重金属镉(Cd)在植物体内的转运途径及其调控机制

重金属镉(Cd)的毒害效应与其由土壤向植物地上部分运输有关,揭示Cd~(2+)转运途径及其调控机制可为提高植物抗镉性以及镉污染的植物修复提供依据。对Cd~(2+)在植物体内的转运途径,特别是限制Cd~(2+)移动的细胞结构和分子调控机制研究进展进行了回顾。Cd~(2+)通过共质体和质外体途径穿过根部皮层进入木质部的过程中,大部分在皮层细胞间沉积,少部分抵达中柱后转移到地上部分。为了免受Cd~(2+)的危害,植物体产生了多种限制Cd~(2+)吸收和转移的生理生化机制:1)环绕在内皮层径向壁和横向壁上的凯氏带阻止Cd~(2+)以质外体途径进入木质部;2)螯合剂与进入根的Cd~(2+)螯合形成稳定化合物并区隔在液泡中;3)通过H+/Cd~(2+)离子通道等将Cd~(2+)逆向转运出根部。植物共质体和质外体途径转运重金属镉的能力以及两条途径的串扰尚待进一步明晰和阐明。

Transport pathways of cadmium (Cd) and its regulatory mechanisms in plant

Heavy metal (HM) toxicity is a worldwide concern because it damages plants by altering their major physiological and metabolic processes. The heavy metal cadmium (Cd) is a nonessential element, and is a valid inhibitor of plant growth. The toxic effect of cadmium is closely related to its transfer from the soil to the plant above ground parts. Understanding the transport pathway and regulatory mechanism of cadmium in plants may improve plant resistance to this heavy metal, in addition to providing a theoretical basis for the phytoremediation soils contaminated by cadmium. In this paper, we reviewed the transport pathways of Cd²⁺ in plants and what limits its mobility based on the cytological structural and molecular regulation mechanism of plants. As the main organ for transporting water and nutrients to the plant body, the plant root is also the main organ that absorbs toxic metals, such as cadmium. During the process of Cd²⁺ transfer from the root cortex to the xylem, most Cd²⁺ is deposited between the cells of the root cortex, with some reaching stele, before being transferred to the plant organs, such as the leaves in the above ground part of the plant. The transport pathway of Cd²⁺ through the root cortex is mainly apoplastic, with the cytoplasmic accumulation of Cd²⁺ possibly causing apoplastic transport towards the vascular cylinder to decline. The transport pathway of Cd²⁺ in the vascular cylinder is also mostly apoplastic, with cytoplasmic accumulation reducing Cd²⁺ transfer to the xylem. Since the aboveground parts of plants are more susceptible to Cd²⁺ poisoning, two cellular strategies to restrict the absorption and transfer of cadmium have evolved. First, the Casparian strip surrounding radial wall and the endodermis wall prevents Cd²⁺ from entering the root xylem via the apoplastic pathway. In addition, the Casparian strip promotes Cd²⁺ transport via the endodermis, leading to vacuolar isolation and cytoplasmic precipitation. Second, heavy metal detoxification occurs by chelating Cd²⁺ to form stable compounds, which are then deposited inside the vacuole. Third, excess cadmium also activates oxidative stress defense mechanisms and the synthesis of heavy metal stress related proteins to minimize metal toxicity, which includes the use of metallothiones and ion channels, such as H⁺/Cd²⁺ binding or sequestrating Cd²⁺ into vacuoles. For systematic improvements in the phytoremediation of heavy metal pollution, a more comprehensive understanding of cellular mechanisms involved in Cd avoidance, uptake, transport, and accumulation is required. Furthermore, the excluder strategy by extensive sequestration and retranslocation of cadmium through symplastic and apoplastic pathways should be confirmed and explored in future studies.