This study was conducted to assess the effects of pyramiding bacterial blight (BB) resistance genes and to develop genes pyramided lines for enhancing the resistance of japonica rice in Korea. Reaction of single resistance genes and their pyramiding effects in the indica and japonica genetic background were investigated against four predominant (K1, K2, K3, and K3a races) and other sixteen Korean BB isolates. Among single resistance genes, Xa1, Xa3, and Xa21 in indica and japonica near-isogenic lines (NILs) exhibited the same or similar level of resistant reaction. On the other hand, Xa4 and xa5 were more effective in resistant reaction in indica than in japonica genetic background. Pyramiding resistance genes was more effective in increasing the resistance relative to single gene or two genes and overcoming susceptibility in both indica and japonica rice. Newly identified combinations in japonica rice, Xa3+Xa21, xa5+Xa21, and Xa3+xa5+Xa21, conferred resistance to all isolates and showed higher resistant reaction than Xa3+xa5 widely used in the current BB resistant breeding program.
Early maturing japonica rice varieties in Korea are more vulnerable to BB than medium and mid-late maturing rice. For enhancing the resistance of early maturing rice against BB, resistance genes pyramided lines were developed. 'Unkwang' carrying Xa3 was used as a recurrent parent with 'SR30075' carrying Xa4+xa5+Xa21 as a donor parent. RL1 (Resistant line, BC1F7), RL2, RL3, RL4, and RL5 (BC2F6) were bred through bio-assay of K3a race inoculation and phenotypic selection of agronomic traits. The presence of introduced resistance genes was confirmed by testing the resistance levels against BB and then was double-checked using DNA markers. RL1 has Xa3+xa5+Xa21. RL2, RL3, and RL5 have Xa3+Xa21 whereas RL4 has only Xa21. The combination of BB resistance genes (Xa3+xa5+Xa21 and Xa3+Xa21) was found to be promising, as the rice lines carrying these genes showed strong resistant reaction against 16 BB isolates. Also, the inoculation of K3a race did not alter the brown rice yield, ripened grain ratio and kernel quality of brown rice compared to the control plot. Although RL1 containing Xa3+xa5+Xa21 showed excellent resistance performance, it is not suitable to cultivation due to lodging, lower yield than Unkwang, and low grain quality. RL5 developed by backcrossing twice with Unkwang was a promising line due to its effective resistance gene combination, Xa3+Xa21 and good agronomic traits such as lodging tolerance, higher yield and quality compared to Unkwang.
Four BB resistance genes, Xa1+Xa3+xa5+Xa21, pyramided elite japonica rice lines were developed. Seven doubled haploid (RDL1-7) and ten F6 lines (RPL1-10) having four genes which were derived from the cross between 'Ilmibyeo', high grain quality japonica rice cultivar carrying Xa1, and 'Iksan575', elite line carrying Xa3+xa5+Xa21, were developed using marker-assisted selection for resistance genes and phenotypic selection for BB resistance and agronomic traits. Among resistance genes combinations in the F2 population, four resistance genes combination, Xa1+Xa3+xa5+Xa21, showed higher resistance than any other combinations of two or three genes. Four genes pyramided lines (RDL and RPL) showed broad-spectrum resistance against 16 Korean bacterial blight isolates and the yield and quality of the lines were not altered by the inoculation of K3a, the most virulent race in Korea. In addition, these lines had excellent plant type and exhibited more yield than previously developed resistant cultivars. Four bacterial blight resistance genes combination, Xa1+Xa3+xa5+Xa21, proved to be efficient and promising combination and developed lines with four genes could be useful materials and will be applied to the breeding programs for enhancing the resistance of japonica rice to bacterial blight.