Basis for the breeding of low-Cd wheat varieties

Among heavy metals, cadmium (Cd) is highly toxic to plants and it is even considered as one of the most toxic elements released into environments at very low concentrations. The development of industry and agriculture have led to the increase of Cd content in soil environment. Cd is released into the soil through application of phosphate fertilizer, animal manures, waste water etc. Cadmium is a non-essential element for plant nutrition but because of its strong toxicity can seriously affects crop growth and development. Due to the high mobility of Cd in soil, the concentration of this element above the critical level can strongly inhibit the growth of plants as well as damage cell structure by interfering with different biochemical and physiological processes. Accumulation of Cd to phytotoxic levels may cause significant growth and yield decrease. If plants are grown in soil contaminated with Cd, they produce products containing this heavy metal, and such plant products are the main source of Cd entering the human body through the trophic chains. Thus, Cd may be an element with high residue, difficult to degrade and easy to accumulate, which may seriously threaten the health of human beings and animals. Cereals such as wheat, rice and maize are the main food crops in the world. Among them, wheat is the source of staple food for more than half of the world's population. Compared with other heavy metals, cadmium is more easily absorbed and accumulated by wheat. This poses a serious threat to human health. Wheat products are the main source of Cd intake by human. Wheat mainly uptakes Cd through the root system, and then it migrates to the above-ground part, and finally accumulates in the wheat grain. Agronomic management practices have been used to reduce Cd uptake and toxicity in wheat. However, these measures could pose some problems, such as large investment, high energy consumption, difficult operation and easy to produce secondary pollution. Low-Cd wheat varieties are the most effective and economic way to reduce the risk of cadmium to human health associated with food consumption. In the traditional breeding process, the selection of Cd-tolerant wheat samples is carried out on the basis of morphological, physiological or biochemical characteristics associated with Cd stress. It is of great significance to study the molecular mechanism of Cd absorption, transport of wheat and the creation of wheat varieties with low Cd accumulation for ensuring food security and food safety. Using molecular breeding technology and their successful integration with traditional breeding methods to select crop varieties with low accumulation of Cd will have a potential impact on the development of low Cd wheat germplasm and important practical significance for ensuring safe agricultural production of Cd contaminated soil. The objective of the present review is to discuss the Cd impact on wheat growth and development, Cd toxicity and tolerance mechanisms and some possible breeding strategies to alleviate Cd toxicity in wheat. The paper reviewed the effects of cadmium on the growth and development of wheat, the absorption, transport and distribution of cadmium in wheat, the tolerance mechanism and the molecular biological level of cadmium in wheat plant. To provide strategies and possible schemes for breeding wheat varieties with low cadmium accumulation