title: Cellular and Molecular Changes Associated with Onion Skin Formation Suggest Involvement of Programmed Cell Death.
abstract: Skin formation of onion (Allium cepaL.) bulb involves scale desiccation accompanied by scale senescence, resulting in cell death and tissue browning. Understanding the mechanism of skin formation is essential to improving onion skin and bulb qualities. Although onion skin plays a crucial role in postharvest onion storage and shelf life, its formation is poorly understood. We investigated the mode of cell death in the outermost scales that are destined to form the onion skin. Surprisingly, fluorescein diacetate staining and scanning electron microscopy indicated that the outer scale desiccates from the inside out. This striking observation suggests that cell death in the outer scales, during skin formation, is an internal and organized process that does not derive only from air desiccation. DNA fragmentation, a known hallmark of programmed cell death (PCD), was revealed in the outer scales and gradually decreased toward the inner scales of the bulb. Transmission electron microscopy further revealed PCD-related structural alterations in the outer scales which were absent from the inner scales.De novotranscriptome assembly for three different scales: 1st (outer), 5th (intermediate) and 8th (inner) fleshy scales identified 2,542 differentially expressed genes among them. GO enrichment for cluster analysis revealed increasing metabolic processes in the outer senescent scale related to defense response, PCD processes, carbohydrate metabolism and flavonoid biosynthesis, whereas increased metabolism and developmental growth processes were identified in the inner scales. High expression levels of PCD-related genes were identified in the outer scale compared to the inner ones, highlighting the involvement of PCD in outer-skin development. These findings suggest that a program to form the dry protective skin exists and functions only in the outer scales of onion.
authors.affiliation: Department of Postharvest Science of Fresh Produce, The Volcani Center, Agricultural Research OrganizationRishon LeZion, Israel; The Robert H. Smith Institute of Field Crops and Vegetables, Robert H. Smith Faculty of Agriculture Food and Environment, The Hebrew University of JerusalemRehovot, Israel.
authors.forename: Ortal
authors.initials: O
authors.lastname: Galsurker
authors.affiliation: Institute of Plant Sciences, The Volcani Center, Agricultural Research Organization Rishon LeZion, Israel.
authors.forename: Adi
authors.initials: A
authors.lastname: Doron-Faigenboim
authors.affiliation: Department of Postharvest Science of Fresh Produce, The Volcani Center, Agricultural Research Organization Rishon LeZion, Israel.
authors.forename: Paula
authors.initials: P
authors.lastname: Teper-Bamnolker
authors.affiliation: Department of Postharvest Science of Fresh Produce, The Volcani Center, Agricultural Research Organization Rishon LeZion, Israel.
authors.forename: Avinoam
authors.initials: A
authors.lastname: Daus
authors.affiliation: The Alexander Silberman Institute of Life Science, Edmond Safra Campus (G Ram), The Hebrew University Jerusalem, Israel.
authors.forename: Yael
authors.initials: Y
authors.lastname: Fridman
authors.affiliation: Department of Postharvest Science of Fresh Produce, The Volcani Center, Agricultural Research Organization Rishon LeZion, Israel.
authors.forename: Amnon
authors.initials: A
authors.lastname: Lers
authors.affiliation: Department of Postharvest Science of Fresh Produce, The Volcani Center, Agricultural Research Organization Rishon LeZion, Israel.
authors.forename: Dani
authors.initials: D
authors.lastname: Eshel
keywords: DNA fragmentation; onion-skin; programmed cell death; scale; transcriptome