Abstract
Rice cultivar ‘Phitsanulok 2’ (‘PSL2’) is popularly grown in lower-northern Thailand, because it has good agricultural traits with high-quality and high-yield grains. However, one significant limitation of rice ‘PSL2’ is its susceptibility to bacterial blight (BB) disease caused by bacteria <i>Xanthomonas oryzae</i> pv. <i>oryzae (Xoo)</i>, leading to considerable yield losses. Effort approaches have been used to enhance the BB resistance of rice ‘PSL2’ by introducing the resistance gene <i>Xa21</i> and using salicylic acid (SA). Thus, the objective of this study was to determine the effects of SA application on BB disease reduction in rice cultivars (‘PSL2’, ‘IRBB21’, and backcrossed lines ‘PSL2-<i>Xa21’</i> in BC<sub>4</sub>F<sub>6</sub>) after<i> Xoo</i>16PK002 inoculation, compared to a mock treatment control. The results showed that the SA-pretreated rice (2mM) exhibited significantly induced upregulation of the <i>Xa21</i> gene expression before the <i>Xoo</i>16PK002-infected rice cultivars ‘IRBB21’ and ‘PSL2-<i>Xa21</i>-BC<sub>4</sub>F<sub>6</sub>’. Additionally, the SA-pretreated rice ‘PSL2-<i>Xa21</i>-BC<sub>4</sub>F<sub>6</sub>’ demonstrated significant suppression of <i>Xoo</i>16PK002 proliferation (from 1.4 to 2.4-fold reduction), compared to the H<sub>2</sub>O pretreatment. Similarly, the SA-pretreated rice ‘PSL2-<i>Xa21</i>-BC<sub>4</sub>F<sub>6</sub>’ exhibited increased resistance against BB disease, i.e. reduced lesion lengths (10.0-10.9 cm at 14 DAI and 24.6-28.0 cm at 21 DAI), compared to the water pretreatment (16.8-18.0 cm at 14 DAI and 34.9-35.1 cm at 21 DAI). In summary, the application of the SA-pretreatment in the rice introgression lines ‘PSL2-<i>Xa21</i>-BC<sub>4</sub>F<sub>6</sub>’ resulted in improved defense mechanisms against bacteria <i>Xoo</i>16PK002 through reduced appearance of symptoms and inhibited <i>Xoo</i> multiplication.