Effects of Arbuscular Mycorrhizal Fungi on the Growth and Root Cell Ultrastructure of Eucalyptus grandis under Cadmium Stress.

Eucalyptus grandis (E. grandis) has been reported to form a symbiosis with arbuscular mycorrhizal fungi (AMF), which plays an important role in improving plant tolerance of heavy metal. However, the mechanism of how AMF intercept and transport cadmium (Cd) at the subcellular level in E. grandis still remains to be researched. In this study, a pot experiment was conducted to investigate the growth performance of E. grandis under Cd stress and Cd absorption resistance of AMF and explored the Cd localization in the root by using transmission electron microscopy and energy dispersive X-ray spectroscopy. The results showed that AMF colonization could enhance plant growth and photosynthetic efficiency of E. grandis and reduce the translocation factor of Cd under Cd stress. After being treated with 50, 150, 300, and 500 µM Cd, the translocation factor of Cd in E. grandis with AMF colonization decreased by 56.41%, 62.89%, 66.67%, and 42.79%, respectively. However, the mycorrhizal efficiency was significant only at low Cd concentrations (50, 150, and 300 µM). Under 500 µM Cd concentration condition, the colonization of AMF in roots decreased, and the alleviating effect of AMF was not significant. Ultrastructural observations showed that Cd is abundant in regular lumps and strips in the cross-section of E. grandis root cell. AMF protected plant cells by retaining Cd in the fungal structure. Our results suggested that AMF alleviated Cd toxicity by regulating plant physiology and altering the distribution of Cd in different cell sites.

Cultivation of arbuscular mycorrhizal Broussonetia papyrifera seedlings by planting the mycorrhizal nurse plant downwards.

Plant mycorrhization can be achieved by transplanting new seedlings with mycorrhizal nurse plants; however, this method inevitably induces plant interactions. Transplanting nurse plants downwards may prevent light competition among new seedlings and nurse plants in the same pot. We hypothesized that seedling mycorrhization via mycorrhizal provision from plants planted downwards would be a feasible and efficient strategy. We used seedlings cultivated for 6 months after inoculation with arbuscular mycorrhizal fungi (AMF) as nurse plants, and seedlings cultivated for 1 month without AMF as recipient plants, transplanting one nurse plant and three recipient plants together in one pot. We compared two approaches for cultivating mycorrhizal Broussonetia papyrifera seedlings: planting mycorrhizal nurse plants upwards (M-NU) and downwards (M-ND). We also planted non-mycorrhizal nurse plants upwards (NM-NU) and downwards (NM-ND) as controls. We analyzed growth parameters and the mycorrhizal colonization status of recipient plants at 45, 60, and 75 days after planting (DAP). As expected, the plant growth, gas exchange, and root morphological parameters of recipient plants with mycorrhizal nurse plants were higher than those of recipient plants with non-mycorrhizal nurse plants at 60 and 75 DAP. Furthermore, the AMF colonization status and physiological growth status of M-ND recipient plants were improved compared with M-NU recipient plants. Our results demonstrate that inducing seedling mycorrhization by planting mycorrhizal nurse plants downwards is a feasible strategy for achieving AMF symbiosis while mitigating negative interactions among plants.

Transcriptional regulation of metal metabolism- and nutrient absorption-related genes in Eucalyptus grandis by arbuscular mycorrhizal fungi at different zinc concentrations.

BACKGROUND: Eucalyptus spp. are candidates for phytoremediation in heavy metal (HM)-polluted soils as they can adapt to harsh environments, grow rapidly, and have good economic value. Arbuscular mycorrhizal fungi (AMF) are the most widely distributed plant symbiotic fungi in nature, and they play an important role in promoting the phytoremediation of HM-polluted soils. However, few studies have evaluated the HM detoxification mechanism of E. spp. in symbiosis with AMF, and thus, the molecular mechanism remains unclear. RESULTS: The gene transcription and metabolic pathways of E. grandis were studied with and without inoculation with AMF and at different zinc (Zn) concentrations. Here, we focused on the transcript level of six HM-related gene families (ZNT, COPT/Ctr, YSL, ZIFL and CE). Under high-Zn conditions, thirteen genes (ZNT:2, COPT/Ctr:5, YSL:3, ZIFL:1, CE:2) were upregulated, whereas ten genes (ZNT:3, COPT/Ctr:2, YSL:3, ZIFL:1, CE:1) were downregulated. With AMF symbiosis under high-Zn conditions, ten genes (ZNT:4, COPT/Ctr:2, YSL:3, CE:1) were upregulated, whereas nineteen genes (ZNT:9, COPT/Ctr:2, YSL:3, ZIFL:4, CE:1) were downregulated. Under high-Zn conditions, genes of three potassium-related transporters, six phosphate transporters (PHTs), and two nitrate transporters (NRTs) were upregulated, whereas genes of four potassium-related transporters,four PHTs, and four nitrogen-related transporters were downregulated. With AMF symbiosis under high-Zn conditions, genes of two potassium-related transporters, six ammonium transporters (AMTs) and five PHTs were upregulated, whereas genes of six potassium-related transporters, two AMTs and five PHTs were downregulated. CONCLUSIONS: Our results indicates that AMF increases the resistance of E. grandis to high-Zn stress by improving nutrients uptake and regulating Zn uptake at the gene transcription level. Meanwhile, our findings provide a genome-level resource for the functional assignments of key genes regulated by Zn treatment and AM symbiosis in six HM-associated gene families and macromineral nutrient-related gene families of E. grandis. This may contribute to the elucidation of the molecular mechanisms of the response to Zn stress in E. grandis with AM symbiosis at the aspect of the interaction between HM tolerance and nutrient acquisition.