The rice disease resistance (R) gene Xa3/Xa26 (having also been named Xa3 and Xa26) against Xanthomonas oryzae pv. oryzae (Xoo), which causes bacterial blight disease, belongs to a multiple gene family clustered in chromosome 11 and is from an AA genome rice cultivar (Oryza sativa L.). This family encodes leucine-rich repeat (LRR) receptor kinase-type proteins. Here, we show that the orthologs (alleles) of Xa3/Xa26, Xa3/Xa26-2, and Xa3/Xa26-3, from wild Oryza species O. officinalis (CC genome) and O. minuta (BBCC genome), respectively, were also R genes against Xoo. Xa3/Xa26-2 and Xa3/Xa26-3 conferred resistance to 16 of the 18 Xoo strains examined. Comparative sequence analysis of the Xa3/Xa26 families in the two wild Oryza species showed that Xa3/Xa26-3 appeared to have originated from the CC genome of O. minuta. The predicted proteins encoded by Xa3/Xa26, Xa3/Xa26-2, and Xa3/Xa26-3 share 91–99% sequence identity and 94–99% sequence similarity. Transgenic plants carrying a single copy of Xa3/Xa26, Xa3/Xa26-2, or Xa3/Xa26-3, in the same genetic background, showed a similar resistance spectrum to a set of Xoo strains, although plants carrying Xa3/Xa26-2 or Xa3/Xa26-3 showed lower resistance levels than the plants carrying Xa3/Xa26. These results suggest that the Xa3/Xa26 locus predates the speciation of A and C genome, which is approximately 7.5?million years ago. Thus, the resistance specificity of this locus has been conserved for a long time.
水稻抗病(R)基因Xa3/Xa26 可以抵御白葉枯病原菌oryzae pv. oryzae (Xoo)。Xa3/Xa26 源于AA基因組栽培水稻，屬于一個多基因家族，在11染色體上成簇排列。該家族編碼富含亮氨酸重復序列(LRR)受體激酶。本研究作者發(fā)現Xa3/Xa26 的同源等位基因Xa3/Xa26-2(CC基因組野生稻O.officinalis)和Xa3/Xa26-3(BBCC基因組O.minuta)也具有白葉枯抗性。Xa3/Xa26-2 和Xa3/Xa26-3 對18個測試的Xoo菌株中的16個具有抗性。對Xa3/Xa26 基因家族的序列比較分析表明Xa3/Xa26 可能源于O.minuta 的CC 基因組。Xa3/Xa26、Xa3/Xa26-2 和Xa3/Xa26-3 預測的編碼蛋白序列具有91-99%的一致性和94-99%的相似性。攜帶有單拷貝的Xa3/Xa26、Xa3/Xa26-2 和Xa3/Xa26-3 的、具有相同遺傳背景的轉基因株系，對不同Xoo菌株表現出類似的抗譜，雖然Xa3/Xa26-2 和Xa3/Xa26-3 株系的抗性水平較Xa3/Xa26 株系低。這些結果表明Xa3/Xa26 座位可能起源于750萬年前，早于A、C基因組物種的分化。因此該座位的特異抗性長久以來都是保守的。