Tyrosine phosphorylation in brassinosteroid signaling

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 kinases.¹ 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 site-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.Key words: brassinosteroids, LRR-RLK, autophosphorylation, tyrosine phosphorylation, signal transduction