BASIS FOR THE BREEDING OF LOW-Cd WHEAT VARIETIES

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

Cereal crops 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, and the annual world output is about 650 million tons. In other words, wheat food is the main source of Cd intake by human. Compared with other cereals, wheat mainly accumulates Cd through the root system, and migrates to the above-ground part, and finally accumulates in the wheat grain ( The problem must be confronted with reducing Cdcontaminated to be solved urgently. In recent years, agronomic management practices including plant growth regulators (PGRs), mineral nutrients, biochar, fertilizers, compost, crop rotation, cropping patterns, and microorganisms have been used to reduce Cd uptake and toxicity in wheat. Therefore, it is very important to study the molecular mechanism of absorption, transport and efflux of Cd from wheat and to develop wheat varieties with low Cd accumulation to ensure food safety.
The objective of the present review is to discuss the Cd influence on wheat growth and development, Cd toxicity and tolerance mechanisms and some possible breeding strategies to alleviate Cd toxicity in wheat.
1. Effects of Cd on growth and development of winter wheat plants

Effects on seed germination and seedling growth of winter wheat
The seed germination and seedling stage are the beginning of plant life cycle. The first organ that comes into contact with Cd in soil is the seed. Therefore, seed germination is the earliest stage to perceive Cd toxicity. In general, low concentration of Cd has little inhibitory effect on seed germination, and even promotes germination of some wheat varieties. With the increase of treatment concentration, Cd had a very strong negative effect on seed germination, which was inhibiting the growth of shoot and root system of wheat seedlings, and the accumulation of dry matter also decreased. (Zhang et al., 2002). The study showed that the treatment concentration of 0.03 mg/kg Cd promoted the growth and dry matter accumulation of wheat. When the concentration of Cd was over 0.03 mg/kg, the growth and dry matter accumulation of wheat were extremely reduced (Zhang et al., 2002). The accumulation of Cd near the growing point of radicle leads to the inhibition of amylase activity and starch hydrolysis in wheat cotyledon. As a result, the nutrients required for the growth of radicle and hypocotyl are not satisfied and the elongation is inhibited. Sfaxi-Bousbih A. (2010) reported that the transport of mineral elements and carbohydrates from cotyledon to cotyledon and radicle of soybean was inhibited, which affected the germination and growth of seeds.

Effects of Cd on the growth of plant organs
Cd toxicity increases with the increase of concentration. Within a certain range of concentration, Cd can promote the growth of some plants. However, with the increase of Cd concentration, the growth and development of plants are significantly inhibited by Cd, which is generally manifested as short stature of plants, dechlorination of leaves, slow growth and decline of biomass. Cd affects plant photosynthesis, membrane system, enzyme system in vivo and metabolism related to the physiological activities, which eventually show a decline in growth and yield (Amirjani et. al Under Cd toxicity, the reoperation amount and reoperation rate of pre-flowering storage substances in wheat leaves and other vegetative organs were significantly reduced, and the thousand seed weight also decreased with the increase of treatment concentration. Cd toxicity inhibited the differentiation of reproductive organs during the period from young spike phase to heading, resulting in the caryatization and abortion. After heading stage, Cd toxicity interferes with and inhibits the synthesis and accumulation of chlorophyll, soluble sugar, soluble protein and starch in wheat flag leaves, and interferes with the migration and redistribution of nutrients in the wheat (Awan et al. 2. Cd uptake, transport and distribution by wheat plant 2.1. Cd absorption by wheat plants Cadmium is high toxic metal and it effects plant growth and development through a number of mechanisms including water nutritional balance and production of reactive oxygen species (Amirjani, 2012;Liu et al., 2015). Among soil factors, soil salinity could shift the soil-solution chemical equilibrium in favour of more soluble Cd compounds like CdCl2 and CdCl + thereby enhancing its availability to plants (Li et al., 2014;Liu et al., 2007;Özkutlu & Kara, 2019). The lower adsorbing ability of these species to soil than free Cd 2+ ions increases Cd mobility at the soil-root interface. Moreover, these complexes can enhance transport of Cd across plasma membrane which results in increased soil-plant transfer of Cd under salinity. It was shown that combined stress of NaCl and Cd caused higher plasma membrane permeability and enhanced production of oxygen radicals and H2O2 in comparison to Cd and NaCl treatments alone in wheat (Muhling & Lauchli, 2003;Shafi et al., 2009).
In general, Cd is absorbed into plants through the root system. It was studied that the absorption of Cd by wheat roots at low Cd concentration was an active absorption process, and the energy required for transport was provided by the hydrolysis of ATP produced in the metabolic process, which was mainly reflected in the highly selective absorption of ions and the energy consumption mechanism (Abedi & Mojiri, 2020). There are various carrier proteins in plant root system, and each ion combines with its corresponding carrier protein (transporter) to form ions. Carrier complexes that transport ions into cells by means of metabolic energy. At high Cd concentrations, Cd absorption is a passive process involving diffusion, ion exchange and chelation. There is a cationic exchange between the internal tissues of the root and the rhizosphere of plants containing cadmium. This part of the absorption of Cd can be without Cd solution desorption from the epidermal cell walls come down. The other part is combined into irreversible macromolecules, and then Cd is absorbed in the root surface. The longer combined into irreversible the higher the proportion of large molecules. The process of diffusion is the process of entering Cd into the cell through the cell wall and cell membrane, which is energy independent and depends on the difference in concentrations of the medium (Greger & Lofstedt, 2004;Huang et al., 2020;Page & Feller, 2015). Root exudates and a series of changes induced by root exudates also affect the uptake of Cd by wheat. The release of root metabolites affect the uptake of Cd in wheat by influencing pH and Cd availability.
In addition, it has been reported that the variation of Cd uptake in over ground part of wheat at different growth stages was as follows: the late growth period was larger than the early growth period, the growth flourishing period was larger than the slow growth period, and the reproductive growth period was larger than the vegetative growth period. The Cd absorption amount and rate jointing stage and heading stage were significantly higher than those in other periods. Cd absorption was significantly positively correlated with dry matter weight gain (r = 0.91633), and absorption rate was significantly positively correlated with dry matter weight gain (r = 0.8003) (Jiang et al., 2007).

Cd transport and distribution in wheat
The accumulation of Cd in wheat depends on the transport of Cd from root to stem, while the accumulation of Cd in seeds depends on the transport of Cd from root to stem and the direct transport of Cd from root to stem to grain through the transport of lignin and phloem. (Abedi & Mojiri, 2020;Harris &Taylor, 2004;Hart et al., 2006). Transpiration and root pressure provide the impetus for this process. Concentration of Cd in wheat grains is not determined by the concentration of Cd in xylem, but mainly by the ability of Cd to be transported from xylem to the phloem of the spike. (Abedi &Mojiri, 2020; Riesen &Feller, 2005). Cd could be transported from the applied leaves to other phloem reservoir organs, such as new leaves (Cakmak et al., 2000).
It was found that Cd was transported in the soybean xylem in the form of cationic complex. This may be because xylem has a large number of amino acids and organic acids, and the metal complex formed by its combination with metal ions can avoid the obstacles to the transport of positively charged metal caused by the strong cation exchange ability of xylem cells, thus making it easier to transport. (Cataldo et al., 1988). Citric acid, low molecular weight dicarboxylic anions and inorganic cations in xylem fluid flow can also affect the transport of Cd. Citric acid can promote the transport of Cd in xylem and reduce the transport of Cd out of xylem (Senden et al., 1990;Senden et al., 1995).
Cadmium entry to the plant is possible through the xylem via symplastic transport and through apoplastic transport under high exposure (Dong et al., 2019). The accumulation of Cd in the grain was mainly transported into the grain by the phloem of flag leaves. The Cd in the leaves and stems can be redistributed to the seeds and the Cd in the seeds hardly transports to other parts. It can be speculated that the transport of Cd may be related to the transport of photosynthetic products. The re-transport of Cd into grains is also related to other metal ions. For example, zinc inhibits the phloem loading and transport of Cd, thus reducing the transfer of Cd from phloem transport to grains. J. J. Hart observed that Cd transported to wheat seeds might be related to phloemmediated Cd transport to the grain (Hart et al. 2006). The phloem is the key Cd transport into wheat seeds. In general cadmium easily reach plants via root uptake and translocation to stems and seeds because of its high mobility (Abedi & Mojiri, 2020).
Cadmium, as mentioned above, enters the plants first through the root system and it is transported to the shoots in an ionic form in the xylem and phloem over transporters and transpiration. Cadmium may cause a wide range of effects on plants, impacts plant metabolism and causes oxidative stress, nutrient uptake disbalance (Huang et al., 2020; Navarro-Leon et al., 2019). Cd can impact the antioxidant defense system of plant organism and induce the formation of reactive oxygen species (ROS), which causes oxidative stress in general, Cd is accumulated in roots or transported to stems, leaves, fruits and other organs after being absorbed by plant from the soil (Ullah Zaid Imdad et al., 2018). However, the accumulation of Cd in plants varies with different parts, varieties, ecotypes and species of the same species. In the same plant object, Cd accumulation caused by roots is usually higher than in stems, leaves, and grains. On the cell level Cd is mainly distributed in the plastids and in the cell wall, and some of it forms carbonate and phosphate precipitation. Leaf, root and stem are easily enriched with Cd in wheat plants, while lower Cd level in seeds is an immovable element, which accumulated more in senescent parts and could not be reused by other non-senescent organs. The filling period and jointing -heading period are the key periods for the control of Cd accumulation. At the early stage of filling, the accumulation of Cd in different organs of wheat is mainly higher in leaves than in stems, leaf sheaths and grains. The accumulated content of leaf in mature stage was higher than that of leaf sheath, grain and stem (Ullah Zaid Imdad et al., 2018).
3. Cd tolerance mechanisms in wheat plant Under the stress of Cd, it can stimulate the antioxidant defense system of plants, remove O 2-and H2O2 generated under the stress of Cd, maintain the balance of reactive oxygen metabolism, and protect the membrane structure, so as to enable plants to endure, reduce or resist stress injury to a certain extent. Therefore, increasing antioxidant enzyme activity is one of the main mechanisms of tolerance to Cd in plants, including wheat . Antioxidant defense systems include enzymes such as superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and non-enzymes such as ascorbic acid (AsA) and glutathione (GSH). It was used the method of direct stress of Cd on wheat seeds during germination to study that the activities of superoxide dismutase (SOD) and peroxidase (POD) increased with the growth of concentration, and malondialdehyde (MDA) content and cell membrane permeability also showed an increasing trend. The activity of POD and CAT increased with the increase of stress intensity, which indicated that the protective enzyme system in wheat was changed, which was also the protective response of plants to the adverse environment (Song et

Marker-Quantitative trait loci (QTL) analysis for Cd tolerance in wheat
The application of molecular marker-assisted selection in breeding has been paid more and more attention. Early in 1999, it was feasible to identify soybean germplasm using markerquantitative trait loci (QTL). Bai Yi Xiong (Bai et al., 2019) analyzed the genetic information of 113 barley materials by using QTL method. The development of molecular markers related to Cd accumulation capacity also has a certain basis. Major genes associated with Cd uptake and transport in Arabidopsis thaliana have been identified as ABC family (Chen, 2004 After digestion of PCR amplification products with restriction enzyme of Hpyl88 I, electrophoresis analysis showed that the marker could be used to detect two alleles of Cd absorption trait genes. About 96 wheat samples were successfully divided into low Cd absorption type or high Cd absorption type. Y.-Y. Kim isolated a TM20 gene from the cDNA library of a wheat root, which produced specific Cd (Cd (II) tolerance (Kim et al., 2008). These genes potentially related to Cd accumulation in wheat, which could be further developed to match molecular markers.
It is a new idea to use molecular marker technology to screen germplasm resources. Molecular markers are developed on the presence of abundant polymorphisms in genomic DNA. It is a new and reliable genetic marker that directly reflects the differences of biological individuals at the level of DNA. DNA Cd pollution hazards can also be controlled through rational management of fields, control of contaminated areas and restricted planting and production. 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 ( For the analysis of the source material, selection methods such as mass selection, pure line and periodic selection methods can be effectively used to obtain wheat varieties with low Cd content. Advances in genetics and molecular biology have expanded the possibilities for many modern selection methods that ensure wheat stability to Cd.
There are two key molecular approaches for estimating Cd stress in wheat: marker-selected and genomic selection The mutant of OsNARMP5 can significantly reduce the absorption of Cd by the root system of rice, thus reducing the content of Cd in the grain to below 3 % of the control. OsHMA3 of the p1b-atpase subgroup is a heavy metal ion pump mainly expressed in the root of rice, which is located on the vacuole membrane and mediated the enrichment of Cd in the vacuole of rice root cells.
The over-expressed plants can selectively reduce the accumulation of Cd in seeds. Low affinity cationic transport protein (LCT1) is a new transport protein cloned from wheat, which is mainly expressed in the root and leaf of wheat . After RNA interference with OsLCT1 gene in rice, there was no significant change in xylem mediated Cd translocation. However, phloem-mediated Cd translocation decreased significantly, and the content of Cd in seeds was reduced to half of the control, indicating that it may be involved in the process of Cd transport from xylem of large vascular bundles to phloem of dispersed small vascular bundles in stem nodes as well as the process of phloem-mediated Cd transport to grains.
The results showed that the study of the genes related to Cd stress played an important role in the development of new varieties of Cd tolerant crops, which laid a foundation for excavating and functional analysis of Cd stress related genes in wheat. Research in this direction provides insight into the screening of functional genes that respond to stress, which can be useful for analyzing and improving the resistance of crops to Cd stresses. Simultaneously, it opens up a new way of high speed, simple operation and low cost for further breeding high Cd tolerance wheat varieties, which is of great significance to improve the grain safety of wheat and promote the sustainable development of agricultural production.
Conclusion. Cd is one of the major inorganic contaminants in the environment. Its presence in the soil or atmosphere has been recognized as a serious threat to agriculture. It is a practical and feasible way to select wheat varieties with low accumulation of Cd to reduce the absorption and accumulation of Cd in crops and thus reduce the content of Cd in agricultural products. In summary, many important achievements have been reported on the injury of Cd to wheat and the tolerance mechanism of Cd to wheat.
However, the effect of Cd on the growth of wheat is very complex. Multiple benefits could be achieved by future critical research efforts including the following: • More detailed studies are needed to have a better understanding of Cd toxicity in wheat at the molecular level.
• The process and mechanism of the uptake, transport and accumulation of Cd by plants should be clarified, and some of these processes should be artificially regulated to improve the tolerance of plants to Cd pollution or reduce the absorption of Cd.
• If the grain yield of wheat varieties with low Cd absorption is low in the planting process.
• Which genes are activated if wheat is subjected to Cd stress ? How these genes work together to synthesize resistant proteins, peptides, amino acids, reducing sugars, and more.
• How to define a physiological index as the standard to measure the resistance and non-resistance to Cd in wheat in many physiological and biochemical reactions ?
If the research in these aspects can achieve breakthrough results, it will be another breakthrough direction of soil pollution treatment and utilization.