Cytokinins in plant senescence: From spray and pray to clone and play

Three approaches have been used to investigate the inhibitory role of the cytokinin class of phytohormones in plant senescence: external application of cytokinins, measurement of endogenous cytokinin levels before and during senescence, and manipulation of endogenous cytokinin production in transgenic plants. In transgenic plant studies, endogenous cytokinin levels are manipulated by expression of IPT, a gene encoding isopentenyl transferase. Transgenic plants expressing IPT from a variety of promoters exhibit developmental and morphological alterations and often display retarded leaf senescence. A recently developed autoregulatory senescence-inhibition system targets cytokinin production quantitatively, spatially and temporally, and results in transgenic plants that exhibit significantly delayed senescence without abnormalities. These transgenic studies not only confirm the regulatory role of cytokinins in plant senescence, but also provide a way to manipulate senescence for potential agricultural applications.