Regulation of Leaf Senescence by Cytokinin, Sugars, and Light
Effects on NADH-Dependent Hydroxypyruvate Reductase

Astrid Wingler^*, Antje von Schaewen, Richard C. Leegood, Peter J. Lea, and W. Paul Quick

Robert Hill Institute and Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, United Kingdom (A.W., R.C.L., W.P.Q.); Pflanzenphysiologie, Fachbereich Biologie/Chemie, Universität Osnabrück, D-49069 Osnabrück, Germany (A.v.S.); and Institute of Environmental and Biological Sciences, Lancaster University, Lancaster LA1 4YQ, United Kingdom (P.J.L.)

The aim of this study was to investigate the interactions between cytokinin, sugar repression, and light in the senescence-related decline in photosynthetic enzymes of leaves. In transgenic tobacco (Nicotiana tabacum) plants that induce the production of cytokinin in senescing tissue, the age-dependent decline in NADH-dependent hydroxypyruvate reductase (HPR), ribulose-1,5-bisphosphate carboxylase/oxygenase, and other enzymes involved in photosynthetic metabolism was delayed but not prevented. Glucose (Glc) and fructose contents increased with leaf age in wild-type tobacco and, to a greater extent, in transgenic tobacco. To study whether sugar accumulation in senescing leaves can counteract the effect of cytokinin on senescence, discs of wild-type leaves were incubated with Glc and cytokinin solutions. The photorespiratory enzyme HPR declined rapidly in the presence of 20 mm Glc, especially at very low photon flux density. Although HPR protein was increased in the presence of cytokinin, cytokinin did not prevent the Glc-dependent decline. Illumination at moderate photon flux density resulted in the rapid synthesis of HPR and partially prevented the negative effect of Glc. Similar results were obtained for the photosynthetic enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase. It is concluded that sugars, cytokinin, and light interact during senescence by influencing the decline in proteins involved in photosynthetic metabolism.

Plant Physiol. (1998) 116: 329-335
Copyright Clearance Center:   0032-0889/98/116/0329/07
© 1998 American Society of Plant Physiologists

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