CYTOKININ METABOLISM AND SIGNALING IN THE SOYBEAN PLANT

Substantial evidence has indicated that cytokinins (CKs) inhibit senescence of a variety of plant organs, especially leaves, but how, or even whether, CK operates as an antisenescence signal in the intact plant needed to be determined. Because soybean has a number of advantageous features, we chose to analyse CK regulation of leaf senescence in that species. Toward the end of podfill, the leaves become visibly yellow and drop off (monocarpic senescence) leading to the death of the plant. This process appears to be mainly under the control of the pods, specifically the seeds, and therefore also needs to be correlated with pod development. Supplying CK solutions in lieu of the roots in explants (a leaf with the associated pod[s] and subtending stem segment) indicates that the roots are the main source of CKs for the leaves. The main conduit from the roots to the leaves is the xylem, and the dominant CKs of the xylem sap are zeatin riboside and dihydrozeatin riboside. These are also the most active in preventing leaf senescence. Early in pod development, before seed fill, the CK flux through xylem drops markedly and this decline is reflected in a decrease in foliar CK activity. This decrease in xylem flux is necessary but not sufficient to cause leaf senescence; however, if the CK flux is maintained in explants, it will largely override the senescence-inducing influence of the pods. Depodding of soybean plants at an early stage prevents the leaf yellowing and abscission and also causes a large resurgence of CK flux in the xylem, while depodding in late podfill does neither. H-3-labelled CKs fed through the xylem flow primarily to the leaves where they are rapidly metabolised, mainly to adenine and its riboside. Thus, a continuous supply of CK is required and modulation of CK flux into the leaf appears to be a sensitive signalling mechanism controlling the senescence of leaves and other parts. The possible roles of the various forms of CK are discussed. In addition to the root-leaf signal (CKs) and seed-leaf signal, a pod-root signal is considered.