Review of research on copper stress in Citrus

Abstract: Copper (Cu) is an element necessary for citrus, which plays an important role in the process of citrus growth and development. Copper fungicides have been widely used to control citrus canker and anthracnose since the 19 th century, resulting in accumulation of copper in soils of citrus orchards.Otherwise, the available copper content of virgin soil in the citrus production areas of China is relatively low. Leaf nutrient of citrus was influenced by soil nutrient status. Copper deficiency and excess would cause stresses in citrus orchards. This paper summarized the effects of copper stress on citrus growth and metabolism, the factors effecting copper stress, and the tolerance of citrus rootstocks to copper stress. The research progress of heavy metal stress-associated proteins was also introduced. Copper participates in several physiological processes of plants. Cu-deficient plants exhibit a reduction in electron transport during photosynthesis due to decreases of plastocyanin synthesis and the contents of chlorophylls and carotenoids. Copper as a cofactor of enzymes, has an influence on the activity of antioxidant enzyme that scavenge reactive oxygen species (ROS) . Copper deficiency affects the lignification process by decreasing lignin, hemicellulose and acetyl contents of cell walls. The uptake of zinc is inhibited in copper-deficient plants. Citrus orchards with high organic matter content, high p H, and rarely use of copper fungicides, are more likely to have Cu-deficient. Visual symptoms of deficiency are characterized by curved or“S-shaped”branches with large, dark green and overdeveloped leaves. In severe cases, reddish brown droplets of gum cover the twigs. As the deficiency becomes acute, the twigs start to die. Copper toxicity in plants affects physiological and biochemical processes, resulting in the inhibition of shoot and root growth. Leaf chlorosis, black and shorter roots have been observed on copper toxicity plants. The excess Cu decreases the concentration of chlorophyll, which results in a reduction in the en-ergy transfer from the antennae of the PSII to the reaction centers. And net photosynthetic rate is significantly reduced. Furthermore, excess copper reduces the ability of the plant to explore the soil for water and essential nutrients. It has been reported that high copper inhibits the uptake and activity of Zn2 +, Fe2 +, Mg2 +, et al. An excess of Cu also leads to serious damage to the root and leaves ultra-structure of citrus. The membrane system of chloroplast and mitochondria is damaged greatly. A thin and twisted cell wall, degeneration of the middle lamella, cellular plasmolysis can be observed in the root cell of copper toxicity plants. Many researches revealed that high copper stresses would result in oxidative stress due to enhanced ROS production. In order to scavenge ROS and alleviate their deleterious effects in cells, plants have increased activities of SOD, CAT and peroxidases (APX and POX) . The availability and mobility of Cu in the soils are related to organic matter content and soil p H. High organic matter content and high p H could reduce the availability of copper in soil. It can effectively alleviate the copper toxicity of citrus when soil p H was 6.5 or over 6.5. The application of fertilizer can also affect the uptake of copper by citrus. Nitrogen (N) fertilizer applied in the NH4+form would result in some degree of soil acidification, which could enhance the leaching of nutrients in sandy soil. Excessive applications of N and P fertilizers in citrus orchards would cause copper deficiency. Previous study showed that increased Ca availability in the rooting environment would ameliorate the effects of Cu phytotoxicity.And sufficient P supply has positive effect on citrus seedling under excess copper. The interaction of rootstock with Cu toxicity in citrus trees has been evaluated in some researches. Results showed that, ‘Swingle citrumelo'rootstock might have better tolerance than‘Sunki mandarin'and‘Rangpur lime'.And rough lemon is more tolerant to Cu phytotoxicity than‘Cleopatra mandarin'. Plants have developed various mechanisms, including avoidance of uptake, chelation, efflux, autoxidation, and intracellular sequestration, to overcome Cu toxicity. The storage of Cu in the root cell walls and vacuole may keep the ion sequestered from the metabolic center. The conversion of Cu2 +to no-toxic complex compound by protein, organic acid or phenolic compounds, is one of the important detoxification mechanisms. The improvement of antioxidant system is also a manifestation of alleviating Cu stress. Heavy metal binding proteins and transporters play important roles in resistance to copper stress. Various protein families related to Cu uptake and transportation were found in plants. Cs MT1 was identified in sweet orange, and mainly expressed in leaves and flowers. COPT gene family encode Cu transporter with high affinity, and 6 COPT family members has been found in Arabidopsis. Researches indicated HMA1 and HMA5-HMA8 were related to Cu transport between organelles. Recently, 12 MTP genes were identified in sweet orange, named as Cit MTP1 and Cit MTP3 to Cit MTP12 based on their sequence similarity to Arabidopsis thaliana MTPs.