In recent years, more and more edible mushrooms have been widely studied for their unique flavors and high biological values [1]. Morels (Morchella spp.), as one of the most highly prized edible mushrooms in China, have been valued for its desirable taste quality, savory flavor and medicinal feature [2]. Morels have been recorded in the Compendium of Materia Medica in China for their ability to promote gastrointestinal health. Now, morels are consumed worldwide as food and food-flavoring agent, which is becoming of highly economic importance [3]. The health-promoting effects and medicinal bioactivities of Morchella species are primarily attributed to the polysaccharides. Many studies demonstrated that Morchella-derived polysaccharides had different functional properties, such as antioxidant activity and immunomodulatory activity [[3], [4], [5]]. Our previous study had shown that native Morchella polysaccharides exhibited dose-dependent antioxidant activities against ABTS, DPPH and hydroxyl radicals [5]. And, this polysaccharide also displayed a significant promotion effect on the growth of macrophages. Due to their beneficial effects on health, morels are currently in use as the functional food in China.

Although natural polysaccharides from morels possess various biological activities, some of them actually show moderate effects. And, commercial cultivation of morels is very limited in China, which cause that natural morel polysaccharides were rare and expensive [2]. Currently, the modification of polysaccharides, especially for selenization modification, has been proven to be an effective method to improve polysaccharides' biological properties and enhance their application prospect [6].

Selenium is an essential trace element found in all life forms, and closely related to the growth, development and health of human bodies [7]. Proper selenium supplementation from foods offers many health benefits to humans, such as preventing cancer, strengthening immunity and reducing oxidative stress [8]. Thus, Se functional agriculture are currently emerging strategies to meet the requirements of people who are deficient in Se.

Selenized polysaccharide is an important form of organic selenium occurring in foods, which is generally formed by the covalent combination of selenium and polysaccharide [8]. Compared with sodium selenite and inorganic Se, the selenized polysaccharide has lower toxicity and stronger biological activity, as it possesses the dual activity of selenium and polysaccharide [9]. Moreover, the selenized polysaccharide is more easily absorbed and utilized by the human body [12,14]. Thus, it is an effective way for humans to obtain selenium by making dietary supplements with selenized polysaccharides. However, selenium-containing polysaccharides are not commonly found in nature [15,16]. Most selenium polysaccharides are mainly obtained through artificial selenylation modifications [17,18]. For example, Schisandra chinensis polysaccharides (SCPs) were selenized by the HNO3-Na2SeO3 method, which could significantly improve the antioxidant activities of SCPs [19]. Wang et al. revealed that artificially selenized Grifola frondosa polysaccharides (SeGFP2) exhibited higher immunomodulatory abilities by promoting lymphocyte proliferation than the natural polysaccharides [10]. And, Li et al. also reported that chemically selenized polysaccharides in Lily bulb (SLP) could significantly improve the index of immune organs and increase the contents of interferon and immunoglobulin in serum as compared to the unmodified polysaccharides [20]. Therefore, the artificial selenization of natural polysaccharides provided additional advantages for the application of polysaccharides.

The Morchella sextelata species has been used in this study owing to its nutraceutical and pharmaceutical importance in China. And, M. sextelata is one of the rarest, edible, and highly priced morels which are not yet cultivated on a large scale in China. Moreover, there are no reports focusing on the selenylation of morel polysaccharides.

In this study, the selenized polysaccharide (SeMSP-4) was prepared from M. sextelata polysaccharides based on the HNO3-Na2SeO3 method. The physicochemical features and structural characteristics of SeMSP-4 were determined. The antioxidant activity, cytotoxicity, and immunomodulatory ability of polysaccharides have been explored. These results will contribute to the development of new functional agents using SeMSP as a major ingredient.