Abstract
The formation of brown protective skin in onion bulbs can be induced by rapid post-harvest heat treatment. Onions that are peeled to different depths and are exposed to heat stress show that only the outer scales form the dry brown skin, whereas the inner scales maintain high water content and do not change color. Our study demonstrates that browning of the outer scale during heat treatment is due to an enzymatic process that is associated with high levels of oxidation components, such as peroxidase and quercetin glucoside. De novo transcriptome analysis revealed differential molecular responses of the outer and inner scales to heat stress. Genes involved in lipid metabolism, oxidation pathways, and cell-wall modification were highly expressed in the outer scale during heating. Defense response-related genes such as those encoding heat-shock proteins, antioxidative stress defense, or production of osmoprotectant metabolites were mostly induced in the inner scale in response to heat exposure. These transcriptomic data led to a conceptual model that suggests sequential processes for the development of browning and desiccation of the outer scale versus processes associated with defense response and heat tolerance in the inner scales.
Highlights
Onion bulbs (Allium cepa L.) are one of the most economically important Allium crops and are highly appreciated by consumers because of their distinctive sensory and beneficial compounds (Mota et al, 2010).The bulb morphology is composed of scales ordered according to their chronological age from inner, younger, to outer, older, scales (Galsurker et al, 2017).A typical onion bulb at maturity has one-to-three dry skins originating from scales that enclose sequential thin outer scales, which in turn enclose several swollen, inner fleshy scales (Brewster, 2008; Galsurker et al, 2017)
High humidity (98% relative humidity (RH)) during heating caused the formation of a dark red-brown integral skin, whereas a cracked and pale yellow-colored skin was observed at 45% RH.The H value of the 1st scale dramatically decreased from 110° to 37° after 22 d of heating under 98% RH, compared to a negligible decrease from 110° to 99° after heating under 45% RH (Fig. 1B)
Based on the observed structural, physiological, and biochemical changes of the different scales and the changes in their transcriptomes following the heat treatment, we suggest that defense responses are significantly activated by heat in the inner scales, generating tolerance, whereas processes related to rapid senescence and skin formation occur in the outer scale
Summary
Onion bulbs (Allium cepa L.) are one of the most economically important Allium crops and are highly appreciated by consumers because of their distinctive sensory and beneficial compounds (Mota et al, 2010).The bulb morphology is composed of scales ordered according to their chronological age from inner, younger, to outer, older, scales (Galsurker et al, 2017).A typical onion bulb at maturity has one-to-three dry skins originating from scales that enclose sequential thin outer scales, which in turn enclose several swollen, inner fleshy scales (Brewster, 2008; Galsurker et al, 2017). Onion-skin formation involves activation of a programmed cell death (PCD) mechanism together with desiccation and development of browning in the outer scales; in parallel, metabolism-maintenance processes occur in the inner, younger, fleshy scales (Galsurker et al, 2017). As the outer and inner scales differ in function and physiological age, they can serve as an excellent system to compare their responses to abiotic stress. Bulbs can be cured either in the field, by allowing them to dry in the sun, or by artificial curing in storage, during which temperature and its rate of change, humidity, and curing duration can be controlled
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