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Title: Tyrosine-610 in the receptor kinase BAK1 does not play a major role in brassinosteroid signaling or innate immunity

item SINGH, VIJAYATA - University Of Illinois
item PERRAKI, ARTEMIS - Sainsbury Laboratory
item Kim, Sang Yeol
item SHRIVASTAVA, STUTI - University Of Illinois
item LEE, JAE HOON - University Of Illinois
item ZHAO, YOUFU - University Of Illinois
item SCHWESSINGER, BENJAMIN - Sainsbury Laboratory
item MARSHALL-COLON, AMY - University Of Illinois
item ZIPFEL, CYRIL - Sainsbury Laboratory
item Huber, Steven

Submitted to: Frontiers in Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/6/2017
Publication Date: 7/6/2017
Citation: Singh, V., Perraki, A., Kim, S., Shrivastava, S., Lee, J., Zhao, Y., Schwessinger, B., Marshall-Colon, A., Zipfel, C., Huber, S.C. 2017. Tyrosine-610 in the receptor kinase BAK1 does not play a major role in brassinosteroid signaling or innate immunity. Frontiers in Plant Physiology. doi: 10.3389/fpls.2017.01273.

Interpretive Summary: Receptor kinases transmit information from the outside of the cell to the inside, allowing an organism to respond to a range of stimuli from hormone signals to invading pathogens. In plants, active signaling complexes form when two different receptor kinases physically interact, with the member primarily binding the signal molecule referred to as the receptor kinase, and the partner referred to as the co-receptor kinase. When the receptor kinase and co-receptor kinase come together in the presence of the signaling molecule, their protein kinase domains become activated by a process involving phosphorylation of serine, threonine, or tyrosine residues. One of the best studied plant co-receptors is BAK1, which interacts with a variety of receptor kinases and thereby participates in a range of plant responses including growth induced by the brassinosteroid hormones. Previous studies had identified a specific tyrosine residue on BAK1 (Tyr-610) as a site of phosphorylation and reported that this modification was essential for the function of BAK1. However, in follow-up studies with newly created and properly genotyped transgenic plants, we now demonstrate that Tyr-610 is not essential for either brassinosteroid-mediated growth or innate immunity. The possibility that phosphorylation of Tyr-610 is required for some other functional role remains a distinct possibility, but these results correct the literature with respect to two major functions of BAK1.

Technical Abstract: The plasma membrane-localized BRI1-ASSOCIATED KINASE1 (BAK1) functions as a co-receptor with several receptor kinases including the brassinosteroid (BR) receptor BRASSINOSTEROID-INSENSITIVE 1 (BRI1), which is involved in growth, and the receptors for bacterial flagellin and EF-Tu, FLAGELLIN-SENSING 2 (FLS2) and EF-TU RECEPTOR (EFR), respectively, which are involved in immunity. BAK1 is a dual specific protein kinase that can autophosphorylate on serine, threonine and tyrosine residues. It was previously reported that phosphorylation of Tyr-610 in the carboxy-terminal domain of BAK1 is required for its function in BR signaling and immunity. However, the functional role of Tyr-610 in vivo has recently come under scrutiny. Therefore, we have generated new BAK1(Y610F) transgenic plants for functional studies. We first produced transgenic Arabidopsis expressing BAK1 (Y610F)-Flag in the homozygous bak1-4 bkk1-1 double null background. In a complementary approach, we expressed untagged BAK1 and BAK1(Y610F) in the bak1-4 null mutant. Both BAK1(Y610F) transgenic lines had no obvious growth phenotype compared to wild-type BAK1 expressed in the same background. In addition, the BAK1(Y610F)-Flag plants responded similarly to plants expressing BAK1-Flag in terms of brassinolide (BL) inhibition of root elongation, and there were only minor changes in gene expression between the two transgenic lines as monitored by microarray analysis and real-time PCR. In terms of plant immunity, there were no significant differences between plants expressing BAK1(Y610F)-Flag and BAK1-Flag in the growth of the non-pathogenic hrpA mutant of Pseudomonas syringae pv. tomato DC3000. Furthermore, untagged BAK1(Y610F) transgenic plants were as responsive as plants expressing BAK1 (in the bak1-4 background) and wild-type Col-0 plants towards treatment with the EF-Tu- and flagellin-derived peptide epitopes elf18- and flg22, respectively, as measured by reactive oxygen species (ROS) production, mitogen-activated protein kinase (MAPK) activation, and seedling growth inhibition. Together, these new results demonstrate that Tyr-610 does not play a role in either BR or immune signaling.