Molecular cloning and expression analysis of EjSWEET15, enconding for a sugar transporter from loquat
Introduction
Loquat (Eriobotrya japonica Lindl.) is a subtropical flowing plant that is widely planted as ornamentals in the family Rosaceae, and native to south-central China. The color of the flesh can serve as a base for segregation of loquat fruit into white-fleshed and red-fleshed types. White-fleshed type is succulent, having a better flavor (Zhou et al., 2007). However, the fruit quality is appreciated based on physical characteristics perceived by human sense (fruit size, skin color, flavor and texture) on one hand, and the organoleptic quality on the other hand, which are determined by physical, chemical and sensorial analyses (Cañete et al., 2007). It has been reported that loquat fruit flavor is a function of taste, odor and texture reflected on the main components of eating quality (Tan, 2000). The main components of fruits flavor comprises of sweetness, sourness and aroma which corresponds to sugars, acids and volatiles, respectively (Kader, 2008). It has been observed that white-fleshed type of loquat fruit has gained more consumer satisfaction and acceptability due to its flavor and sweet taste over the red-fleshed type of loquat.
It is worth noting that sweetness is determined by the concentrations of the common disaccharide sugars sucrose naturally produced in plant and a combination of the two monosaccharide fructose and glucose (Hong et al., 2014). Understanding the influential cues for sweetness in fruits of plant is of paramount importance for plant breeders with interest on altering fruit flavor to better suit human desire. Although, total sugar in fruit is a function of genetic, nutritional, environmental and developmental factors, a change in the expression of metabolic enzymes for sugar metabolism has a huge influence on sugar compositional changes (Nookaraju et al., 2010). Sugar transporters is a general term attributed to proteins that are responsible for sugar transport, and play a key physiological role in the metabolism, growth and development of organisms (Baker et al., 2012; Chen et al., 2012). SWEET is a sugar transporter protein type member reported in recent years in eukaryotes and comprises of two typical MtN3/saliva domains responsible for the transportation of sucrose and fructose (Chen et al., 2015a; Yuan et al., 2014). In maize it is known as ZmSWEET4c and its rice ortholog OsSWEET4 were likely recruited during domestication to enhance sugar import into the endosperm (Sosso et al., 2015). Also, AtSWEET15 is expressed at its peak in mature and senescent leaves, it is considered to be a key member of carbohydrate transport during organ aging in Arabidopsis (Chen et al., 2015b; Gao et al., 2016). However, the mechanism by which SWEET transports soluble sugars in loquat is not clear.
Here, a mutant white-flesh loquat with significantly higher soluble sugar content fruit as compared to that of the wild-type red-flesh loquat was isolated. We further elucidate that EjSWEET15 protein plays an important role in regulating sugar transport in loquat.
Section snippets
Plant materials and growth conditions
The mutant white-flesh loquat plant of about 20 years was discovered and identified in Mao Xian, Sichuan province; China in 2003 and its wild-type red-flesh loquat were used as test materials. These plants were tag and samples collected at different time intervals beginning at the onset of flowering stage. Sampling was done four times at different stages from the start of flowing: (i) fruit core-hardening, (ii) fruit swelling, (iii) a week before fruit ripening and (iv) mature which were set at
Changes in sugar content of mutant white-flesh loquat
In our previous work a mutant type of Dong ting loquat with the wild-type fruit presenting a red-flesh loquat was described. This tree has a single bud at the middle part of the trunk which gives rise to a white peel loquat (Fig. 1A-B). The fruits resulting from the bud is refer to as the mutant which gives rise to white-flesh loquat against the red-flesh loquat from the wild-type as presented through Fig. 1C to D. There was a gradual and progressive color change (phenotype) observed with the
EjSWEET15 is a typical plant SWEET sugar transporter
Photosynthetic products in plant leaves are the main source of sugar in fruits (Ludwig, 2016). As a "reservoir", the fruit requires sugar transporters to be involved in unloading and storing sugar. Sugar transporters are a collective name for a group of proteins responsible for sugar transport in plants. These sugar transporters can be divided into different groups in relation to the type of product involve or to be transported. We thus talk of monosaccharide transporters and polysaccharide
CRediT authorship contribution statement
Jing Li: Conceptualization, Writing - original draft. Dong Chen: Investigation, Visualization. Guo-liang Jiang: Data curation, Formal analysis. Hai-yan Song: Writing - review & editing. Mei-yan Tu: Supervision, Project administration. Shu-xia Sun: Conceptualization, Resources, Funding acquisition.
Declaration of Competing Interest
We declare that all author have no known competing financial interests or personal relationship that could have appeared to influence the work reported in this paper.
Acknowledgments
The study was supported by Breakthrough Fruit Tree Breeding Materials and Methods Innovation-fruit Tree Excellent Genetic Resources Exploration, Conservation and Utilization (2016NYZ0043-01), the Financial Innovation Capacity Improvement Project for White Loquat Project of Sichuan Province (2016TSCY-002) and Chengdu Technology Innovation Research and Development Project (2019-YF05-01378-SN).
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These authors contributed equally to this work.