Exogenous brassinosteroids activate cytokinin signalling pathway gene expression in transgenic Arabidopsis thaliana

Transgenic Arabidopsis thaliana plants carrying the GUS reporter gene fused to the promoter of the gene of primary response to cytokinins (CKs), ARR5, were used to estimate the influence of several brassinosteroids (BRs): brassinolide (BL), epibrassinolide (EBL), homobrassinolide (HBL), and 6-o-carboxymethyloxohomocastasterone (CHC) on the expression of CK signalling genes. BRs tested differed in their ability to activate the ARR5 gene promoter in 4-day-old seedlings and 3-week-old plants. BL caused the most prominent effect, yet it was considerably less than that of 6-benzylaminopurine (BA). An increase in GUS activity was observed in both dark and light conditions; however, the rate of elevation was higher in dark conditions. The activation of the P (ARR5) :GUS fusion was accompanied by a moderate induction of the P (AHK) :GUS constructs, in which the reporter GUS gene was fused to the promoter of one of the CK receptor histidine kinases. The effects of BL on the AHK gene promoters were organ specific and correlated with the ability of a particular AHK gene to respond to BA treatment. BL activated the AHK3 promoter in 4-day-old seedlings and in shoots and roots of 3-week-old plants without any effect in detached leaves. The AHK2 gene promoter was activated by BA and BL only in seedlings, whereas the AHK4 gene promoter was activated only in roots. BL treatment caused the coordinate elevation of the CK levels in leaves to the same degree as the activation of the P (AHK) :GUS construct, suggesting that the accumulation of CKs was the reason for the activation by BRs of the CK signalling genes. The data obtained provide the evidence for the involvement of BRs in the regulation of the genes of the CK signalling pathway through an increase in the CK levels. However, the exact molecular mechanisms underlying BR-induced elevation of the CK content are unclear and warrant identification in the future.