Increased cytokinin levels in transgenic P_SAG12–IPT tobacco plants have large direct and indirect effects on leaf senescence, photosynthesis and N partitioning

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.

Functional Significance of Shade‐Induced Leaf Senescence in Dense Canopies: An Experimental Test Using Transgenic Tobacco

“…n p 12 reported for solitarily grown SAG plants (Gan and Amasino 1995;Wingler et al 1998;Jordi et al 2000), we also observed a higher LAI (table 2) and more uniform N distribution ( fig. 2D) of SAG plants compared to WT plants in the dense leaf canopies, allowing us to test these model predictions.…”

“…Wisconsin 38) and transgenic hemizygous P SAG12 -IPT (SAG) tobacco (Gan and Amasino 1995;Jordi et al 2000).…”

“…In this study, we have chosen to make use of transgenic P SAG12 -IPT tobacco (SAG) plants with a strong delay in leaf senescence (Gan and Amasino 1995). Such plants develop a greater LAI than normally senescing wild-type (WT) plants and have a more uniform distribution of leaf N and associated photosynthetic capacity, at least when grown solitarily (Gan and Amasino 1995;Wingler et al 1998;Jordi et al 2000). This phenotype is caused by production of the senescence-inhibiting phytohormone cytokinin, targeted specifically to tissues at the onset of senescence (Gan and Amasino 1995).…”

Photosynthetic Productivity: Can Plants do Better?

“…The P SAG12 :IPT plants retained their leaves longer than WT plants as expected based upon previous observations (Boonman et al, 2006;Jordi et al, 2000;Gan & Amasino, 1995). The difference between the total number of nodes on the stem, a measure of all the leaves that had ever been produced, and the live leaves present at the time of the harvest is a measure of the number of leaves that had been lost in the canopy due to senescence and abscission ( Table 2).…”

“…Canopy recycling of N L is particularly important for plants grown in nitrogen poor soils. Jordi et al (2000) demonstrated the detrimental effects of prolonged leaf lifespan on N L and photosynthesis in upper canopy leaves of WT and P SAG12 :IPT plants grown under N-starvation conditions. As in the present study, Jordi et al (2000) found that older leaves in WT plants dried, yellowed, and abscised whereas older leaves on the P SAG12 :IPT plants remained intact, green, and photosynthetically competent.…”