The C4 protein of Beet curly top virus (BCTV-B [US:Log:76]) induces hyperplasia in infected phloem tissue and tumorigenic growths in transgenic plants. The protein offers an excellent model for studying cell cycle control, cell differentiation and plant development. Expression of the C4 transgene, under an inducible promoter, in Arabidopsis thaliana plants (line IPC4-28) lead to extensive cell division in all tissues types examined, and radically altered tissue layer organization and organ development. Induced seedlings failed to develop shoot and root meristems, vascular tissue, true leaves and lateral roots. Exogenous application of brassinosteroid and abscisic acid weakly rescued the C4-induced phenotype, while induced seedlings were hypersensitive to gibberellic acid and kinetin.The C4 protein interacts in planta with three closely related members of the Arabidopsis SHAGGY-like protein kinase (AtSK21, AtSK22 and AtSK23) family, which are negative regulators of brassinosteroid (BR) hormone signaling. AtSK21 is known to hyperphosphorylate BES1 and BZR1 transcription factors, thereby inhibiting their function. The expression of C4 in IPC4-28 plants resulted in dephosphorylation of the BES1 transcription factor and changes in the expression levels of BES1 and BZR1 target genes. These changes were similar to those that occur when AtSK21 is inactivated by BR hormone treatment. Using several approaches a putative phosphorylation site (Ser at residue 49) on C4 is shown to be required for the C4/AtSK interaction. Mutation of Ser49 to Ala abolishes the C4/AtSK interaction, loss of the C4-expression phenotype, and failure to induce changes in the BR signaling pathway. Substitution of Ser49 by Thr restores both the C4/AtSK interaction and the effects of C4 on plant development and BR signaling. An active kinase is also required for the interaction, as AtSK kinase dead mutants fail to interact with C4. Taken together this indicates that C4 is a substrate for AtSK kinase activity and functions, at least in part, through disruption of the BR signaling pathway.