black dots indicate the positions of Wx^b and ALK^A-GC genes from the donor parent of HHZ. The blue dot indicates the position of the Pigm gene from the donor parent of I135. The yellow dot indicates the position of the Xa23 gene from the donor parent of I488.

Figure 4

Analysis of grain appearance quality in improved lines. (a) The appearance of head rice. Scale bar, 1 cm. (b) HD, analysis of head rice (n = 3). (c) CGP, analysis of chalky grain percentage (n = 3). (d) CGG, analysis of chalky grain grade (n = 3). (e) LWR, analysis of length–width ratio (n = 10). (f) AC, analysis of amylose content (n = 3). (g) GC, analysis of gel consistency (n = 3). (h) ASV, analysis of alkali spreading value (n = 3). (i) CTS, analysis of comprehensive taste score (n = 3). (j) AS, analysis of appearance score (n = 3). (k) TS, analysis of taste score (n = 3). (l) Analysis of hardness (n = 3). (m) Analysis of viscosity (n = 3). The significant comparison object is R600. A two−tailed Student’s t-test was used to generate p-values. ** indicate a significant difference at p < 0.01; Values are means ± SD. Error bars represent SDs.

Figure 5

The RVA characteristic curves of improved lines. (a) The RVA characteristic curves. (b) GT, analysis of gelatinization temperature (n = 3). (c) HPT, analysis of hot paste viscosity (n = 3). (d) CPT, analysis of cool paste viscosity (n = 3). (e) BV, analysis of breakdown values (n = 3). (f) SV, analysis of setback values (n = 3). (g) CV, analysis of consistency values (n = 3). The significant comparison object is R600. A two−tailed Student’s t-test was used to generate p-values. ** indicate a significant difference at p < 0.01. Values are means ± SD. Error bars represent SDs.

Figure 6

Evaluation and analysis of disease resistance of improved lines. (a) Rice blast is naturally induced. (b) Incidence of leaf blast. Scale bar, 1 cm. (c) Disease analysis of rice blast (n = 3). (d) Bacterial blight inoculation. (e) Incidence of bacterial blight. Scale bar, 1 cm. (f) Analysis of leaf blight spot length (n = 10). The significant comparison object is R600. The red arrow represents the inoculated leaves. A two−tailed Student’s t-test was used to generate p-values. ** indicate a significant difference at p < 0.01. Values are means ± SD. Error bars represent SDs.

Figure 7

Evaluation of main agronomic characters in improved lines. (a) Main panicles. Scale bar, 10 cm. (b) Plant morphology. Scale bar, 20 cm. (c) YP, yield per plant (n = 10). (d) TN, tiller number (n = 10). (e) PH, plant height (n = 10). (f) SGP, spike grain per panicle (n = 10). (g) SR, setting rate (n = 10). (h) SL, spike length (n = 10). (i) PB, primary branches (n = 10). (j) SD, spikelet density (n = 10). (k) 1000-grain weight (n = 10). The significant comparison object is R600. A two−tailed Student’s t-test was used to generate p-values. * and ** indicate a significant difference at p < 0.05 and 0.01. Values are means ± SD. Error bars represent SDs.

Figure 8

Analysis of rice quality in hybrid combinations. (a) Whole-milled rice appearance of 608A/R600. Scale bar, 1 cm. (b) Whole-milled rice appearance of 608A/iR600. Scale bar, 1 cm. (c) CGP, chalky grain percentage of hybrid combinations (n = 3). (d) CGG, chalky grain grade of hybrid combinations (n = 3). (e) CTS, comprehensive taste score of hybrid combinations (n = 3). (f) AC, amylose content of hybrid combinations (n = 3). (g) GC, gel consistency of hybrid combinations (n = 3). (h) ASV, alkali spreading value of hybrid combinations (n = 3). The significant comparison objects are the cross combinations of R600 corresponding to male sterile line. A two-tailed Student’s t-test was used to generate p-values. ** indicate a significant difference at p < 0.01. Values are means ± SD. Error bars represent SDs.

Figure 9

Major agronomic phenotypes of hybrid combinations. (a–c) Field growth of hybrid combinations. (d) Yield of hybrid combination (n = 3). (e) Plant height of hybrid combination (n = 10). (f) Tiller number of hybrid combinations (n = 10). Values are means ± SD. Error bars represent SDs. Different lowercase letters indicate significant differences (p ≤ 0.05; one−way ANOVA).