Wider resistance level was observed in combination of 4 resistance genes (Xa4, xa5, xa13, and Xa21) against Xoo strains (Kumar et al.
These studies concluded that combination of four genes (i.e., Xa4, xa5, xa13, Xa21) was having durable resistance against various pathogen strains (Nayak et al.
The inoculum preparation and inoculation methods were the same as those described earlier for Xa4. At the maximum tillering stage, each plant was inoculated with the above three strains of Xoo using the leaf clipping method (Kauffman et al., 1973).
Xa4 is clearly expressed against PXO61, in both IRBB4 and IR50, since they have almost the same lesion length, but the mean lesion length of IR50 is still shorter than that of IRBB4.
It is interesting to find that nontransformed IR50 showed a lesion length equal to that of the resistant reaction when inoculated with Race 10, as IR50 contains the endogenous Xa4 gene.
This may be due to the additive effect or pyramiding of the endogenous Xa4 gene along with the transformed Xa21 gene.