Cellular and Molecular Changes Associated with Onion Skin Formation Suggest Involvement of Programmed Cell Death

Abstract

Skin formation of onion (Allium cepa L.) 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 novo transcriptome 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.

Laterals take it better – Emerging and young lateral roots survive lethal salinity longer than the primary root in Arabidopsis

“…The PR zone of stressed Arabidopsis plants were excised under trinocular stereo microscope (SZ61-OLYMPUS) using sharp razor blade and fixed in 5% Glutaraldeyde in 0.1 M Cacodylate buffer (pH 7.4) 3 hours at room and transferred to 4 °C for continuation of fixation overnight. The samples were further processed according to Galsurker et al 55 . The final sections were placed on grids and sequentially stained with uranyl acetate and Lead citrate for 10 minutes each and viewed with Tecnai 12 TEM 100 kV (Phillips, Eindhoven, the Netherlands) equipped with MegaView II CCD camera and Analysis version −3.0 software (SoftImaging System GmbH, Münstar, Germany).…”

Mechanism of Allium Crops Bulb Enlargement in Response to Photoperiod: A Review

“…Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as transfer RNA (tRNA) or small nuclear RNA (snRNA) genes, the product is a functional RNA [156][157][158][159][160][161][162]. In onion, the physiology of the bulb beginning was precisely defined by Mettananda and Fordham [163]; photoperiodic conditions mark bulb beginning, similar to the photoperiodic switch of flowering in other species [20,164].…”

The Harvest Season Changes the Organoleptic Properties of Onion during Storage

“…The cure reduce moisture by allowing skin color changes and volatile sulfur compounds to enhance aroma and reduce bulb susceptibility to microorganisms, characteristics that change according to the genotype). The cure can be natural (done in the field, for a period of three to ten days, depending on the weather) or artificially (fans with natural or heated air are used) [9].…”

“…Some commercial varieties of onion have large variations in maturation due to factors ranging from asynchronous growth and seed development to photoperiod requirements [8]. This results in crops with plants at different stages, making it difficult to determine the harvesting time and reducing bulb quality during storage [9]. The quality of the onion bulbs is evaluated basically by the color of the bulb and the contents of sugars, organic acids and phenolic compounds that confer characteristic flavor, aroma and color [10,9,11].…”

“…This results in crops with plants at different stages, making it difficult to determine the harvesting time and reducing bulb quality during storage [9]. The quality of the onion bulbs is evaluated basically by the color of the bulb and the contents of sugars, organic acids and phenolic compounds that confer characteristic flavor, aroma and color [10,9,11]. These quality characteristics are influenced by the harvest period.…”