Research PaperEnhanced Cd efflux capacity and physiological stress resistance: The beneficial modulations of Metarhizium robertsii on plants under cadmium stress
Graphical Abstract
Introduction
Along with the rapid agricultural and industrial development, soil pollution by cadmium (Cd), a non-essential element for living organisms, has become increasingly serious. The electronic configuration of Cd resembles the trace essential element zinc, a cofactor of hundreds of proteins, and it is thus able to reduce or even eliminate activities of the zinc proteins (Wolfgang, 2016). Soil Cd poisons or even kills plants by reducing carbon assimilation, etiolating leaves, inhibiting photosynthesis, and promoting oxidative stress (Ding et al., 2016, Gutsch et al., 2020, He et al., 2019, Khanna et al., 2019). In addition, due to its high migration capacity, Cd can be accumulated in crops that in turn could toxify humans and animals. Therefore, Cd pollution in soil has been threatening food security (Wu et al., 2010).
Physical and chemical remediation approaches have been developed to reduce Cd concentration in polluted soil to a biological safety level (Liu et al., 2018, Wang et al., 2020). However, these approaches are usually time-consuming and costly, could also release new pollutants to soil. It has been recently documented that some plant-associated microbes are able to reduce Cd bioaccumulation and promote plant growth in Cd polluted soil (Gutsch et al., 2018, Jiang et al., 2016). These microbes can be developed as efficient and environmentally friendly biological agents for bioremediation of Cd-polluted soil. With a better understanding of the mechanisms by which plant-associated microbes impact Cd accumulation in plant cells, new strategies could be obtained to improve microbial efficacy of controlling risk of Cd-polluted soil.
Some microbes sequestrate Cd in rhizosphere soil by absorbing this heavy metal into cells, adsorbing it on the cell wall or precipitation, thereby directly obliviating Cd toxicity stress to plants (Chen and Cutright, 2003, Wu et al., 2010, Xu et al., 2012). Some other microbes reduce Cd bioaccumulation in plant cells by decreasing cell permeability to Cd via suppressing the expression of Cd transporters. For example, the endophytic bacterium Stenotrophomonas maltophilia R5–5 down-regulated the expression level of the Cd transporters OsNramp5 and OsHMA2 of Oryza sativa, resulting in the reduction of Cd accumulation in the root and blade (Zhou et al., 2020). AMF was also found to decrease the expression of the transporter Nramp5 in rice and thus reduce Cd concentration in the grain (Yang et al., 2020). In addition to Cd sequestration directly by microbes and microbe-induced reduction in plant cell permeability to Cd, microbes were also reported to reduce Cd toxicity to plant cells by Cd compartmentalization and enhancement of antioxidant system (Khan et al., 2017).
Metarhizium fungi are well-known insect pathogens, and some species are also plant symbionts with a variety of hosts including Arabidopsis thaliana, wheat, corn, soybean (Lahey et al., 2020, Liao et al., 2017; Leger St and Wang, 2020). The symbiotic Metarhizium species can facilitate plant growth and promote plant health even under abiotic stresses such as salt stress and poor nutrition (Liao et al., 2017, Liao et al., 2014). It has been recently documented that M. anisopliae is also able to absorb Cd and Pb in waste water (Khalid et al., 2011), and that some other Metarhizium strains could improve plant tolerance to heavy metals, and thus boost plant growth (Farias et al., 2019). In this study, we found M. robertsii reduced Cd accumulation in roots and leaves of A. thaliana and O. sativa and promoted plant health by increasing PCR Cd efflux proteins and the total antioxidant capacity.
Section snippets
Fungi and plants
M. robertsii ARSEF2575 was obtained from United States Department of Agriculture/Agricultural Research Service (USDA/ARS) collection. A. thaliana Columbia-0 was obtained from Weidi Biotech (Shanghai, China), and the mutants of the genes PCR3, PCR9 and PCR12 were obtained from the Arashare Center (Fuzhou, China); the mutants are Salk T-DNA homozygous lines, which was further confirmed in this study using PCR with the primers suggested by the supplier (Fig. S1). All the primers used in this study
Colonization by M. robertsii promoted plant growth and reduced Cd accumulation in A. thaliana
After 14 d growth on the solid 1/2 MS medium containing Cd (50 μM), the A. thaliana leaves were etiolated, an important sign of Cd toxicity, while the leaves of the plants colonized by M. robertsii were obviously greener (Fig. 1a). Consistent with this result from the visual observation, the SPAD value of the leaves of the plants colonized by M. robertsii was 26.02 ± 1.33, which was significantly higher than that (24.10 ± 1.60) of the leaves without the fungal treatment (Fig. 1b). The
Discussion
A variety of plant-associated microbes including fungi and bacteria have been documented to reduce Cd accumulation in plants, and they achieve this either through sequestrating Cd by themselves or modifying plant physiological status. Although the impacts of the plant-microbe interactions on plant physiological status have been extensively studied, the mechanisms by which plant-associated microbes reduce Cd accumulation remain to be fully understood. In this study, we showed that the plant
CRediT authorship contribution statement
Xiaohan Jiang: Conceptualization, Investigation, Data curation, Writing − original draft. Jin Dai: Investigation, Methodology. Xing Zhang: Investigation, Methodology. Hanxin Wu: Investigation. JianHao Tong: Investigation. Jiyan Shi: Conceptualization, Supervision, Writing − review & editing, Funding acquisition. Weiguo Fang: Conceptualization, Supervision, Writing − review & editing, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This research was supported by the National Natural Science Foundation of China (41721001, 42077119 and 32172470).
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