A Family of Avirulence Genes from Xanthomonas Oryzae pv. Oryzae is Involved in Resistant Interactions in Rice

Abstract

Xanthomonas oryzae pv. oryzae, the bacterial blight pathogen of rice, contains at least three avirulence genes (avrxa5, avrXa7, and avrXal0) that are related to avrBs3, an avirulence gene from the pepper pathogen X. campestris pv. vesicatoria. The gene avrXal0, which specifies the resistant phenotype on rice cultivars with the bacterial blight resistance gene Xa-10, like avrBs3, contains a 102-bp repeated sequence, although avrXal0 contains fewer copies than avrBs3 (15.5 vs. 17.5). Each 102-bp repeat contains a six-nucleotide variable region; the order of the repeats within avrXal0 is distinct from avrBs3. The number of repeat copies in avrXa7 was estimated at 25. The physiological events associated with resistant interactions were investigated using the cloned avirulence genes and near-isogenic rice cultivars with single bacterial blight resistance genes. Resistance in rice to X. oryzae pv. oryzae is characterized by a light-dependent increase in the extracellular activity of three peroxidases, lignin deposition, and a decrease in the rate of bacterial multiplication. The timing of these events is dependent on the specific avirulence gene-resistance gene interaction, that is, the responses associated with resistance occur later in interactions involving avrxa5-xa5 and avrXa7-Xa7 than those between avrXal0 and Xa-10.