The three factors WRKY1, WRKY2, and WRKY3 are highly homologous to each other and Xb10. Phylogenetic analysis indicates that together they comprise a family (referred to as IIa) of WRKY's that contains an N-terminal coiled-coil domain or leucine zipper with high conservation of domains. In Arabidopsis at least one of the group IIa WRKY��s (WRKY18) has been shown to function as a transcription factor in developmentally regulated disease resistance (resistant proteins of this type are only found in adult plants) (Eulgem T. et al, 2000). Xa21 has also been shown to be developmentally regulated, further supporting group IIa��s role in the STP.
Another receptor kinase called FLS-2 also requires WRKY��s (a separate group named wrky22/29) for defense gene induction (Asai et al., 2002). Although FLS2 requires WRKY's to function, it has not be shown to interact with them. This fact allows group IIa (Xb10, WRKY1, WRKY2, and WRKY3) to be the first rice WRKY-receptor interaction characterized. Moreover, each group of WRKY��s may represent a distinct signaling pathway and the WRKY��s within each group may have redundant functions.
Transcription factors in rice resistance are generally known to homodimerize (one factor activating one DNA segment.) However, Xb10 interacts with other WRKY factors in group IIa. Although heterodimerization occurs in other systems, this is the first time that it has been discovered amongst rice WRKY factors. Interacting transcription factors may be advantageous in the STP. Assuming Xb10 acts alone to activate defense genes in the Xa21avr pathway, it would have to bind to all the promoter sites of defense genes to activate a multitude of responses. Forming transcription factor complexes increases the efficiency of the cascade reaction and allows more proteins to act as activators. Different factors can bind to numerous defense genes during or after interaction with Xb10.