Bacterial blight (BB) caused by Xanthomonas oryzae pv. oryzae is one of the most devastating diseases
of rainfed lowland rice in Thailand. The gene Xa21 shows broad-spectrum resistance and has been
widely utilized to improve BB resistance in rice worldwide. However, Xa21 is not fully expressed in
the early stages of development (seedling stage). In this study, we attempted to improve the Thai jasmine
rice variety KDML105 to obtain non-age-related broad-spectrum resistance to BB. The Xa21 gene
and seedling resistance genes from rice variety IR1188 (a variety with non-age-related broad-spectrum
resistance to BB) were introgressed into KDML105 through three rounds of marker-assisted backcrossing
(MAB) and phenotypic selection. Sixty KDML105 backcross introgression lines (KBILs) carrying the Xa21
gene were successfully developed. They were used to evaluate seedling resistance against thirteen Xoo
strains. Three seedling resistance (SR) loci inherited from IR1188 were identified on rice chromosomes
1 (RM302-RM212), 8 (RM210-RM149) and 11 (RM287-RM224). The agronomic characters of the KBILs
were assessed by planting these lines in the paddy field at Kasetsart University in 2003. Phenotypic variation
was observed in the agronomic traits of these lines. Seven KBILs carrying the Xa21 and multiple
SR loci and also having a similar plant type to the original KDML105 were chosen for testing in multilocation
trials at research stations in rainfed lowland environments. The trials were conducted in 10
and 11 locations in the North and Northeast of Thailand, respectively, between 2005 and 2006. The yield,
agronomic traits, cooking quality and important diseases were examined and compared with those of the
original KML105. All of the 7 KBILs had a cooking quality profile (aroma, amylose content, gel consistency
and alkaline spreading value) and agronomic performance similar to the original KDML105. In 2007, four
KBILs were planted in farmers’ fields at 5 locations. All BILs and the original KDML105 did not differ significantly
in their agronomic performance. In this study, multiple loci for broad-spectrum seedling resistance
were identified from the KBIL population developed by the integrated marker-assisted and phenotypic
selection procedures (MAS and PS). These results facilitated the successful improvement of non-agerelated
broad-spectrum BB resistance in KDML105. The established non-age-related broad-spectrum
BB-resistant KDML105 is currently recommended by Kasetsart University for planting in farmers’ fields
where the crop is vulnerable to BB.