Role of Cytokinins in Senescence, Antioxidant Defence and Photosynthesis

Abstract

Cytokinins modulate a number of important developmental processes, including the last phase of leaf development, known as senescence, which is associated with chlorophyll breakdown, photosynthetic apparatus disintegration and oxidative damage. There is ample evidence that cytokinins can slow down all these senescence-accompanying changes. Here, we review relationships between the various mechanisms of action of these regulatory molecules. We highlight their connection to photosynthesis, the pivotal process that generates assimilates, however may also lead to oxidative damage. Thus, we also focus on cytokinin induction of protective responses against oxidative damage. Activation of antioxidative enzymes in senescing tissues is described as well as changes in the levels of naturally occurring antioxidative compounds, such as phenolic acids and flavonoids, in plant explants. The main goal of this review is to show how the biological activities of cytokinins may be related to their chemical structure. New links between molecular aspects of natural cytokinins and their synthetic derivatives with antisenescent properties are described. Structural motifs in cytokinin molecules that may explain why these molecules play such a significant regulatory role are outlined.

Keywords: antioxidant; antioxidant enzymes; antisenescent; cytokinin; derivative; genes; photosynthesis; plant defence; structure and activity relationship.

Figure 1

Evaluation of the biological activity of (A) classical cytokinins, (B) 3-fluoro derivatives of BAP and BAPR and (C) urea based cytokinins in the wheat leaf senescence assay performed in the dark (according to [19]). The dotted line indicates where the chlorophyll content in the leaves is 50% of that in fresh control leaves, while 100% represents the chlorophyll content in fresh control leaves. Dashed lines indicate values for the control treatment (DMSO control) with no added compound. Error bars show the S.D. of the mean for four replicate determinations.