Mycorrhizal infection rate is an index to measure the affinity between mycorrhizae and host plants [27]. The infection rate is affected by various factors, such as soil nutrients, types and concentrations of pollutants, and the combination of strains and host plants [28]. In this study, we found that in the light rare earth elements La and Ce polluted soil, the plants developed symbiotic relationships with mycorrhizae, indicating that the mycorrhiza could survive and reproduce under the certain concentration of LREE contamination in the soil [26]. The infection rate of ectomycorrhiza in P. tabuliformis ranged from BLS 4.36–64.81%, ILK 2.57–42.23%, RFP 0.51–39.03%, and LSF1.18 -34.16%. The mycorrhizal infection rates in this study were slightly lower in comparison to previously published studies. Studies have shown that the infection rate of endomycorrhiza in Zea mays in Ce contaminated soil ranges from 5.29 to 78.79% [13]. Similarly, the infection rate of mycorrhiza in Glycine max L. in Ce contaminated soil is 67.46% [29]. The primary reason for this variation lies in the high concentration of LREE in the soil tested in this study. In addition, this study found that both the biomass of above-ground organs and the roots of the plants decreased significantly with the increase of LREE concentration. When the concentration of element La increased by about 19 times and the concentration of element Ce increased by about 11 times, the dry weight of above-ground organs in P. tabuliformis increased by approximately 36.25%, the dry weight of roots increased by approximately 21.31%, the dry weight of above-ground organs in L. bicolor increased by 31.66–47.21% and the dry weight of roots increased by 32.61–40.06%. Our study showed a lower biomass increase compared to previously published data. The dry weight of above-ground organs and roots of Lolium perenne and Cynodon dactylon. Inoculated with mycorrhiza Glomus aggregatum and Glomus constrictum increased by 203% vs. 482% and 119% vs. 291%, respectively [29]. Inoculation of mycorrhiza Glomus versiforme increased the dry weight of organs and roots of Elymus dahericus by 83% and 103% [30]. The fresh weight of Brassica campestris decreased by about 76% when the Ce element concentration increased from 5 mg/L to 100 mg/L [31]. Increased Ce content in soil resulted in decreased biomass of host plant Raphanus sativus L. [32]. In this study and other studies, the "Hormesis" effect of mycorrhizal infection rate increasing first and decreasing later appeared with the increase of LREE concentration [33]. Therefore, it is feasible to repair the symbiosis between mycorrhiza and host plants in the soil contaminated with a certain concentration of LREE.
The results of this study showed that after the inoculation of mycorrhiza in P. tabuliformis and L. bicolor, the content of La and Ce LREE in the plants increased with the content of LREE in the soil. Other studies have found that the La element significantly increased in host plants after mycorrhiza inoculation in soils contaminated with La element [34]. When the soil contains a low concentration of La element, mycorrhiza inoculation can increase the content of La in Astragalus sinicus [35]. The above results show that mycorrhizal inoculation can increase the host plant's LREE absorption at certain LREE concentrations. The absorption mechanism may be that mycorrhizae increase the channels through which host plants absorb nutrients such as phosphorus, and at the same time increase the absorption of light rare earth elements. Therefore, the mechanism and interaction of mycorrhizal absorption of LREE should be further investigated.
In this study, we found that P. tabuliformis reduced the transfer coefficients and increased the root retention coefficients following mycorrhizal inoculation. Some studies have found that in soils contaminated with elements La and Pb (lead) at different concentrations, mycorrhizal inoculation significantly reduced the Pb content in above-ground organs of maize and increased the Pb content in roots, effectively inhibiting the transfer of Pb pollutants from roots to above-ground organs [36]. Other studies have shown that endomycorrhizal inoculation in host plants can catalyze the synthesis of chelated synthase (PCs), chelate heavy metal ions in the cytoplasm, transfer heavy metal ions to plant vacuoles, and effectively improve host plant tolerance [37]. Under the chelation of mycorrhizal hyphae and glomamycin, heavy metal ions are fixed to produce filtration effect, reducing the heavy metal ions entering the host plants, thus reducing the transfer ability [38]. Mycorrhizae influence Pb enrichment in plants, and a large amount of Pb is fixed in the root by mycelia and vesicles, which reduce the transfer rate from root to above-ground organs [39], consistent with the results obtained in this study. Therefore, mycorrhizae have a negative effect on the host plant's transfer of LREE in La and Ce polluted soil, leading to a decrease in the transfer ability of hyperaccumulators. This may be related to the interaction between mycorrhizae, host plant, LREE, and heavy metal ions. It is therefore necessary to study the mechanism of the absorption, transfer, and retention of mycorrhizae-inoculated hosts in LREE contaminated soil.