Advances in the intercropping remediation of heavy metal polluted soil

Phytoextraction is an efficient, novel, economic, green, and low-risk method for metal-polluted soil remediation that harvests metal hyperaccumulators to remove heavy metals from the soil. The cultivation of a single hyperaccumulator for the remediation of heavy metal-polluted soil not only interrupts agricultural production, leading to economic loss, but also results in low remediation efficiency owing to many disadvantages, such as low biomass and long remediation cycle. As a traditional agronomic management method, intercropping can improve the utilization efficiency of resources and increase the quality of co-planted crop species by using the principles of ecological niche and biodiversity. For the remediation of moderately or lightly metal-polluted soil, an intercropping system can be used to increase the concentrations of heavy metals in hyperaccumulators by regulating the growth and development of the hyperaccumulators and crops. Furthermore, the antioxidative ability of the hyperaccumulators and crops is also improved, which decreases the contents of peroxidation products, such as malondialdehyde and reactive oxygen species, in the cell membrane lipids. Intercropping generally enhances low molecular weight organic acid (LMWOA) secretion from the roots of heavy metal hyperaccumulators, decreases the pH value of rhizospheric soil, increases the activity of heavy metals, and consequently promotes heavy metal uptake by hyperaccumulators. However, LMWOA secretion from the crop roots is inhibited, resulting in decreased heavy metal uptake and improved crop yield and quality. Decreased heavy metal uptake by crops reduces the risk to human health, and the increased metal accumulation in hyperaccumulators enhances the removal of heavy metals from the soil. Moreover, the benefits to farmers are not affected or may even increase when using intercropping remediation technology. Therefore, the land utilization rate and economic benefits increase based on the "production while remediated" approach. This study systematically reviewed changes in biomass, physiological and biochemical responses, heavy metal uptake, translocation, and accumulation in hyperaccumulators and crops, as well as the effects of intercropping on soil environmental quality. While many studies examining the effects of intercropping systems on heavy metal hyperaccumulators and crops had focused on growth and development, metal uptake, translocation, accumulation, and physiological and biochemical responses to heavy metal stress, little information was available on the underlying molecular mechanisms of the physiological and biochemical processes. Additionally, the effects of intercropping on the microbial composition of the rhizosphere of heavy metal hyperaccumulators and crops and the related ecological implications and main function mechanisms remained unclear. From these unsolved questions, future perspectives in this field, such as the signal transduction and molecular mechanisms of the intercropping system of hyperaccumulators and crops, the different and functional mechanisms of rhizosphere microorganisms of two plants, and how to construct an efficient intercropping system to improve the remediation efficiency of heavy metal-polluted soil, were also proposed.