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Planta

, Volume 221, Issue 6, pp 801–814 | Cite as

Effects of senescence-induced alteration in cytokinin metabolism on source-sink relationships and ontogenic and stress-induced transitions in tobacco

  • A. Keith. Cowan
  • Michael Freeman
  • Per-Olof Björkman
  • Björn Nicander
  • Folke Sitbon
  • Elisabeth Tillberg
Original Article

Abstract

Senescence and reserve mobilization are integral components of plant development, are basic strategles in stress mitigation, and regulated at least in part by cytokinin. In the present study the effect of altered cytokinin metabolism caused by senescence-specific autoregulated expression of the Agrobacterium tumefaciens IPT gene under control of the PSAG12 promoter (PSAG12-IPT) on seed germination and the response to a water-deficit stress was studied in tobacco (Nicotiana tabacum L.). Cytokinin levels, sugar content and composition of the leaf strata within the canopy of wild-type and PSAG12-IPT plants confirmed the reported altered source–sink relations. No measurable difference in sugar and pigment content of discs harvested from apical and basal leaves was evident 72 h after incubation with (+)-ABA or in darkness, indicating that expression of the transgene was not restricted to senescing leaves. No difference in quantum efficiency, photosynthetic activity, accumulation of ABA, and stomatal conductance was apparent in apical, middle and basal leaves of either wild-type or PSAG12-IPT plants after imposition of a mild water stress. However, compared to wild-type plants, PSAG12-IPT plants were slower to adjust biomass allocation. A stress-induced increase in root:shoot ratio and specific leaf area (SLA) occurred more rapidly in wild-type than in PSAG12-IPT plants reflecting delayed remobilization of leaf reserves to sink organs in the transformant. PSAG12-IPT seeds germinated more slowly even though abscisic acid (ABA) content was 50% that of the wild-type seeds confirming cytokinin-induced alterations in reserve remobilization. Thus, senescence is integral to plant growth and development and an increased endogenous cytokinin content impacts source–sink relations to delay ontogenic transitions wherein senescence in a necessary process.

Keywords

Cytokinin Germination PSAG12-IPT Senescence source–sink relations Tobacco Water stress 

Abbreviations

ABA

Abscisic acid

IPT

Isopentenyl transferase

2iP

Isopentenyl adenine

2iPR

Isopentenyl adenosine

2iPM

Isopentenyl adenosine −5’-monophosphate

SAG

Senescence-associated gene

SLA

Specific leaf area

Z

Zeatin

ZMP

Zeatin riboside-5’-monophosphate

Z9G

Zeatin-9-glucoside

ZR

Zeatin riboside

Notes

Acknowledgements

This work was supported by a grant from The Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (Dnr 310588/9). We thank Jan Fahlesson for help with digital photography. Keith Cowan is grateful to the Department of Plant Biology and Forest Genetics, SLU, Uppsala for partial financial support and the facilities that allowed for completion of this project.

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Copyright information

© Springer-Verlag 2005

Authors and Affiliations

  • A. Keith. Cowan
    • 1
    • 3
  • Michael Freeman
    • 2
  • Per-Olof Björkman
    • 1
  • Björn Nicander
    • 1
  • Folke Sitbon
    • 1
  • Elisabeth Tillberg
    • 1
  1. 1.Department of Plant Biology and Forest GeneticsSwedish University of Agricultural SciencesUppsalaSweden
  2. 2.Department of Ecology and Environmental ResearchSwedish University of Agricultural SciencesUppsalaSweden
  3. 3. MiddletonUSA

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