|M.Sc Student||Edison-Tkach Natalia|
|Subject||Identification and Characterization of USP (Universal Stress|
Protein) in Arabidopsis and its Involvement in
|Department||Department of Biology||Supervisor||Professor Emeritus Shimon Gepstein|
|Full Thesis text|
Plant senescence, the final stage of its development, is a form of programmed cell death, a genetically controlled system of self destruction. Leaf senescence is an active process and is characterized by massive decomposition of macromolecules. The degradation products are recycled and are used by young leaves, fruits and seeds. Both environmental and autonomous factors regulate leaf senescence. The autonomous factors include age, reproductive development, and phytohormone levels, whereas the environmental cues include different stresses. Stress may cause premature senescence and on the other hand senescing plants and organs are more sensitive to stresses. The inter-relationship between stress and senescence is also reflected by a large overlap of gene expression in these two processes.
We have identified a gene encoding a Universal Stress Protein (USP) that may serve as a stress signal for premature flowering, senescence and death of monocarpic plants. USP has been described widely in prokaryotes, where its production is stimulated by large variety of stresses. In eukaryotes, however, the USPs and their function have not been elucidated yet. Sequences of the USP gene family have been identified in the Arabidopsis genome databases but their role has not been determined yet.
Temporal pattern of the USP gene expression has revealed the highest levels of the USP transcripts during the initiation of the inflorescence stem, a stage which is closely associated also with the initiation of leaf senescence. Stress-induced senescence revealed a dramatic up-regulation of the USP transcript especially in detached Arabidopsis leaves in darkness. Kinetic studies of the USP transcript levels have revealed a periodic rhythm, with peak in the afternoon.
Exogenous plant hormones are involved in the USP regulation, where ACC caused up-regulation, whereas cytokinins and ozone caused down-regulation of the USP transcript.
Over expressing plants containing the USP gene-13XMyc construct driven by 35S-promoter, exhibited significant acceleration of life cycle as reflected by premature flowering and advanced leaf senescence.
Sub-cellular localization analysis of the USP gene product indicated its residing within the chloroplasts.
Taken together, the results suggest possible association of the USP gene product in processes related both to development and to the plant responses to environmental stresses.
The specific premature senescence syndrome caused by extreme environmental stresses raises the possibility that the USP gene serves as a link between stress and the senescence syndrome.