A novel evaluation of the effect of lanthanum exposure on plant populations

Addressing lanthanum toxicity in plants: Sources, uptake, accumulation, and mitigation strategies

Lanthanum (La), the second most abundant rare earth element (REE) is emerging as an environmental issue, with the potential to impact ecosystems and human health. Major sources of soil contamination by La include agricultural, and industrial activities. Lanthanum is non-essential for plant growth but accumulates in various plant parts. The uptake of La by plants is intricately influenced by various factors such as soil pH, redox potential, cation exchange capacity, presence of organic acids and rhizosphere composition. These factors significantly impact the availability and absorption of La ions. Lanthanum impact on plants depends on soil characteristics, cultivated species, developmental stage, La concentration, treatment period, and growth conditions. Excessive La concentrations affect cell division, DNA structure, nutrient uptake, and photosynthesis and induce toxicity symptoms. Plants employ detoxification mechanisms like vacuolar sequestration, osmolyte synthesis, and antioxidant defense system. However, higher concentrations of La can overwhelm these defense mechanisms, leading to adverse effects on plant growth and development. Further, accumulation of La in plants increases the risk for human exposure. Strategies to mitigate La toxicity are, therefore, vital for ecosystem protection. The application of phytoremediation, supplementation, chelation, amendments, and biosorption techniques contributes to the mitigation of La toxicity. This review provides insights into La sources, uptake, toxicity, and alleviation strategies in plants. Identifying research gaps and discussing advancements aims to foster a holistic understanding and develop effective strategies for protecting plant health and ecosystem resilience against La contamination.

Biostimulant Response of Foliar Application of Rare Earth Elements on Physiology, Growth, and Yield of Rice

Rare earth elements (REEs) have been intentionally used in Chinese agriculture since the 1980s to improve crop yields. Around the world, REEs are also involuntarily applied to soils through phosphate fertilizers. These elements are known to alleviate damage in plants under abiotic stresses, yet there is no information on how these elements act in the physiology of plants. The REE mode of action falls within the scope of the hormesis effect, with low-dose stimulation and high-dose adverse reactions. This study aimed to verify how REEs affect rice plants' physiology to test the threshold dose at which REEs could act as biostimulants in these plants. In experiment 1, 0.411 kg ha-1 (foliar application) of a mixture of REE (containing 41.38% Ce, 23.95% La, 13.58% Pr, and 4.32% Nd) was applied, as well as two products containing 41.38% Ce and 23.95% La separately. The characteristics of chlorophyll a fluorescence, gas exchanges, SPAD index, and biomass (pot conditions) were evaluated. For experiment 2, increasing rates of the REE mix (0, 0.1, 0.225, 0.5, and 1 kg ha-1) (field conditions) were used to study their effect on rice grain yield and nutrient concentration of rice leaves. Adding REEs to plants increased biomass production (23% with Ce, 31% with La, and 63% with REE Mix application) due to improved photosynthetic rate (8% with Ce, 15% with La, and 27% with REE mix), favored by the higher electronic flow (photosynthetic electron transport chain) (increase of 17%) and by the higher Fv/Fm (increase of 14%) and quantum yield of photosystem II (increase of 20% with Ce and La, and 29% with REE Mix), as well as by increased stomatal conductance (increase of 36%) and SPAD index (increase of 10% with Ce, 12% with La, and 15% with REE mix). Moreover, adding REEs potentiated the photosynthetic process by increasing rice leaves' N, Mg, K, and Mn concentrations (24-46%). The dose for the higher rice grain yield (an increase of 113%) was estimated for the REE mix at 0.72 kg ha-1.

Foliar spraying of lanthanum activates endocytosis in lettuce (Lactuca sativa L.) root cells, increasing Cd and Pb accumulation and their bioaccessibility

Cadmium (Cd) and lead (Pb) accumulate easily in leafy vegetables and can harm human health. Lanthanum (La) have been used to improve agricultural yield and quality, but the effect of La application on Cd/Pb enrichment in leafy vegetables remains incomplete currently. A previous study reported that the endocytosis in lettuce leaf cells can be activated by La, leading to an increase in Pb accumulation in lettuce leaves. However, it has not been investigated whether foliar application of La enhances root cellular endocytosis and promotes its uptake of Cd and Pb. In this study, the influence of La on the uptake of Cd and Pb, Cd bioaccessibility, and the safety risks of cultivating lettuce under Cd and Pb stress were explored. It was found that La increased Cd (16-30 % in shoot, 16-34 % in root) and Pb (25-29 % in shoot, 17-23 % in root) accumulation in lettuce. The increased accumulation of Cd and Pb could be attributed to La-enhanced endocytosis. Meanwhile, La enhanced the toxicity of both Cd and Pb, inhibited lettuce growth, and aggravated the damage to the photosynthetic and antioxidant systems. Finally, gastrointestinal simulation experiments showed that La increased the Cd bioaccessibility in both gastric and intestinal phase by 7-108 % and 9-87 %, respectively. These results offer valuable insights into the safety of REEs for agricultural applications.

Foliar application of lanthanum promotes growth and phytoremediation potential Solanum nigrum L

The hormetic effect of rare earth elements (REEs) has been found in a variety of crops and has been promoting crop growth for decades. Spraying leaves with REEs can enhance the endocytosis of plant roots. The non-selectivity of endocytosis is conducive to the direct absorption of environmental pollutants. The hyperaccumulator Solanum nigrum L. (S. nigrum), as a plant with high biomass and heavy metal tolerance, is a good candidate for phytoremediation. La(III), as a typical light REE, also has an obvious hormetic effect on S. nigrum. At 10 μM La(III), the biomass of S. nigrum reached the maximum, which was 89% greater than the control, and La(III) concentration was much lower than the previously reported optimum of 56 μM for general plants. In the present study, enhanced endocytosis after foliar spraying of La(III) was firstly observed in the root cell of hyperaccumulation plants, and La(III) increased the biomass of S. nigrum by improving the photosynthetic system, and promoting nutrient uptake and root development. The antioxidant defense system improved by La(III) contributed to the tolerance of S. nigrum to heavy metals. Applying a reasonable range of La(III) is beneficial to improving S. nigrum growth and tolerance of heavy metals. Compared with spraying deionized water, the translocation factor and bioaccumulation factor value of S. nigrum to cadmium increased by 15% and 21% respectively when spraying 10 μM La(III). Our findings provide a reference for improving hyperaccumulator plant growth and biomass, which improves phytoremediation efficiency.

Physiochemical responses of earthworms (Eisenia fetida) under exposure to lanthanum and cerium alone or in combination in artificial and contaminated soils

Rare earth elements inevitably release into the soil due to their widespread application. However, it is unclear how they affect the soil animals. The study surveyed the growth and physiological responses of earthworm (Eisenia fetida) exposed into artificial soils spiked with La, Ce, and their mixture, and actual mine soil collected from an abandoned La-Ce mining area (Mianning, Sichuan). The results showed that the 1000-1200 mg/kg combined exposure in two soils induced significant histopathological and phenotypic changes of earthworms. Concentration significantly affected the superoxide dismutase (SOD), peroxidase (POD), malondialdehyde (MDA), and protein of E. fetida and the effects differentiated with the prolonging duration. These indicators were negatively affected under the La stress ≥800 mg/kg (SOD, POD, and protein), the 1200 mg/kg (SOD), Ce stress ≥1000 mg/kg (protein), and the combination ≥800 mg/kg (SOD, POD) and ≥1000 mg/kg (protein). Artificial combination had -15.04% (SOD), 8.87% (POD), 5.64% (MDA), and -8.34% (protein) difference compared with the contamination soil, respectively. Overall, E. fetida respond sensitively under the La and Ce stress, the antioxidant defense system and the lipid peroxidation were stimulated, and the artificial soil might overestimate eco-toxicological effect.

Effects of exogenous lanthanum(III) exposure on the positive interaction between mutually beneficial populations

Rare earth elements (REEs) are widely used in various fields, and their accumulation has been reported to pose environmental risks. Most studies confirmed the damage of excessive REE exposure to individual plants; however, little attention has been given to their effects on plant populations. A positive interaction indicates a mutually beneficial relationship between two populations, which is beneficial to the survival and growth of the populations. However, it remains unknown whether exogenous REEs affect the positive interactions between populations. This study investigated the effects of exogenous lanthanum(III) [La(III)] exposure on the positive interaction between soybean (Glycine max L.) and wheat (Triticum aestivum L.) populations by their modules. At normal nutrient level (½-strength Hoagland), the inhibition of excessive La(III) on population modules decreased with increasing population density. Decreases of 39.26 to 1.05% for soybean and 41.45 to 2.41% for wheat indicated the inhibition of La(III) on the positive interaction of both populations weakened with increasing population density. At low nutrient level (¼-strength Hoagland), the inhibition of excessive La(III) on population modules increased with increasing population density. Decreases of 5.82-57.14% for soybean and 4.22-59.04% for wheat indicated the inhibition of La(III) on the positive interaction of both population was strengthened with increasing population density. In summary, the inhibitory effects of exogenous La(III) exposure on the positive interaction between populations vary with both nutrient level and population density. This is a new factor that needs to be considered when evaluating the safety risks of REEs in the environment.

A living plant cell-based biosensor for real-time monitoring invisible damage of plant cells under heavy metal stress

Heavy metals inevitably cause invisible or visible damage to plants, leading to significant economic losses. Therefore, it is necessary to develop a method for timely monitoring the damage of plants under the stress of heavy metals. Here, vitronectin-like proteins (VN) on the surface of plant cells is as an important biomarker for monitoring damage of plants under the stress of heavy metals. A living plant cell-based biosensor is constructed to monitor invisible damage of plant cells induced by cadmium [Cd(II)] or lead [Pb(II)]. To fabricate this sensor, l-cysteine was first modified on the glassy carbon electrode followed by the modification of anti-IgG-Au antibody. Then, the living plant cells, incubated with the anti-VN, were modified onto the electrode. The sensor worked by determining the change in electrochemical impedance. Cd(II) and Pb(II) was detected in the linear dynamic range of 45-210 and 120-360 μmol·L^-1, respectively. And the detection limit of Cd(II) and Pb(II) of this biosensor was 18.5 nmol·L^-1 [with confidence interval (95%) 18.4-18.6 nmol·L^-1] and 25.6 nmol·L^-1 [with confidence interval (95%) 25.4-25.8 nmol·L^-1], respectively. In both Arabidopsis and soybean, when the content of VN increased by about 20 times under the stress of Cd(II) or Pb(II), which means when the electron-transfer resistance increased by 35%, chlorophyll content showed significant decrease about 17%. Therefore, by establishing a quantitative relationship among the content of biomarker, the electron-transfer resistance and chlorophyll content in plant cells, the invisible damage of plants under the stress of heavy metals was detected. These results can provide a reference method for early-onset warning systems for heavy metal pollution in the environment.

Effects of nutrient level and planting density on population relationship in soybean and wheat intercropping populations

A positive interaction between plant populations is a type of population relationship formed during long-term evolution. This interaction can alleviate population competition, improve resource utilization in populations, and promote population harmony and community stability. However, cultivated plant populations may have insufficient time to establish a positive interaction, thereby hindering the formation of the positive interaction. As current studies have not fully addressed these issues, our study established soybean/wheat intercropping populations beneficial for growth and explored the effects of nutrient level and planting density on the positive interaction between the two crops. Changes across population modules in both sole cropping and intercropping populations of soybean and wheat were analyzed. Results using nutrient levels of ½- or ¼-strength Hoagland solution indicated that soybean/wheat intercropping population modules significantly increased at low planting densities (D20 and D26) and significantly decreased at high planting densities (D32 and D60). Therefore, as planting density increased, the modules of both intercropping populations initially increased before decreasing. Similarly, positive interaction initially strengthened before weakening. Moreover, at an intermediate planting density, the population modules reached their maxima, and the positive interaction was the strongest. Under the same planting density, ¼-strength Hoagland solution recorded better growth for the soybean/wheat intercropping population modules compared to results using the ½-strength Hoagland solution. These findings indicated that low nutrient level can increase the positive interaction of intercropping populations at a given planting density, and that environmental nutrient level and population planting densities constrain the positive interaction between soybean and wheat populations in the intercropping system. This study highlights issues that need to be addressed when constructing intercropping populations.

Effects of elevated ultraviolet-B radiation on root growth and chemical signaling molecules in plants

Ozone layer depletion leads to elevated ultraviolet-B (UV-B) radiation, which affects plant growth; however, little is known about the relationship between root growth and signaling molecules in roots. Therefore, in this work, simulated UV-B radiation was used to study the effects of elevated UV-B radiation on root growth of soybean seedlings and changes in the content of signaling molecules in roots. The results showed that compared with the control, the 2.63 kJ m-2 d-1 and 6.17 kJ m-2 d-1 elevated UV-B radiation treatments inhibited root growth, and root growth parameters (total root length, root surface area, root volume, average diameter, root tip number, and root dry weight) all decreased. For root signaling molecules, the content of nitric oxide, reactive oxygen species, abscisic acid, salicylic acid, and jasmonic acid increased, and the content of auxin, cytokinin, and gibberellin decreased. The above indices changed more significantly under the 6.17 kJ m-2 d-1 treatment. After withdrawal of the exposure, the above indices could be restored to a certain extent. These data indicated that UV-B radiation interfered with root growth by affecting the content of signaling molecules in roots, and the degree of the effects was related to the intensity of UV-B radiation. The results from this study provide a theoretical basis for studying the preliminary mechanism of elevated UV-B radiation on root growth and possible pathways that can mitigate UV-B radiation damage for root growth. ONE SENTENCE SUMMARY: The effects of elevated UV-B on root growth of soybean seedlings were regulated by signaling molecules, and the degree of the effects was related to the intensity of UV-B radiation.