Phenotypic Broad Spectrum of Bacterial Blight Disease Resistance from Thai Indigenous Upland Rice Germplasms Implies Novel Genetic Resource for Breeding Program

Abstract

Bacterial blight (BB) disease, caused by Xanthomonas oryzae pv. oryzae (Xoo), is one of the most devastating diseases of rice worldwide. Breeding for BB resistance has been utilized to overcome this constraint of rice production; however, limited genetic resources of BB resistance or non-desirable genetic linkage between BB disease resistance and agronomic traits have become major obstacles. Interestingly, indigenous upland rice cultivars cultivated across Thailand are considered to be novel genetic resources of BB resistance for rice cultivar improvement through breeding programs. In this study, we screened for BB disease resistance among 256 indigenous upland rice cultivars using individual inoculation of two virulent Xoo isolates; NY1-1 and MS1-2, under greenhouse conditions. The results showed that 19 indigenous rice cultivars demonstrated BB disease resistance abilities after inoculation. These 19 upland rice cultivars were further examined for broad-spectrum resistance (BSR) performance through five individual Xoo isolate inoculations, under greenhouse conditions. Moreover, a mixed five Xoo isolate inoculation, including NB7-8, CM3-1, CN2-1, MS1-2, and NY1-1, was conducted to assess the BSR for BB resistance of those 19 cultivars under field conditions. Simultaneously, independent plants of the 19 varieties were grown without inoculation in the field to observe the disease reactions from the natural infection caused by local Xoo isolates. The results of the three experiments herein showed that five indigenous upland rice cultivars—ULR024, ULR029, ULR172, ULR207, and ULR356—consistently expressed 100% BSR to BB disease, as well as the resistance check varieties, IRBB5. This further illustrated that ULR024, ULR029, ULR172, ULR207, and ULR356 upland rice cultivars were phenotypically resistant to five Xoo isolates, within those (a) inoculated with five individual isolates under greenhouse conditions or (b) inoculation with five mixed isolates under field conditions. Moreover, the ULR024, ULR029, ULR172, ULR207, and ULR356 revealed BB disease-resistance abilities to natural infection. These results present novel genetic resources from indigenous upland rice cultivars in further breeding program of BB resistance in varied rice cultivars.