Increased cytokinin levels in transgenic PSAG12–IPT tobacco plants have large direct and indirect effects on leaf senescence, photosynthesis and N partitioning

TLDR: Interactions between cytokinin metabolism, Rubisco and protein levels, photosynthesis and plant nitrogen partitioning were studied in transgenic tobacco plants showing delayed leaf senescence through a novel type of enhanced cytokinIn synthesis, targeted to senescing leaves and negatively auto-regulated (PSAG12‐IPT), thus preventing developmental abnormalities.

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 synthesis, i.e. targeted to senescing leaves and negatively auto-regulated (PSAG12‐IPT), thus preventing developmental abnormalities. Plants were grown with growthlimiting 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 senescenceassociated declines in N, protein and Rubisco levels and photosynthesis rates were delayed. Senescing leaves accumulated more ( 15 N-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.