The broad bacterial blight resistance of rice line CBB23 is triggered by a novel transcription activator‐like (TAL) effector of Xanthomonas oryzae pv. oryzae

Abstract

Bacterial blight (BB), caused by Xanthomonas oryzae pv. oryzae (Xoo), is not only a disease devastating rice production worldwide, but also an ideal model system for the study of the interaction between plants and their bacterial pathogens. The rice near-isogenic line (NIL) CBB23, derived from a cross between a wild rice Oryza rufipogon accession (RBB16) and a susceptible indica rice variety (Jingang 30), is highly resistant to all field Xoo strains tested so far. Although the BB resistance of CBB23 has been widely used in rice breeding programmes, the mechanism of its extremely broad-spectrum resistance remains unknown. Here, we report the molecular cloning of an avirulence gene, designated as avrXa23, from Xoo strain PXO99(A) . We validate that AvrXa23, a novel transcription activator-like effector, specifically triggers the broad-spectrum BB resistance in CBB23. The prevalence of avrXa23 in all 38 Xoo strains surveyed may explain the broad-spectrum feature of BB resistance in CBB23. The results will significantly facilitate the molecular cloning of the corresponding resistance (R) gene in the host, and provide new insights into our understanding of the molecular mechanism for broad-spectrum disease resistance in plants.

XA23 Is an Executor R Protein and Confers Broad-Spectrum Disease Resistance in Rice

“…By examining the RVDs of AvrXa23 (Wang et al, 2014a) with the TALE recognition code, which governs the interaction between TALE RVDs and their target nucleotides (Boch et al, 2009;Moscou and Bogdanove, 2009), we could visually corroborate the prediction of EBE for AvrXa23 in the promoter region of Xa23 using the TALE-NT program (https://tale-nt.cac.cornell.edu/) ( Figure 6A). In the promoter of Xa23, a DNA sequence that partially matched (19 of 27 nucleotides), the predicted binding sequence was found; most of the matching nucleotides were located in the 5 0 region of the putative binding element ( Figure 6A).…”

“…To further corroborate the involvement of ORF113 in Xa23-mediated resistance, we reduced ORF113 expression by RNA interference (RNAi) in Zhongwu1 (ZW1), a japonica rice variety harboring the Xa23 locus introduced from CBB23 by back-crossing (Wang et al, 2014a). The ORF113-RNAi plants (e.g., R350) derived from ZW1 became susceptible to PXO99 A ( Figure 4C and Supplemental Figure 2B), and the susceptibility is correlated well to the suppression of ORF113 expression ( Figure 4D).…”

“…CBB23 has been widely adopted in rice breeding programs (Zhou et al, 2009;Huang et al, 2012), and several Xa23-containing hybrid rice varieties have been released to farmers in recent years. In previous work, we speculated that the Xa23-mediated resistance might depend on the transcriptional activation of Xa23 by AvrXa23, the function of which is highly conserved in all tested Xoo strains, and that Xa23 might be an executor type R gene (Wang et al, 2014a). However, the molecular nature of Xa23 remains to be elucidated.…”

“…For example, CBB23, a near isogenic line obtained through transfer of the Xa23 locus from the wild rice into the cultivated indica rice (O. sativa ssp. indica) JG30 (Zhang et al, 2001), displays the broadest resistance to Xoo strains isolated from different ricegrowing regions (Wang et al, 2014a). CBB23 has been widely adopted in rice breeding programs (Zhou et al, 2009;Huang et al, 2012), and several Xa23-containing hybrid rice varieties have been released to farmers in recent years.…”

Genome editing for crop improvement: Challenges and opportunities

“…NHEJ are used to study gene function and for trait development even for polyploidy plants (Curtin et al, 2013;Qi et al, 2013). Gene knockout using SSNs has been used to elucidate plant gene of important genes that could be used for crop improvement in diverse plant species, including several important crops, such as barley, soybean, maize, rice, wheat, and sorghum (Voytas and Gao, 2014;Curtin et al, 2013;Qi et al, 2013;Wendt et al, 2013;Gao et al, 2010;Lyznik et al, 1991;Yang et al, 2009;Shukla et al, 2009;Liang et al, 2013;Jiang et al, 2013;Wang et al, 2013;.…”