טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentYaela Golumbic
SubjectCytokinin's Role in Abiotic Stress Tolerance and its
Distribution in Planta
DepartmentDepartment of Biology
Supervisor Professor Emeritus Gepstein Shimon
Full Thesis textFull thesis text - English Version


Abstract

Senescence in an age-dependant deterioration process, that occurs in the whole plant, at the organ or at the cell level, and leads to their death. During leaf senescence, leaf cells undergo changes in cell structure, metabolism and gene expression, an active process that is well regulated by various endogenous and environmental factors.

Cytokinin is the most significant hormone in senescence retardation, its level drops in developing tissues causing senescence initiation. Exogenous application of cytokinins delays senescence, and transgenic plants expressing IPT (a key enzyme in cytokinin biosynthesis) demonstrate delayed senescence. Recently, it has been shown that transgenic plants expressing IPT under the regulation of the SARK (senescence gene) promoter also provide high resistance to drought conditions.

Distribution of cytokinins was examined using plants containing a cytokinin target gene fused to GUS (pARR5:GUS). Our results demonstrate the decrease of CK level during senescence and show the highest level of CK in the young leaves. In late developmental stages, the CKs decrease and are concentrated in the reproductive organs. We also showed that CK distribution is different between plant species, suggesting different locations of activity, biosynthesis and plants growth methods.

The role of CK in plant resistance to abiotic stress was examined in transgenic plants expressing IPT under the regulation of a stress promoter of the metallothionein gene. Stress conditions induce the metallothionein promoter causing the expression of IPT and the biosynthesis of CK. Transgenic plants grown under drought conditions with 50% of the optimal water requirements produced similar biomass, in comparison to plants with optimal watering. The biomass of the wild type (WT) plants was reduced by 20% in response to these drought conditions. This data implies that drought resistance was conferred by the enhanced level of IPT and CK.

Young M-IPT transgenic plans that were exposed to several severe abiotic stresses (drought, salinity, heat, cold), all showed enhanced tolerance and high recovery from the stress as opposed to the WT plants.

The results obtained in this research demonstrate, for the first time, CKs' role in conferring resistance of plants against a variety of abiotic stresses, a discovery that may have a great practical significance in agriculture in harsh environments. 

The exact mechanism of the stress resistance provided by the CK is not yet known. However, the new genomics and proteomics methodologies offer appropriate tools for deciphering the mechanism underlying this phenomenon.