טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentBanner Idit
SubjectDevelopment of Resistant Plants against Abiotic Stresses
Containing Controlled System of Cytokinin
Biosynthesis
DepartmentDepartment of Biology
Supervisor Professor Emeritus Shimon Gepstein
Full Thesis textFull thesis text - English Version


Abstract

Abiotic stresses, the most prominent threat to agricultural production worldwide, accelerates leaf senescence, loss in photosynthesis and reduced yields. On the basis of the assumption that senescence is a type of cell death program that could be inappropriately activated during stress, we hypothesized that it may be possible to enhance abiotic stress tolerance by delaying stress induced leaf senescence. A most dramatic transgenic intervention leading to a delay senescence phenotype concerns the manipulation of levels of the plant hormones cytokinins, which promote cell division and growth and delay the senescence of leaves. Reports on an effective approach in delaying senescence of tobacco plants based on the autoregulation of cytokinin synthesis  controlled by  a senescence  promoter  have contributed to our understanding of the role of cytokinins on plant senescence . Surprisingly, this technology did not delay leaf senescence in the model plant Arabidopsis. Moreover, exogenous application of cytokinins has no or little delaying effect on the senescence of Arabidopsis attached leaves. In this study, we show two promising strategies for delaying senescence by cytokinins in detached and intact leaves of tobacco and Arabidopsis. The key gene of cytokinin synthesis - isopentenyl transferase (IPT) gene was fused to the bean promoter of the Senescence-Associated Receptor Kinase (SARK) gene for plants transformation. Based on RT-PCR results, IPT expression is autoregulated by SARK promoter. Tobacco plants containing PSARK::IPT showed a dramatic increase in the life span of plants. Remarkably, the suppression of drought-induced leaf senescence in PSARK::IPT Arabidopsis plants resulted in outstanding drought tolerance. In addition, the autoregulatory IPT gene expression driven by PSARK caused the delayed leaf senescence of transgenic tobacco plants grown under low nutrients supply. Under extremely severe nutrients deficiency stress  the transgenic plants showed  longer  survival  as visualize  by delay of yellowing and the appearance of senescence  molecular markers. The survival of plants was correlated with higher contents of bioactive cytokinins in the middle leaves. Another approach for creating transgenic plants  was to fuse the IPT gene to estrogen-inducible XVE system.. The transgenic plants displayed characteristic developmental phenotypes  of seedlings and delayed leaf senescence Our developed  systems may provide a useful tool for studying the role of cytokinins in  developmental and senescence processes. Not less important is the potential biotechnological application emerged from our research. In our growing populated globe, where regions of agriculture lands are diminishing, our  new approach that allows growing crops in mineral deficient non agricultural lands is extremely important.