| dc.description.abstract | Arabidopsis BRI1-associated receptor kinase1 (BAK1) is a signaling partner of multiple receptors involved in plant immunity, growth and cell death control. BAK1 contains an extracellular leucine rich repeat (LRR) domain, transmembrane (TM) domain, juxta-membrane (JM) domain and kinase domain (K). Here we report the existence of at least one carboxyl-terminal BAK1 fragment (BAK1-C') consisting of TM, JM and K domains, originating from proteolytic cleavage. Production of BAK1-C' is largely reduced by brassinosteroid (BR)-induced BRI1-BAK interaction. An aspartic acid-to-alanine mutation at the site 287 (D287A) of BAK1 largely reduces BAK1 cleavage and BAK1D287A mutant is not able to activate multiple immune responses in Arabidopsis plants. Our data also indicate that the D287 site of BAK1 is required for its functions in BR signaling and cell death control. The D287A mutation of BAK1 does not affect BAK1 kinase activity, flagellin (flg22)-induced FLS2 (receptor for flg22)-BAK1 interaction or BR-induced BRI1 (receptor for BR)-BAK1 interaction. Taken together, our study suggests that the proteolytic cleavage of BAK1 may play roles in plant immunity, growth and cell death control.
The duration and intensity of immune responses must be tightly regulated to combat infections without causing deleterious side effects. The plant U-box E3 ubiquitin ligases PUB12/13 associates with FLS2 in a BAK1-dependent manner upon flg22 perception, and poly-ubiquitinates FLS2 for degradation, thereby down-regulating flagellin signaling. PUB13 consists of UND, U-box and ARM domains, and the ARM domain interacts with the FLS2/BAK1. Over-expression of the ARM domain of PUB13 supresses FLS2-PUB13 association and FLS2 degradation, indicating that ectopic expression of the ARM domain of PUB13 in planta generates dominant negative effects via blocking the PUB12/13-mediated ubiquitination activity. Similarly to the pub12pub13 mutant plants, the transgenic plants over-expressing the PUB13 ARM domain show enhanced immune responses compared to wild-type plants. Furthermore, pub12pub13 mutant plants and PUB13ARM transgenic plants are more sensitive to stress-induced senescence. Thus, the resemblance between PUB13ARM transgenic plants and pub12pub13 mutant plants provides evidences that the ectopic expression of the ARM domain of PUBs serves as an alternative method to dissect the overlapping functions of PUB genes. | en |
