Role of Cytokinins in Senescence, Antioxidant Defence and Photosynthesis.

Martin Hönig,Alexandra Husičková,Lucie Plíhalová,Karel Doležal,Jaroslav Nisler

doi:10.3390/ijms19124045

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.

Highlights

Occurring cytokinins (CKs) are purine based plant growth regulators that influence almost all of the developmental stages of plant life, e.g., development of vasculature, differentiation of embryonic cells, maintenance of meristematic cells, shoot formation and leaf senescence delay
CKs are effective in delaying the breakdown of chlorophyll, suggesting that they may play a role in maintaining the photosynthetic apparatus of plants [5]
In agreement with previously published data, it was shown that CK derivative treatment upregulated CK response regulators and other CK responsive genes, such as ARR15, ARR5, ARR8, ARR7, ARR4, ARR6, ARR9, CKX4, CRE1/AHK4, CRF2 and CRF5, genes encoding components of the photosystem II light harvesting complex (LHCII), i.e., At2g05070, At5g54270, At1g44575, At3g55330 and At2g39470, even though BAP had a negligible effect on these genes [44]

Summary

Introduction

Occurring cytokinins (CKs) are purine based plant growth regulators that influence almost all of the developmental stages of plant life, e.g., development of vasculature, differentiation of embryonic cells, maintenance of meristematic cells, shoot formation and leaf senescence delay. Synthetic CK analogues, 6-alkynyl and 6-alkenylaminopurines, some of which were substituted at the N9 atom of the purine moiety, have been tested for their antioxidant activity as potential diphenylpicrylhydrazyl (DPPH) scavengers and as inhibitors of 15-lipoxygenase enzyme, together with naturally occurring CKs BAP, Kin and tZ [46]. Some derivatives of BAP, BAPR, 9-THF (THP)—BAP and Kin exhibit low CK activity at the receptor level, lower or average activity in other CK bioassays, increased antisenescence activity [34,39,43] An example of such phenomena—the presence of an oxygen containing group and lack of relation of CK activity to the antisenescent activity—was provided recently by Nisler et al [49]. Some have been identified and well described, whereas others await discovery

Antisenescent Activity of Urea Based CKs and Their Derivatives

CKs and Photosynthesis

Conclusions