M. oryzae Avr gene pairs (Pita/AvrPita, Pik/Avr-Pik, Piz-t/AvrPiz-t, Pia/Avr-Pia,
and Pi-CO39/Avr1-CO39) are well studied for rice blast resistance. Most of the
23 cloned M. oryzae R genes are dominant NB-LRR (nucleotide-binding
(NB) domain, leucine-rich repeat (LRR)-containing receptors) genes (Chen et al.
2010).
A single Avr gene may be identified by one or more R-gene products or
conversely one R-gene may be involved in the recognition of multiple Avr proteins.
Pita/AvrPita and Piz-t/AvrPiz-t are two common and well-studied examples of
recognition of a single Avr gene by a single dominant R gene (Jia et al. 2000).
While Pik-, Pi5-, and Pia/Pi-CO39 require two NB-LRR-type R-gene including
Pik-1 and Pik-2, Pi5-1 and Pi5-2, a locus called Pia or Pi-CO39 consisting of
RGA4 and RGA5, respectively (Ashikawa et al. 2008; Cesari et al. 2013; Lee
et al. 2009; Okuyama et al. 2011). In contrast, identification of two Avr proteins
including Avr1-CO39 and Avr-Pia, which share no sequence similarity, by RGA5-A
is an example of recognition of two different Avr proteins by a single R-gene
product. A total of four atypical R genes (Pi21, Ptr, BSR-D1, and BSR-K1) have
been isolated from rice involved in the non-race-specific resistance (Zhao et al. 2018;
Zhou et al. 2018).
12.5 Rice Blast Resistance Breeding
The development of disease resistance cultivar has been proven to be the most
effective and economical method for disease control. There can be either complete
or true resistance or partial or field resistance in rice (Parlevliet 1979). While
complete resistance is race-specific, partial resistance is not. In addition, complete
resistance is controlled by a single dominant or recessive R gene and partial
resistance is controlled by Quantitative Trait Locus (QTLs), which implies more
general mechanisms and is thought to be more durable (Skamnioti and Gurr 2009).
Molecular marker technology offers the opportunity to improve the efficiency and
resolution of genetic analysis of resistance genes as well as to select the lines
harboring proper resistance genes for durable resistance breeding. Moreover, gene
and individual QTL pyramiding should be considered for durable resistance to blast
fungus. QTLs for the rice resistance against M. oryzae are co-localized with Pi loci or
with other QTLs involved in the rice resistance against other pathogens (Paterson
et al. 1991). Some SSR (simple sequence repeat markers) (RM168, RM8225,
RM1233, RM6836, RM5961, and RM413) have been found by Ashkani et al. that
could be used in MAS (marker-assisted selection) programs (Ashkani et al. 2011).
MAS is used for screening of selected populations to track introgression of resistance
genes Pib, Pik, Pii, Piz, and Pita (Jia et al. 2002). Also, it is possible to pyramid Pi-ta
with either of these major resistance genes to achieve broad-spectrum resistance in
the improved germplasm. Pyramiding three blast R genes, Pi1, Piz-5, and Pita-2, into
cultivars provides broad-spectrum resistance to many isolates of M. oryzae (Jia et al.
2002). It is mandatory to facilitate the discovery and transfer of new DNA markers to
breeders for the development of disease-resistant cultivars. In rice, GWAS
12 Unraveling the Molecular Mechanism of Magnaporthe oryzae Induced. . .
373
and Pi-CO39/Avr1-CO39) are well studied for rice blast resistance. Most of the
23 cloned M. oryzae R genes are dominant NB-LRR (nucleotide-binding
(NB) domain, leucine-rich repeat (LRR)-containing receptors) genes (Chen et al.
2010).
A single Avr gene may be identified by one or more R-gene products or
conversely one R-gene may be involved in the recognition of multiple Avr proteins.
Pita/AvrPita and Piz-t/AvrPiz-t are two common and well-studied examples of
recognition of a single Avr gene by a single dominant R gene (Jia et al. 2000).
While Pik-, Pi5-, and Pia/Pi-CO39 require two NB-LRR-type R-gene including
Pik-1 and Pik-2, Pi5-1 and Pi5-2, a locus called Pia or Pi-CO39 consisting of
RGA4 and RGA5, respectively (Ashikawa et al. 2008; Cesari et al. 2013; Lee
et al. 2009; Okuyama et al. 2011). In contrast, identification of two Avr proteins
including Avr1-CO39 and Avr-Pia, which share no sequence similarity, by RGA5-A
is an example of recognition of two different Avr proteins by a single R-gene
product. A total of four atypical R genes (Pi21, Ptr, BSR-D1, and BSR-K1) have
been isolated from rice involved in the non-race-specific resistance (Zhao et al. 2018;
Zhou et al. 2018).
12.5 Rice Blast Resistance Breeding
The development of disease resistance cultivar has been proven to be the most
effective and economical method for disease control. There can be either complete
or true resistance or partial or field resistance in rice (Parlevliet 1979). While
complete resistance is race-specific, partial resistance is not. In addition, complete
resistance is controlled by a single dominant or recessive R gene and partial
resistance is controlled by Quantitative Trait Locus (QTLs), which implies more
general mechanisms and is thought to be more durable (Skamnioti and Gurr 2009).
Molecular marker technology offers the opportunity to improve the efficiency and
resolution of genetic analysis of resistance genes as well as to select the lines
harboring proper resistance genes for durable resistance breeding. Moreover, gene
and individual QTL pyramiding should be considered for durable resistance to blast
fungus. QTLs for the rice resistance against M. oryzae are co-localized with Pi loci or
with other QTLs involved in the rice resistance against other pathogens (Paterson
et al. 1991). Some SSR (simple sequence repeat markers) (RM168, RM8225,
RM1233, RM6836, RM5961, and RM413) have been found by Ashkani et al. that
could be used in MAS (marker-assisted selection) programs (Ashkani et al. 2011).
MAS is used for screening of selected populations to track introgression of resistance
genes Pib, Pik, Pii, Piz, and Pita (Jia et al. 2002). Also, it is possible to pyramid Pi-ta
with either of these major resistance genes to achieve broad-spectrum resistance in
the improved germplasm. Pyramiding three blast R genes, Pi1, Piz-5, and Pita-2, into
cultivars provides broad-spectrum resistance to many isolates of M. oryzae (Jia et al.
2002). It is mandatory to facilitate the discovery and transfer of new DNA markers to
breeders for the development of disease-resistant cultivars. In rice, GWAS
12 Unraveling the Molecular Mechanism of Magnaporthe oryzae Induced. . .
373