**5. Prospects for engineered broad and durable resistance in rice to BB**

Traditional resistance breeding has identified many useful *R* genes and introgressed the genes into elite cultivars. Further, development of molecular markers allows the pyramiding of multiple genes into single lines. The development of designer TALENs and CRISPR-Cas genome editing brings greater flexibility and rapidity to the development of resistant germplasm. A continuous provision of novel *R* genes in breeding programs is possible. Of course, the adoption and utility of different approaches is dependent on the regulatory climate. Introduction of novel or alien genes may be prohibitive in the foreseeable future. Classification of genome editing techniques will also vary depending on the individual country. In the rice system, our understanding allows numerous approaches for the enhancement of resistance beyond classical breeding. TALe biology, specifically, can be exploited (**Figure 2**). Least intrusive is targeted genome editing of *S* genes. *OsSWEET14* is targeted by unrelated TALEs, AvrXa7, PthXo3, Tal5 and TalC from different *Xoo* strains and which in some cases overlap their EBEs [43, 45, 46]. *OsSWEET14* was made unresponsive to TALEs AvrXa7 and Tal5, when their respective EBEs were mutated using TALENs in otherwise susceptible rice cv. Kitake [85, 86]. Thus, recessive resistance obtained by the genome editing of *OsSWEET14* is expected to be broad and contribute to durability given the apparent few major TALes in the extant population. Future efforts will be to target all EBE/*S* gene combinations in single elite lines. Fusion of EBEs to a variety of R and E genes has

### **Figure 2.**

*Super promoters: pyramiding of EBEs of multiple TALEs upstream of an E gene for broad and durable resistance. Subscript of each EBE corresponds to the respective TALEs. Blocks under each EBE represent the respective TALes with blunt ends as their N termini, and arrowheads as their C-termini flanking the binding repeats in center.*

**117**

**Author details**

Tariq Mahmood1

United States

provided the original work is properly cited.

and Frank F. White2

\*Address all correspondence to: ffwhite@ufl.edu

*Disease Resistance and Susceptibility Genes to Bacterial Blight of Rice*

gene can be broadened by linkage to general inducible defense genes

been demonstrated to provide resistance [68, 87]. The functional specificity of an E

[69, 88]. Approaches are not limited to TALe-associated responses. The RLK immunity receptor EFR from *Arabidopsis* [89, 90], as well as XA21/EFR fusion proteins function in rice [91]. Thus, the sky is the limit for the engineering of broad and

*DOI: http://dx.doi.org/10.5772/intechopen.86126*

durable resistance in rice to BB.

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

\*

1 Department of Agriculture, Hazara University Mansehra, Pakistan

2 Department of Plant Pathology, University of Florida, Gainesville, FL,

*Disease Resistance and Susceptibility Genes to Bacterial Blight of Rice DOI: http://dx.doi.org/10.5772/intechopen.86126*

*Protecting Rice Grains in the Post-Genomic Era*

editing of rice genes [82–84].

of each repeat preferentially binds to the respective nucleotides in the EBEs of target gene, such that HD, NG, NI and NN bind to C, T, A, and G, respectively in the effector binding elements (EBEs) of the promoter of a target gene [71–73]. The TALe recognition code allowed custom-engineer of DNA binding domains, also called designer TALes (dTALes), with novel specificity to the user-chosen DNA sequences [74–76]. dTALes provide a useful tool box to transiently activate host genes of interest for their functional analysis and assess the associated effect on host phenotype and physiology during rice-*Xoo* interaction. TALENs are fusions between dTALes and the nuclease domain of restriction enzyme FokI [77–80]. Other C-terminal domains have also been used [81]. Target site recognition and TALEN dimerization triggers a double-strand break (DSB) and generates small random insertions or deletions at the cleavage site, resulting in an edited sequence. CRISPR-Cas editing approaches have circumvented the need to construct dTALes and achieved wide general use, including

**5. Prospects for engineered broad and durable resistance in rice to BB**

*Super promoters: pyramiding of EBEs of multiple TALEs upstream of an E gene for broad and durable resistance. Subscript of each EBE corresponds to the respective TALEs. Blocks under each EBE represent the respective TALes with blunt ends as their N termini, and arrowheads as their C-termini flanking the binding* 

Traditional resistance breeding has identified many useful *R* genes and introgressed the genes into elite cultivars. Further, development of molecular markers allows the pyramiding of multiple genes into single lines. The development of designer TALENs and CRISPR-Cas genome editing brings greater flexibility and rapidity to the development of resistant germplasm. A continuous provision of novel *R* genes in breeding programs is possible. Of course, the adoption and utility of different approaches is dependent on the regulatory climate. Introduction of novel or alien genes may be prohibitive in the foreseeable future. Classification of genome editing techniques will also vary depending on the individual country. In the rice system, our understanding allows numerous approaches for the enhancement of resistance beyond classical breeding. TALe biology, specifically, can be exploited (**Figure 2**). Least intrusive is targeted genome editing of *S* genes. *OsSWEET14* is targeted by unrelated TALEs, AvrXa7, PthXo3, Tal5 and TalC from different *Xoo* strains and which in some cases overlap their EBEs [43, 45, 46]. *OsSWEET14* was made unresponsive to TALEs AvrXa7 and Tal5, when their respective EBEs were mutated using TALENs in otherwise susceptible rice cv. Kitake [85, 86]. Thus, recessive resistance obtained by the genome editing of *OsSWEET14* is expected to be broad and contribute to durability given the apparent few major TALes in the extant population. Future efforts will be to target all EBE/*S* gene combinations in single elite lines. Fusion of EBEs to a variety of R and E genes has

**116**

**Figure 2.**

*repeats in center.*

been demonstrated to provide resistance [68, 87]. The functional specificity of an E gene can be broadened by linkage to general inducible defense genes [69, 88]. Approaches are not limited to TALe-associated responses. The RLK immunity receptor EFR from *Arabidopsis* [89, 90], as well as XA21/EFR fusion proteins function in rice [91]. Thus, the sky is the limit for the engineering of broad and durable resistance in rice to BB.
