Increased cytokinin levels in transgenic PSAG12–IPT tobacco plants have large direct and indirect effects on leaf senescence, photosynthesis and N partitioning |
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Authors: | W Jordi A Schapendonk E Davelaar G M Stoopen C S Pot R De Visser J A Van Rhijn S Gan & R M Amasino |
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Institution: | Research Institute for Agrobiology and Soil Fertility (AB), PO Box 14, 6700 AA Wageningen, The Netherlands,; RIKILT, Post Box 230, 6700 AE Wageningen, The Netherlands,; Department of Agronomy and Tobacco and Health Research Institute, University of Kentucky, Lexington, KY 40546, USA, and; Department of Biochemistry, 433 Babcock Dr, University of Wisconsin, Madison, WI 53706-1544, USA |
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Abstract: | We studied the impact of delayed leaf senescence on the functioning of plants growing under conditions of nitrogen remobilization. Interactions between cytokinin metabolism, Rubisco and protein levels, photosynthesis and plant nitrogen partitioning were studied in transgenic tobacco (Nicotiana tabacum L.) plants showing delayed leaf senescence through a novel type of enhanced cytokinin syn‐thesis, i.e. targeted to senescing leaves and negatively auto‐regulated (PSAG12–IPT), thus preventing developmental abnormalities. Plants were grown with growth‐limiting nitrogen supply. Compared to the wild‐type, endogenous levels of free zeatin (Z)‐ and Z riboside (ZR)‐type cytokinins were increased up to 15‐fold (total ZR up to 100‐fold) in senescing leaves, and twofold in younger leaves of PSAG12–IPT. In these plants, the senescence‐associated declines in N, protein and Rubisco levels and photosynthesis rates were delayed. Senescing leaves accumulated more (15N‐labelled) N than younger leaves, associated with reduced shoot N accumulation (–60%) and a partially inverted canopy N profile in PSAG12–IPT plants. While root N accumulation was not affected, N translocation to non‐senescing leaves was progressively reduced. We discuss potential consequences of these modified sink–source relations, associated with delayed leaf senescence, for plant productivity and the efficiency of utilization of light and minerals. |
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Keywords: | canopy N profile chlorophyll cytokinins isopentenyl transferase N-15 photosynthesis Rubisco remobilization senescence-associated gene sink activity transgenic plants |
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