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.
Highlights
The onion (Allium cepa L.) bulb is considered to be one of the most important vegetable crops in the world (Hoffman, 1933; Griffiths et al, 2002; Mallor et al, 2011)
The sequential scales of the onion bulb, which have a common origin as leaf bases, differed in various characteristics
Onion scales contain quercetin 4 -glucoside and quercetin 3,4 -diglucoside as major phenolics, with their concentration increasing from the inner to outer scales (Tsushida and Suzuki, 1995; Hirota et al, 1998)
Summary
The onion (Allium cepa L.) bulb is considered to be one of the most important vegetable crops in the world (Hoffman, 1933; Griffiths et al, 2002; Mallor et al, 2011). The onion plant’s leaves grow in a circular pattern from a flattened stem, giving rise to older leaves on the outside and younger leaves on the inside (Hoffman, 1933). The base of the leaf sheath swells to form the fleshy scales and the young developing leaves cease to form blades, developing instead into swollen, bladeless scales (Brewster, 1994, 2008). This provides the typical bulb structure, which is ordered according to the physiological age of the scales from inner younger to outer older scales. One to three outer scales dry out and develop into thin, brown protective skins (Jones and Mann, 1963)
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