Zinc is an essential trace element for human body, which can promote the growth and development of human body and enhance the immune function of human body(Jia et al.,2004). Zinc deficiency can lead to the decrease of lymphocytes and the irrecoverable damage of lymphatic system. However, nearly one third of the world’s population is facing health problems caused by zinc deficiency. Zinc in normal diet can not meet the normal needs of human body. In order to ensure the health of human body and prevent related diseases, adequate zinc intake should be provided. The sources of zinc include inorganic zinc and organic zinc. Inorganic zinc is not easy to be absorbed by the body, and excessive intake will produce toxicity. Biological organic zinc has the advantages of high absorption and utilization, safety and non-toxic, so the development of bio organic zinc health products is a safe and effective way to provide zinc source for the body. In order to transform zinc into organic zinc, it is necessary to find suitable zinc enrichment and transformation carriers. The results showed that Flammulina velutipes was an effective carrier for zinc enrichment and transformation (Liu et al.,2014). Our study found that Dendrobium also has the ability to enrich and transform zinc. Zinc can be distributed in different parts of Dendrobium and form zinc polysaccharide with Dendrobium polysaccharides, which provides a new way for zinc enrichment in plants and a new method for obtaining organic zinc from biological sources.

Dendrobium nobile is a kind of precious Chinese herbal medicine, which is traditionally used to nourish the body. Existing studies have shown that Dendrobium nobile has a variety of biological activities, including enhancing immunity. And Dendrobium polysaccharide is the main active component of Dendrobium nobile to enhance immunity. The data show that zinc plays an important role in maintaining the structure and function of the central immune organs (thymus, supraluminal SAC) and peripheral immune organs (lymph nodes, spleen, tonsil). Therefore, the zinc polysaccharide synthesized by zinc and Dendrobium polysaccharide can not only convert inorganic zinc into organic zinc, improve the absorption and utilization rate of zinc, but also greatly promote the enhancement of human immunity.

Therefore, a new plant Dendrobium nobile, which can enrich and transform zinc, has been found in the experiment. At the same time, Dendrobium polysaccharide, a new carrier for the transformation of inorganic zinc into organic zinc, has also been found.

4.5Dendrobiumnobile can reduce the damage of zinc stress to plants by chelating zinc with polysaccharide in vivo

Root is an important organ for plants to absorb water and mineral nutrients, and it is also an important part of soil heavy metal stress. We found that the zinc content in the root of Dendrobium nobile was significantly higher than that in the stem. For example, in the high concentration group (2000 μmol/L), the zinc content in the root was 6.3 times higher than that in the stem.

Heavy metal stress has a certain effect on plant growth. Reactive oxygen species (ROS) produced by plant cells under heavy metal stress can cause oxidative damage. In response to stress, plants produce antioxidants to resist ROS damage to cells. At the same time, plant cells will actively accumulate some soluble substances, such as soluble protein and soluble sugar, to reduce the intracellular osmotic potential, so as to ensure the normal water supply under heavy metal stress and maintain the normal physiological function of cells (Zhang et al., 2011). In addition, heavy metal chelation is also an important way for plants to alleviate heavy metal injury. For example, phytochelatins (PC) (Gupta et al., 2013) uses a large number of thiol groups in its structure to coordinate with heavy metal ions to form heavy metal PC chelates, which make heavy metal ions lose activity (Rama et al.,2010; Park et al., 2012), thus reducing the toxicity of heavy metals to plants. At present, some studies have found that the formation of PC can be promoted by adding Se, so as to alleviate the stress of heavy metals (Salt et al., 1995; Rana et al.,1996). Intracellular polyphosphate bodies can also chelate heavy metals. Under the stress of heavy metals, the number of polyphosphate bodies increases, and the phosphorus concentration in plants increases, and the increase of phosphorus concentration can reduce the toxicity of heavy metals to plants (Rai et al.,1981).

The results showed that the photosynthesis and carbohydrate synthesis of Dendrobium nobile increased significantly under the influence of zinc stress, and the biosynthesis of polysaccharide increased. The polysaccharide can combine with zinc well to form polysaccharide zinc chelate, which makes zinc ion lose its activity. This chelate has no toxicity to plants, and the polysaccharide-zinc chelate is transported through the vacuole membrane to be stored in the vacuole under the action of ATP, so as to reduce the damage to Dendrobium nobile and protect it (Fig. 7).