Here, it used to be an "arsenic land" where no grass grows,
Later, restored lush,
Comparison before and after the restoration of Shimen mining area (Image source: Hunan Daily · New Hunan Client News 2021-03-24 Correspondent Shen Yuqi)
It used to be the Shimen Xionghuang mining area in Hunan, the largest single arsenic mine in Asia, and the only medicinal realgar production area in China, with a history of more than 1,500 years of realgar mining. In 2011, the Shimen mining area was closed in accordance with the law. However, the waste gas, waste water and waste residue produced by long-term mining have a serious impact on the soil in the mining area and the surrounding 35 square kilometers of the mining area. Thanks to the great attention of the state and provinces (cities and counties), these more serious environmental pollution problems are being effectively controlled. An important part of the treatment project is the centipede grass.
So, what is so special about these centipede grasses that allows them to grow on the "arsenic ground"?
The centipede grass of "Dumei" in the real-yellow mining area
In the 90s of the last century, when collecting soil and plant samples at the realgar mine in Shimen County, researchers had already noticed a special fern that was widely distributed in the local realgar mining area. In the "arsenic field", where there is almost no grass, this gifted plant can grow freely. It is characterized by the slender leaves of the near-opposite pair like a pair of centipede feet, turned to the back of the leaves, the reddish-brown sporangia group and the common centipede pattern color is very similar, therefore, this plant has a very vivid name "centipede grass".
Centipede grass in the greenhouse of the South Garden of the National Botanical Garden (Photo source: Photo by Zhang He Zifan)
According to the botanical records, the scientific name of centipede grass (Pteris vittata L.) is "Centipede phoenix fern", commonly known as "cockscomb phoenix fern" and "centipede fern", which is a plant of the family Pterniaceae of the Fern phylum Equisetum Equisetum Hydrokeletidae Anchovy fern. Centipede grass is widely distributed in the tropical and subtropical regions of the continent, with the southern slope of the Qinling Mountains as the northern boundary, and the pantropical areas of the Old Continent. It is an indicator plant of calcareous soil and limestone; On calcareous soil or limestone, up to 2000 meters above sea level, it is also often found in stone crevices or walls, and the size of the body varies greatly in different habitats.
Realgar is the common name of tetraarsenic tetrasulfide, which can be oxidized into arsenic trioxide, a highly toxic ingredient in the air after heating to a certain temperature. Arsenic is a highly toxic element and a strong carcinogen, and the main component of "arsenic" is the compound of arsenic (arsenic trioxide), which is one of the oldest poisons. Ordinary plants wilt and die in the realgar mines, while centipede grass grows well in the realgar mines and appears to be completely unaffected by arsenic. This anomaly has attracted the attention of scientific researchers.
Possess special skills to become a natural arsenic super-accumulation plant
Through in-depth research on centipede grass, researchers found that centipede grass is a natural arsenic superaccumulation plant, which has three characteristics: "resistant", "more" and "fast".
"Resistance" means that centipede grass has strong arsenic resistance, and on soil with arsenic concentrations as high as 1500 mg/kg (25 times that specified in the Soil Environmental Quality Standards), centipede grass can still grow normally, and arsenic at a concentration of 1-5 mg/kg can cause arsenic poisoning symptoms in ordinary plants such as rice;
"More" means that centipede grass has a strong arsenic accumulation capacity, ordinary plants can usually only accumulate < 10 mg/kg of arsenic, the highest arsenic content in centipede grass is as high as 10,000 mg/kg, 200,000 times higher than that of ordinary plants, and the arsenic concentration in its feather leaves is higher than that of roots, and even higher than the arsenic concentration in soil, reaching more than 2% of dry weight;
"Fast" means that centipede grass has efficient arsenic transport efficiency, and a large amount of arsenic can accumulate in the aboveground part of centipede grass within 8 weeks, and most of it is stored in the aboveground feather leaves.
除蜈蚣草外，与之同属的Pteris longifolia（番蜈蚣凤尾蕨）、Pteris umbrosa（荫生凤尾蕨）和Pteris cretica（欧洲凤尾蕨/大叶井口边草）等和与之同科的Pityrogramma calomelanos（粉叶蕨）均可以超富集环境内的砷。
So, how does centipede grass absorb arsenic and ensure that it is not poisoned by arsenic? The accumulation of arsenic in Centipede grass mainly involves three links: arsenic absorption, long-distance transportation of arsenic, and separation of arsenic in leaves. And centipede grass has some special proteins, which make it a master of arsenic collection in every link.
1. Arsenic absorption
Aquaporin is a protein located on the cell membrane, which forms "pores" on the cell membrane, which can control the entry and exit of water, glycerol, ions, etc. in the cell. Researchers from the Institute of Botany, Chinese Academy of Sciences found that, compared with other plants, the aquaporin PvTIP4 of centipede grass; 1 It has stronger permeability, so it can absorb more arsenic.
2. Long-distance transportation of arsenic
In the roots, arsenic is transported from the epidermis and cortex to the column in the middle of the root, where it is loaded in the xylem and then moves up the xylem duct to the aboveground parts. Among them, xylem loading proteins play an important role in the efficient long-distance transport of arsenic in C. centipede. By efficiently loading arsenic into the xylem, the "highway" of arsenic from the underground to the aboveground is unimpeded, and arsenic is quickly accumulated into the leaves.
3. Arsenic differentiation in leaves
The vast majority of arsenic transported to the aerial parts in centipede grass is stored in vacuoles of pinnae cells. In this process, the vacuolar membrane acts as a "sandbox", isolating the contact between the organelles of centipede grass and arsenic, and ensuring that the organelles of centipede grass are not harmed by arsenic. In Centipede grass, PvACR3 is involved in root segmentation and is responsible for the lateral transport of As(III) in roots and xylem loading. PvACR3; 1. Participate in the differentiation of roots; PvACR3; 3. Participate in the segmentation of above-ground parts. The combination of multiple genes ensures that arsenic does not affect the normal life activities of centipede grass.
Fig.2 Schematic diagram of the process of arsenic hyper-enrichment in centipede grass (Adapted from Zhang Tian et al., Research Progress on the Molecular Mechanism of Arsenic Super-enrichment in Centipede Grass, Chinese Journal of Biotechnology, 2020)
The special protein leads to a stronger arsenic enrichment efficiency in C. centipede, and its super-accumulation characteristics may be related to the high arsenic environment during the Cretaceous-Paleogene boundary period: Centipede grass appeared around the Cretaceous-Paleogene boundary period, when the arsenic concentration in the geochemical environment was abnormally high, and in order to cope with the threat of arsenic to growth and development in the arsenic-rich environment, C. centipede evolved a mechanism to efficiently distinguish arsenic. The researchers also found that other plants of the genus Anchovy Fern also had arsenic superaccumulation characteristics at the same time, but the mechanism of this has yet to be studied.
Fix the soil, plants in action
In general, the pollution in the soil is difficult to remove through natural purification, and conventional physical and chemical methods such as soil replacement, landfill and chemical sedimentation are costly and inefficient, difficult to apply on a large scale and easy to cause secondary pollution. Phytoremediation is a kind of biotechnology that uses the metabolic activities of plant life to absorb and accumulate pollutants in the environment and reduce their toxicity, which has the advantages of good treatment effect, low cost, simple operation, small environmental disturbance, simple post-treatment and no secondary pollution.
In order to solve the problem of arsenic pollution in the Shimen mining area, since 2013, the research team of the Institute of Geographic Sciences of the Chinese Academy of Sciences has carried out the "R&D Demonstration of Heavy Metal Pollution Risk Zoning and Remediation Technology in Southern China" project in the Xionghuang mining area of Heshan Village.
After 1 year of planting Centipede grass, the arsenic content of the soil decreased by 10%, while the arsenic content of harvested Centipede grass leaves was as high as 0.8%; After 3 years, the soil arsenic content further decreased by about 30%; After 5 years of restoration, the average arsenic content of the soil met the requirements of the "Soil Environmental Quality Standard" (GB 15618-1995), and the farmland could be safely planted with ordinary crops after restoration.
Under the vigorous promotion of the Changde Municipal Government and the Chinese Academy of Sciences, the 4,476 acres of contaminated farmland around the Xionghuang mining area were intensively restored with centipede grass-activator and centipede grass-citrus intercropping restoration technology. Through the comprehensive treatment measures, the environmental quality of the Xionghuang mining area has been significantly improved, and the green vegetation has gradually recovered. After the successful pilot in Hunan, the phytoremediation technology for arsenic contaminated soil was further extended to 10 places across the country, including Yunnan, Guangxi, Henan, Hebei and Sichuan, to help solve the problem of arsenic pollution in the local area.
Centipede grass planting site of the first phase of soil remediation in male yellow mine (Photo source: May 2, 2018 Changde All Media)
However, the growing conditions of centipede grass itself are relatively demanding. As a fern, centipede grass grows mainly in the humid and warm environments of the south, which severely limits its application as a phytoremediation material. Therefore, screening and cultivating engineered plants with large biomass, fast growth rate and wide environmental adaptability has become a new research goal in the field of phytoremediation.
In the future, researchers can introduce super-enriched molecular elements from super-accumulated plants such as centipede grass into plant carriers with large biomass and wide adaptability from multiple perspectives to create "super phytoremediation engineering plants" with stronger resistance and higher accumulation capacity, and provide better "plant" solutions for soil arsenic pollution.
Phytoremediation concept art
Bibliography:
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Author: Zhang He Zifan, Yan Huili, He Zhenyan
Author Affilications:Institute of Botany, Chinese Academy of Sciences