Abstract
Watercore is a physiological disorder that commonly occurs in sand pear cultivars. The typical symptom of watercore tissue is transparency, and it is often accompanied by browning, breakdown and a bitter taste during fruit ripening. To better understand the molecular mechanisms of watercore affecting fruit quality, this study performed transcriptome and metabolome analyses on watercore pulp from “Akibae” fruit 125 days after flowering. The present study found that the “Akibae” pear watercore pulp contained higher sorbitol and sucrose than healthy fruit. Moreover, the structure of the cell wall was destroyed, and the content of pectin, cellulose and hemicellulose was significantly decreased. In addition, the content of ethanol and acetaldehyde was significantly increased, and the content of polyphenol was significantly decreased. Watercore induced up-regulated expression levels of sorbitol synthesis-related (sorbitol-6-phosphate dehydrogenase, S6PDH) and sucrose synthesis-related genes (sucrose synthesis, SS), whereas it inhibited the expression of sorbitol decomposition-related genes (sorbitol dehydrogenase, SDH) and sorbitol transport genes (sorbitol transporter, SOT). Watercore also strongly induced increased expression levels of cell wall-degrading enzymes (polygalactosidase, PG; ellulase, CX; pectin methylesterase, PME), as well as ethanol synthesis-related (alcohol dehydrogenase, ADH), acetaldehyde synthesis-related (pyruvate decarboxylase, PDC) and polyphenol decomposition-related genes (polyphenol oxidase, PPO). Moreover, the genes that are involved in ethylene (1-aminocyclopropane- 1-carboxylate oxidase, ACO; 1-aminocyclopropane- 1-carboxylate synthase, ACS) and abscisic acid (short-chain alcohol dehydrogenase, SDR; aldehyde oxidase, AAO) synthesis were significantly up-regulated. In addition, the bitter tasting amino acids, alkaloids and polyphenols were significantly increased in watercore tissue. Above all, these findings suggested that the metabolic disorder of sorbitol and sucrose can lead to an increase in plant hormones (abscisic acid and ethylene) and anaerobic respiration, resulting in aggravated fruit rot and the formation of bitter substances.
Highlights
Watercore is an internal physiological disorder affecting apples and pears, in which the intercellular air spaces of the flesh become filled with liquid, resulting in tissues with a translucent appearance [1]
The excessive accumulation of sorbitol is considered to be the direct cause of the watercore symptom, and it is closely related to the influence of environmental factors, such as temperature
Hayama found that the watercore disorder in the Japanese pear “Niitaka” is increased by high fruit temperatures during fruit maturation [5]
Summary
Watercore is an internal physiological disorder affecting apples and pears, in which the intercellular air spaces of the flesh become filled with liquid, resulting in tissues with a translucent appearance [1]. A large number of studies have examined environmental, physiological and biochemical factors involved in watercore development [3]. Most studies have suggested that the accumulation of sorbitol in the intercellular spaces is a possible cause of watercore in apples [4]. A wide range of environmental and physiological factors have been implicated in the incidence of watercore. There are three main factors contributing to watercore incidence: (1) Abnormal temperature. Watercore fruit in the “Hosui” and “Niitaka” pear species was found to increase significantly under higher air temperatures during fruit maturation [5]. Watercore fruit in the “Hosui” and “Niitaka” pear species was found to increase significantly under higher air temperatures during fruit maturation [5]. (2) Plant hormones
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