**4. Implication of interactions between TALes and the corresponding host genes**

Due to the large reservoir of TALes in each strain of *Xoo* and the diverse roles of TALes in pathogenesis, the BB of rice represents an excellent plant/pathogen system for studying the biology of TALes. The apparent reason for the broad activity of *Xa27* and *Xa23* is the presence of the cognate TALes *avrXa27* and *avrXa23* in a large number of strains from southeast Asia, including Korea, China, Japan and the Philippines [37, 39]. On the other hand, the loss of *avrXa27*, *avrXa23*, or *avrXa10*, for that matter, does not appear to have an apparent fitness cost to the pathogen, and populations of *Xoo* may lose *avrXa27* if *Xa27* is widely deployed [37–39]. AvrXa7 is an important virulence factor for some strains of *Xoo*, and strains with AvrXa7 are incompatible on rice lines harboring the *Xa7*. In this case, loss of *avrXa7*, which is a major TALe for *OsSWEET14*, may result in strains that are weakly virulent or, essentially, nonpathogenic, if no other SWEET inducing TALes are present [43, 58]. A variety of other TALe genes are present in *Xoo* populations that can restore full virulence to strains missing *avrXa7* [59]. Evasion of *Xa7*-mediated resistance is possible by loss of the gene, rearrangement of the central repeats or recombination among different TALe genes [60, 61]. However, despite rapid adaptation of bacteria by genetic changes and gene flow, field studies in the Philippines indicated that deployment of *Xa7* was durable in test plots for more than 10 years [62]. Therefore, strains may have other limitations due to geographical location or rice genotype. Nevertheless, pyramiding broadly effective *R* genes with cognate TALes that are wide-spread in the pathogen populations should provide a degree of broad and durable resistance.

In the case of *xa13*, induction of the dominant allele *SWEET11* is mediated by the TALe PthXo1 [42]. However, strains of *Xoo* that solely rely on PthXo1 cannot induce *xa13* allele, and rice homozygous for *xa13* is symptomless. *xa13*-dependent recessive resistance is phenotypically and qualitatively different from resistance provided by the dominant *R* gene *Xa7* [42, 63]. Quantitatively, however, resistance mediated by *xa13* and *Xa7* are approximately equal with respect to bacterial growth and lesion length [42, 58, 64]. *Xa7* resistance is the result of the presence of the appropriate AvrXa7 in the pathogen and dominant, while *xa13* resistance is dependent on the absence of an effective virulence factor and recessive. The mechanism of XA7 mediated resistance is as yet unknown.

Type III effectors, in general, are hypothesized to interfere with host defense and defense signaling mechanisms. Strains of *Xoo* have other type III effectors, differing from TALes, and, therefore, not entirely dependent on TALes for suppression of host defenses [65]. *Xoo* strains lacking major TALes are still capable of causing water-soaking, if syringe inoculated, which is in contrast to type III secretion system (Hypersensitive reaction/pathogenicity or Hrp*<sup>−</sup>* ) mutants. Hrp<sup>−</sup> mutant strains are incapable of secreting any type III effectors, including TALes, and are virtually symptomless [66]. The mechanism by which SWEET transporters condition susceptibility is unknown. One hypothesis is that the transporters allow cells to leak sucrose, providing the pathogen with nutrients. SWEET function may interfere with normal plant defense functions or, possibly, allow transport of other nutrients or disease promoting compounds [41]. However, little empirical evidence for the nutrition model exists at present.

Sequencing of *Xoo* genomes has revealed the full complement of TALes is now known [17–23]. The individual TALe genes are distinguishable on the basis of the number of repeats in the central repetitive region and by polymorphisms within each repeat sequence, particularly, at the 12th and 13th codons. Strains of the Asian lineage contain upwards of 16–19 TALe genes in each genome [18]. The large numbers of TALe genes in these species may reflect the evolutionary investment in utilizing the TALes for virulence and are essential, to the ecological niche these bacteria occupy. The maintenance of a large repertoire of TALe genes may increase the frequency of recombination between, and diversity of TALecgenes within the pathogen population [60]. Pathogen may then adapt faster to the changing host genotypes as exemplified by the appearance of *pthXo5,* which avoids Xa7 recognition and appears to be a hybrid between *avrXa7* and *pthXo6* [61].

Not all TALE genes of *Xoo*, however, are just substrates for new major TALEs. Two other TALE genes from PXO99 strain of *Xoo*, in addition to *pthXo1*, contribute to virulence, known to elevate the expression of two host genes distinct from *SWEET11*. PthXo6 elevates the expression of *OsTFX1*, which contributes to approximately 35% of the disease [67]. Many strains induce *OsTFX1*. The gene *pthXo7* of PXO99 elevates the expression of *OsTFIIAγ1* and would appear to be an adaption to host genotypes containing the *xa5* allele of *TFIIAγ5* [67]. However, introduction of *pthXo7* to other strains does not restore full virulence on *xa5*/*xa5* plants and may provide only an incremental fitness benefit [67]. All Asian strains also carry a set of truncated TALes, the inhibitory or iTALes, which function to suppress *Xa1* mediated resistance [32].

### **4.1 Executor** *R* **genes and super promoters**

*Xa10*, *Xa23* and *Xa27* are representatives of the new class of E genes, so-named because the induction of these genes executes a response of programed cell death (PCD) in the host. *Xa10* induced PCD in plant species rice and *N. benthamiana,* and mammalian HeLa cells [38]. No cognate *S* genes for AvrXa10, AvrXa23, or AvrXa27 in compatible host cultivars have been reported, though the presence of AvrXa27 and AvrXa23 in many extant strains of *Xoo* may portend either a defeated function or an unknown cryptic function in *S* gene expression. Nonetheless, *E* genes hold great potential for broad and durable resistance in rice against extant *Xoo* population. A super promoter consisting of multiple EBEs, corresponding to specific TALes in extant population of *Xoo*, have been constructed (**Figure 1**). [68–70]. Addition of multiple EBEs to a pathogen strain specific rice BB resistance gene makes it effective against additional strains of *Xoo*. The EBEs of TALes PthXo1, PthXo6 and Tal9a when conjugated to E gene *Xa27*, showed resistance against PXO99 and a derivative strain lacking *AvrXa27* [68]. A similar scenario was

**115**

before deployment.

**Figure 1.**

**4.2 Targeted genome regulation and editing**

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

accomplished using E gene *Xa10* [69]. The study suggested that broad-spectrum and potentially durable resistance is possible by stable integration of an E gene engineered in a way to respond to multiple TALes from different strains or even different pathogens. Design of a super promoter, however, needs to be done carefully. Risk that an added EBE might coincidently contain a *cis* regulatory element could induce the E gene expression in response to particular stimuli and cause cell death without challenge by TALes. Amended promoters should be tested thoroughly

*(B) five types of TALe interactions affecting outcome of Xoo and rice interaction.*

*Xoo TALe-dependent resistance and susceptibility in BB of rice. (A) Schematic of typical TALe from Xoo and* 

Central to TALe function is the discovery of the DNA recognition cipher of TALEs [71, 72]. The central domain of a TALe, also known as binding domain, consists of variable number of tandem repeats, each consisting 33–35 amino acid residues. The 12th and 13th amino acid residues (known as repeat variable di-residues, RVDs)

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

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

### **Figure 1.**

*Protecting Rice Grains in the Post-Genomic Era*

nutrition model exists at present.

mediated resistance [32].

**4.1 Executor** *R* **genes and super promoters**

tion system (Hypersensitive reaction/pathogenicity or Hrp*<sup>−</sup>*

tion and appears to be a hybrid between *avrXa7* and *pthXo6* [61].

Type III effectors, in general, are hypothesized to interfere with host defense and defense signaling mechanisms. Strains of *Xoo* have other type III effectors, differing from TALes, and, therefore, not entirely dependent on TALes for suppression of host defenses [65]. *Xoo* strains lacking major TALes are still capable of causing water-soaking, if syringe inoculated, which is in contrast to type III secre-

strains are incapable of secreting any type III effectors, including TALes, and are virtually symptomless [66]. The mechanism by which SWEET transporters condition susceptibility is unknown. One hypothesis is that the transporters allow cells to leak sucrose, providing the pathogen with nutrients. SWEET function may interfere with normal plant defense functions or, possibly, allow transport of other nutrients or disease promoting compounds [41]. However, little empirical evidence for the

Sequencing of *Xoo* genomes has revealed the full complement of TALes is now known [17–23]. The individual TALe genes are distinguishable on the basis of the number of repeats in the central repetitive region and by polymorphisms within each repeat sequence, particularly, at the 12th and 13th codons. Strains of the Asian lineage contain upwards of 16–19 TALe genes in each genome [18]. The large numbers of TALe genes in these species may reflect the evolutionary investment in utilizing the TALes for virulence and are essential, to the ecological niche these bacteria occupy. The maintenance of a large repertoire of TALe genes may increase the frequency of recombination between, and diversity of TALecgenes within the pathogen population [60]. Pathogen may then adapt faster to the changing host genotypes as exemplified by the appearance of *pthXo5,* which avoids Xa7 recogni-

Not all TALE genes of *Xoo*, however, are just substrates for new major TALEs. Two other TALE genes from PXO99 strain of *Xoo*, in addition to *pthXo1*, contribute to virulence, known to elevate the expression of two host genes distinct from *SWEET11*. PthXo6 elevates the expression of *OsTFX1*, which contributes to approximately 35% of the disease [67]. Many strains induce *OsTFX1*. The gene *pthXo7* of PXO99 elevates the expression of *OsTFIIAγ1* and would appear to be an adaption to host genotypes containing the *xa5* allele of *TFIIAγ5* [67]. However, introduction of *pthXo7* to other strains does not restore full virulence on *xa5*/*xa5* plants and may provide only an incremental fitness benefit [67]. All Asian strains also carry a set of truncated TALes, the inhibitory or iTALes, which function to suppress *Xa1*-

*Xa10*, *Xa23* and *Xa27* are representatives of the new class of E genes, so-named because the induction of these genes executes a response of programed cell death (PCD) in the host. *Xa10* induced PCD in plant species rice and *N. benthamiana,* and mammalian HeLa cells [38]. No cognate *S* genes for AvrXa10, AvrXa23, or AvrXa27 in compatible host cultivars have been reported, though the presence of AvrXa27 and AvrXa23 in many extant strains of *Xoo* may portend either a defeated function or an unknown cryptic function in *S* gene expression. Nonetheless, *E* genes hold great potential for broad and durable resistance in rice against extant *Xoo* population. A super promoter consisting of multiple EBEs, corresponding to specific TALes in extant population of *Xoo*, have been constructed (**Figure 1**). [68–70]. Addition of multiple EBEs to a pathogen strain specific rice BB resistance gene makes it effective against additional strains of *Xoo*. The EBEs of TALes PthXo1, PthXo6 and Tal9a when conjugated to E gene *Xa27*, showed resistance against PXO99 and a derivative strain lacking *AvrXa27* [68]. A similar scenario was

) mutants. Hrp<sup>−</sup> mutant

**114**

*Xoo TALe-dependent resistance and susceptibility in BB of rice. (A) Schematic of typical TALe from Xoo and (B) five types of TALe interactions affecting outcome of Xoo and rice interaction.*

accomplished using E gene *Xa10* [69]. The study suggested that broad-spectrum and potentially durable resistance is possible by stable integration of an E gene engineered in a way to respond to multiple TALes from different strains or even different pathogens. Design of a super promoter, however, needs to be done carefully. Risk that an added EBE might coincidently contain a *cis* regulatory element could induce the E gene expression in response to particular stimuli and cause cell death without challenge by TALes. Amended promoters should be tested thoroughly before deployment.
