The role of BRI1-associated kinase 1 (BAK1) in the regulation of plant innate immunity : functional and genetic characterization of BAK1 overexpression in Arabidopsis thaliana

The role of BRI1-ASSOCIATED KINASE 1 (BAK1) in the regulation of plant innate immunity: functional and genetic characterization of BAK1 overexpression in Arabidopsis thaliana
SUMMARY
BAK1 (BRI1-ASSOCIATED KINASE 1) is an intensively studied member of the large leucine-rich repeat (LRR)-receptor-like kinase (RLK) family in Arabidopsis. It was initially identified as interacting partner of the brassinosteroid receptor BRI1 (BRASSINOSTEROID INSENSITIVE 1), which perceives the plant hormone brassinolide and thereby regulates a wide set of developmental and physiological processes in plants. In addition, BAK1 has been discovered to play an important role in one aspect of the active defense of plants against pathogens, the so-called pattern-triggered immunity (PTI). This involves perception of conserved microbe-associated molecular patterns (MAMPs) by so called pattern recognition receptors (PRRs). Some of these PRRs interact with BAK1 immediately after MAMP recognition, and this promotes receptor phosphorylation and initiates PTI.
In this work BAK1 was overexpressed in Arabidopsis in order to better understand its role in innate immunity (Chapter 1). Surprisingly, constitutive overexpression of BAK1 led to stunted plant stature, leaf necrosis and premature death of the plant. Using an inducible system to express BAK1, it was revealed that this phenotype was probably due to the constitutive activation of defense responses triggered by the accumulation of BAK1. As a consequence, these plants displayed an enhanced resistance to the bacterial pathogen Pseudomonas syringae pv tomato DC3000. Likewise, overexpression of BAK1 homologs, SERK1 and SERK4, also induced constitutive activation of defense responses, supporting the idea of a redundant function for SERK proteins in innate immunity. Mutation of a defense related gene SOBIR1 (SUPPRESSOR OF BIR1-1) almost entirely reverted the BAK1 overexpression phenotype. The SOBIR1 protein kinase appears to be involved in PTI as well as in a second aspect of the plant’s active defense, effector-triggered immunity (ETI).
Since overexpression of BAK1 always produced putative truncated forms of the BAK1 protein, the possible connection between the accumulation of these protein fragments and the overexpression phenotype was studied in more detail (Chapter 2). Apparently, the extracellular domain anchored to the plasma membrane as well as the activity of the kinase domain both contribute to the BAK1 overexpression phenotypes.
Our study demonstrates that BAK1 over-accumulation causes a constitutive defense phenotype likely due to constitutive PTI activation. However we can not exclude that in addition ETI gets constitutively activated in these conditions. Moreover, the presence of the BAK1 extracellular domain as well as its kinase function appears to be crucial for its ability to induce defense responses.