Auxin–cytokinin interactions in the regulation of correlative inhibition in two-branched pea seedlings

A model system of 10–12 day-old, two-branched (2-B) pea (Pisum sativum L. cv. Adagumsky) seedlings was used to study the roles of endogenous auxin indole-3-acetic acid (IAA) and cytokinins (CKs) in the interaction between the shoots. The IAA export activity (IEA) from shoots was 2-fold higher in one-branched (1-B) plants with one shoot removed than in the 2-B plants, while tZ-type cytokinin contents in xylem sap were 4-fold greater in the 1-B plants than in 2-B plants. Exogenous 6-benzylaminopurine introduced into the vascular stream of one shoot enhanced its IEA. Therefore, xylem cytokinin appears to control both growth and IEA in branches. In the hypocotyls of 1-B and 2-B plants, IAA contents were equal in both cases, while the levels of tZ-type cytokinins were different. These data do not agree with the well-supported role of auxin in down-regulating CK content. The observed paradox may be explained by assuming that a steady-state IAA level in the hypocotyls is feedback regulated via xylem cytokinin, which controls the delivery of IAA from the shoots. As a result, the level of IAA in the hypocotyl is most likely maintained at a threshold below which a decrease in auxin content can switch on CK synthesis that will increase xylem cytokinin levels, thereby stabilizing the level of IAA in the hypocotyl. Therefore, our results suggest that correlative inhibition in the 2-B pea system is a function of an IAA/CK feedback loop, in which cytokinin essentially acts as a second messenger for IAA.