Evaluation of bacterial blight resistance reaction

Bacterial blight (BB) is a critical disease of rice caused by the bacterial pathogen Xanthomonas oryzae pv. oryzae (Xoo) resulting in heavy yield losses. Application of host resistance is the most effective approach to solve this problem. To date, 27 major resistance genes (R genes) have been identified (Table 1) and utilized for breeding of resistant varieties. However, it is highly probable that the resistance of cultivars having only a single major R gene would easily be broken down by the attack of diverse and rapidly evolving pathotypes of pathogenic bacteria (over 30 pathotypes have been reported). To overcome such problems, discovery of new wider-spectrum resistance genes and understanding the physiology of infection and the defense response at the molecular level are the most important and urgent issues. The availability of diverse genetic resources is essential in these studies. IRRI (international Rice Research Institute) performed a large screening of the resistance genes in wild rice germplasm (Heinrichs et al. 1980). As one of the second term activities, we have evaluated the BB resistance reaction among the wild rice accessions collected in the first term NBRP-Rice Resources and are aiming to identify new source of resistance genes.

Table1 Genes conferring resistance to the bacterial blight pathogen Xanthomonas oryzae pv.oryzae.

Gene Chromosome*1 Inheritance*2 Source*3
Xa1 4 D Kogyoku
Xa2 4 D Tetep
Xa3 11 D Wase Aikoku 3
Xa4 11 D TKM 6
xa5 4 r Aus Boro lines (e.g.DZ192)
Xa7 6 D DV85
xa8 7 r PI231129
Xa10 11 D Cas 209
Xa11 ND D IR8, IR944
Xa12 4 D Kogyoku
xa13 8 r BJ1 (Aus Boro)
Xa14 4 D TN1
xa15 ND r M41, a Harebare mutant line
Xa16 ND D Tetep
Xa17 ND D Asominori
Xa18 ND D IR24, Toyonishiki
xa19 ND r XM5
xa20 ND r XM6
Xa21 11 D O. longistaminata
Xa22 11 D Zhachanglong
Xa23 11 D O. rufipogon
xa24 ND r DV86, DV85, Aus 295
Xa25(a) 4 D HX-3, a somaclonal mutant of Minghui 63
Xa25(b) 12 D Minghui 63
Xa26 11 D Minghui 63
Xa27 6 SD O. minuta
xa28 ND r Lota Sail
Xa29(t) 1 D O. officinalis

*1: ND ,not determined.
*2: D,dominant; r,recessive; SD,semidominant.
*3: Oryza sativa cultivar (roman type),or Oryza species(italics).