|Oh, Man Ho|
Submitted to: Plant Signaling and Behavior
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/10/2009
Publication Date: 12/1/2009
Citation: Oh, M., Clouse, S.D., Huber, S.C. 2009. Tyrosine Phosphorylation in Brassinosteroid Signaling. Plant Signaling and Behavior. 4(12):77-81. Interpretive Summary: Brassinosteroids are plant growth hormones that control biomass accumulation and also tolerance to many stress conditions and hence are of relevance to agriculture. Brassinosteroid action starts with binding of the hormone to a receptor protein known as BRASSINOSTEROID INSENSITIVE 1 (BRI1) and numerous studies have identified the gene that encodes this protein as an intrinsic yield gene. Unfortunately, the molecular determinents that regulate the ability of BRI1 to regulate plant growth are not known. However, results of the present study suggest that a specific tyrosine residue on BRI1 plays a special role in regulating the activity of the receptor protein. Transgenic Arabidopsis plants expressing a mutant BRI1 protein where the naturally occurring tyrosine residue at position 831 was substituted with phenylalanine grow larger and accumulate more biomass than plants expressing the native protein. Alteration of the other tyrosine residues of the protein did not result in a similar effect. The results indicate that the tyrosine residue at position 831 specifically functions to regulate the growth-promoting activity of brassinosteroid hormones, and suggest that similar strategies may be effective to control brassinosteroid signaling in crop plants to increase yield and/or biomass accumulation.
Technical Abstract: Brassinosteroids (BRs) regulate plant growth and development through a complex signal transduction pathway involving BRASSINOSTEROID INSENSITIVE 1 (BRI1), which is the BR receptor, and its co-receptor BRI1-ASSOCIATED KINASE 1 (BAK1). Both proteins are classified as Ser/Thr protein kinases. Recently, we reported that recombinant cytoplasmic domains (CD) of BRI1 and BAK1 also autophosphorylate on tyrosine residues and thus are dual-specificity kinases1. Two sites of Tyr autophosphorylation were identified that appear to have different effects on BRI1 function. Tyr-831 in the juxtamembrane domain is not essential for kinase activity but has a regulatory role, with phosphorylation of Tyr-831 causing inhibition of growth and delay of flowering. In contrast, Tyr-956 is located in subdomain IV of the kinase domain and is essential for kinase activity, and we are speculating that the free hydroxyl group at this position is essential and thus phosphorylation of Tyr-956 would inhibit BRI1 kinase activity. Expression of BRI1(Y831F)-Flag in the weak allele bri1-5 rescued the dwarf phenotype but plants had rounder leaves, increased shoot biomass, and flowered earlier than plants expressing the BRI1(wild type)-Flag in the bri1-5 background. To further elaborate on earlier results, we present additional phenotypic analysis of transgenic Arabidopsis plants expressing BRI1(Y831F)-Flag or directed mutants of other Tyr residues within the kinase domain. The results highlight the unique role of Tyr-831 in regulation of BR signaling in vivo. Elucidating the molecular basis for increased biomass accumulation in plants expressing BRI1(Y831F)-Flag may have applications for agriculture.