Molecular-Marker-Based Design for Breeding Indica–Japonica Hybrid Rice with Bacterial Blight Resistance

<b>Background/Objectives</b>: To overcome the limitations imposed by bacterial blight on widely adopted indica-japonica hybrid rice, this study employed molecular design breeding strategies to develop a resistant germplasm. <b>Methods</b>: Through conventional backcross breeding combined with molecular-marker-assisted selection, the <i>Xa23</i>-carrying material XR39 was hybridized with the wide-compatibility restorer line R5315 harboring the <i>S5n</i> gene. Progeny selection integrated evaluations of agronomic traits, disease resistance identification, and test-crossing with sterile lines. <b>Results:</b> Five wide-compatibility restorer lines simultaneously incorporating the <i>Xa23</i> and <i>S5n</i> genes were successfully developed, demonstrating outstanding bacterial blight resistance and restoration ability. The selected hybrid combinations, A3/RP1, A1/RP4, and A4/RP4, exhibited yield increases of 2.6-8.6% compared to the control. <b>Conclusions</b>: This study not only established a novel germplasm for developing bacterial blight-resistant indica-japonica hybrid rice varieties, but also established a model for gene design breeding for rice improvement.