Xylem-Translocated Cytokinin: Metabolism and Function

Abstract

The concept that cytokinin synthesized in the root moves via the xylem to the shoot to regulate some aspects of development, and especially leaf senescence, is often proposed [2]. However, studies of the distribution and metabolism of radioactive cytokinins supplied to the xylem or root have been invariably inadequate for one or more of the following reasons: (1) cytokinins endogenous to the xylem of the species under study were not supplied and often synthetic (unnatural) cytokinins were used; (2) unphysiological and even toxic levels (e.g. mM) were given; (3) the radioactive metabolites in the shoot were not identified and attempts to do so were based on inadequately prepared extracts, and employed crude fractionation procedures and chromatography without appropriate markers. A study of the metabolism of xylem cytokinins in blue lupin, which avoids these problems, has been reported [2] and further results are described below. A similar study, in relation to sequential leaf senescence of tobacco, is also outlined. In order to evaluate the role of xylem cytokinins in shoot development, it is necessary to develop more natural methods which would allow manipulation of the endogenous cytokinin level in the transpiration stream. This has been achieved using Rhizobium mutants and cytokinin levels have been related to shoot development as discussed below.