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Abstract

Abscission is a highly regulated process in which various organs, including leaves, flowers and fruits, are separated from the mother plant as a natural stage of plant development. Abscission occurs specifically in the abscission zone (AZ) tissue. AZ is located at the base of the organ to be shed, between fruit, flower or leaf and the plant body and is comprised of few layers of differentiated cells. AZ has a crucial role in the process. Not much is known about the molecular regulation of abscission. However, we have recently shown that the programmed cell death (PCD) is involved in the abscission of tomato leaves and flowers and suggested that various abscission-related processes occur asymmetrically between the AZ proximal and distal sides.   The proposed research is aimed to further characterize the identified asymmetry in the AZ and investigate its functional significance for the abscission process of flowers and leaves in tomato. We will investigate: (a) presumably high metabolic activity and membrane trafficking of cells at the proximal side of AZ, including the process of endoreduplication; (b) PCD type in AZ; (c) ethylene biosynthesis and perception; (d) cell wall metabolism/modification-related gene expression and global transcriptomic changes at the late stage of abscission; and (e) if inhibition of PCD leads to inhibition of abscission. We will examine changes of expression of specific genes using quantitative RT-PCR (qPCR), and use in situ hybridization and immunolocalization to determine in what AZ cells the gene transcripts and synthesized proteins are localized. Genes examined will be LX RNase, ACC oxidase (involved in ethylene biosynthesis), polygalacturonases, vacuolar protease with caspase-like activity, TBN1 nuclease and NADPH oxidase. Ultrastructural changes in AZ of leaves and flowers will be further examined using transmission electron microscopy (TEM). For achievements of some goals the approaches of functional genomics will be applied.   The results of the proposed research will supply information required for determining the functional significance of the observed asymmetry in the AZ and confirmation of the suggested hypotheses. The results could reveal novel and significant insights in three aspects: (1) the functional organization of the AZ in respect to the late execution stage of the process, and the significance of separating the execution machinery of abscission from the other involved factors (ethylene, LX RNase and PCD); (2) the role of a PCD process in ethylene-related processes or in another, yet unclear, way in the execution mechanism of abscission; (3) the role of ethylene as a signal for inducing LX RNase and PCD in abscission. The findings of this study will contribute to the overall basic knowledge on the process of abscission that is still extremely rudimentary. Results will be directly applicable in agriculture and biotechnology for controlled abscission of fruits and manipulation of abscission time.   The research will be conducted by three highly complementary and experienced research groups: two partner organizations in Slovenia – Laboratory for Experimental Botany at the Biotechnical Faculty, University of Ljubljana and the Department for Biotechnology and Systems biology at the National Institute of Biology. The third partner research group is the laboratory of Dr. Amnon Lers at the Volcani Center, Agricultural Research Organization, Department of Postharvest Science of Fresh Produce in Israel. All three partner laboratories have state-of the art equipment capable of producing high quality results that assure the total feasibility of the proposed project.

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