The role of arbuscular mycorrhizal symbiosis in improving plant water status under drought

Advances and mechanisms of fungal symbionts in improving the salt tolerance of crops

Soil salinization leads to reduced crop yields and waste of land resources, thereby impacting global food security. To meet the increasing demand for food and simultaneously alleviate pressure on soil resources, the development of sustainable agriculture is imperative. In contrast to physical and chemical methods, bioremediation represents an efficient and environmentally friendly approach. Fungal symbionts have been found to be associated with most plants in natural ecosystems, colonizing and residing within the internal tissues of host plants. Moreover, the potential of fungal symbionts in improving saline-alkaline soil has been widely recognized and confirmed. Numerous reports have documented the effectiveness of arbuscular mycorrhizal fungi in alleviating salt stress in plants. Meanwhile, research on other endophytic fungi for mitigating plant salt stress has emerged in recent years, which contributes to refining mechanisms for enhancing plant salt tolerance. In this review, we summarized various mechanisms by which endophytic fungi enhance plant salt tolerance. We also provided an overview of the challenges and development directions in the field of fungal symbiosis, with the aim of offering a viable strategy for the bioremediation of saline-alkali soils.

Contrasting drought tolerance traits of woody plants is associated with mycorrhizal types at the global scale

It is well-known that the mycorrhizal type of plants correlates with different modes of nutrient cycling and availability. However, the differences in drought tolerance between arbuscular mycorrhizal (AM) and ectomycorrhizal (EcM) plants remains poorly characterized. We synthesized a global dataset of four hydraulic traits associated with drought tolerance of 1457 woody species (1139 AM and 318 EcM species) at 308 field sites. We compared these traits between AM and EcM species, with evolutionary history (i.e. angiosperms vs gymnosperms), water availability (i.e. aridity index) and biomes considered as additional factors. Overall, we found that evolutionary history and biogeography influenced differences in hydraulic traits between mycorrhizal types. Specifically, we found that (1) AM angiosperms are less drought-tolerant than EcM angiosperms in wet regions or biomes, but AM gymnosperms are more drought-tolerant than EcM gymnosperms in dry regions or biomes, and (2) in both angiosperms and gymnosperms, variation in hydraulic traits as well as their sensitivity to water availability were higher in AM species than in EcM species. Our results suggest that global shifts in water availability (especially drought) may alter the biogeographic distribution and abundance of AM and EcM plants, with consequences for ecosystem element cycling and ultimately, the land carbon sink.

Structural insights into rice KAI2 receptor provide functional implications for perception and signal transduction

KAI2 receptors, classified as plant α/β hydrolase enzymes, are capable of perceiving smoke-derived butenolide signals and endogenous yet unidentified KAI2-ligands (KLs). While the number of functional KAI2 receptors varies among land plant species, rice has only one KAI2 gene. Rice, a significant crop and representative of grasses, relies on KAI2-mediated Arbuscular mycorrhiza (AM) symbioses to flourish in traditionally arid and nutrient-poor environments. This study presents the first crystal structure of an active rice (Oryza sativa, Os) KAI2 hydrolase receptor. Our structural and biochemical analyses uncover grass-unique pocket residues influencing ligand sensitivity and hydrolytic activity. Through structure-guided analysis, we identify a specific residue whose mutation enables the increase or decrease of ligand perception, catalytic activity, and signal transduction. Furthermore, we investigate OsKAI2-mediated signaling by examining its ability to form a complex with its binding partner, the F-box protein DWARF3 (D3) ubiquitin ligase and subsequent degradation of the target substrate OsSMAX1, demonstrating the significant role of hydrophobic interactions in the OsKAI2-D3 interface. This study provides new insights into the diverse and pivotal roles of the OsKAI2 signaling pathway in the plant kingdom, particularly in grasses.

Unraveling root and rhizosphere traits in temperate maize landraces and modern cultivars: Implications for soil resource acquisition and drought adaptation

A holistic understanding of plant strategies to acquire soil resources is pivotal in achieving sustainable food security. However, we lack knowledge about variety-specific root and rhizosphere traits for resource acquisition, their plasticity and adaptation to drought. We conducted a greenhouse experiment to phenotype root and rhizosphere traits (mean root diameter [Root D], specific root length [SRL], root tissue density, root nitrogen content, specific rhizosheath mass [SRM], arbuscular mycorrhizal fungi [AMF] colonization) of 16 landraces and 22 modern cultivars of temperate maize (Zea mays L.). Our results demonstrate that landraces and modern cultivars diverge in their root and rhizosphere traits. Although landraces follow a 'do-it-yourself' strategy with high SRLs, modern cultivars exhibit an 'outsourcing' strategy with increased mean Root Ds and a tendency towards increased root colonization by AMF. We further identified that SRM indicates an 'outsourcing' strategy. Additionally, landraces were more drought-responsive compared to modern cultivars based on multitrait response indices. We suggest that breeding leads to distinct resource acquisition strategies between temperate maize varieties. Future breeding efforts should increasingly target root and rhizosphere economics, with SRM serving as a valuable proxy for identifying varieties employing an outsourcing resource acquisition strategy.

Arbuscular mycorrhizal fungi attenuate negative impact of drought on soil functions

Although positive effects of arbuscular mycorrhizal (AM) fungi on plant performance under drought have been well documented, how AM fungi regulate soil functions and multifunctionality requires further investigation. In this study, we first performed a meta-analysis to test the potential role of AM fungi in maintaining soil functions under drought. Then, we conducted a greenhouse experiment, using a pair of hyphal ingrowth cores to spatially separate the growth of AM fungal hyphae and plant roots, to further investigate the effects of AM fungi on soil multifunctionality and its resistance against drought. Our meta-analysis showed that AM fungi promote multiple soil functions, including soil aggregation, microbial biomass and activities of soil enzymes related to nutrient cycling. The greenhouse experiment further demonstrated that AM fungi attenuate the negative impact of drought on these soil functions and thus multifunctionality, therefore, increasing their resistance against drought. Moreover, this buffering effect of AM fungi persists across different frequencies of water supply and plant species. These findings highlight the unique role of AM fungi in maintaining multiple soil functions by mitigating the negative impact of drought. Our study highlights the importance of AM fungi as a nature-based solution to sustaining multiple soil functions in a world where drought events are intensifying.

Soil compaction reversed the effect of arbuscular mycorrhizal fungi on soil hydraulic properties

Arbuscular mycorrhizal fungi (AMF) typically provide a wide range of nutritional benefits to their host plants, and their role in plant water uptake, although still controversial, is often cited as one of the hallmarks of this symbiosis. Less attention has been dedicated to other effects relating to water dynamics that the presence of AMF in soils may have. Evidence that AMF can affect soil hydraulic properties is only beginning to emerge. In one of our recent experiments with dwarf tomato plants, we serendipitously found that the arbuscular mycorrhizal fungus (Rhizophagus irregularis 'PH5') can slightly but significantly reduce water holding capacity (WHC) of the substrate (a sand-zeolite-soil mixture). This was further investigated in a subsequent experiment, but there we found exactly the opposite effect as mycorrhizal substrate retained more water than did the non-mycorrhizal substrate. Because the same substrate was used and other conditions were mostly comparable in the two experiments, we explain the contrasting results by different substrate compaction, most likely caused by different pot shapes. It seems that in compacted substrates, AMF may have no effect upon or even decrease the substrates' WHC. On the other hand, the AMF hyphae interweaving the pores of less compacted substrates may increase the capillary movement of water throughout such substrates and cause slightly more water to remain in the pores after the free water has drained. We believe that this phenomenon is worthy of mycorrhizologists' attention and merits further investigation as to the role of AMF in soil hydraulic properties.

Mycorrhization enhances plant growth and stabilizes biomass allocation under drought

Plants and their symbionts, such as arbuscular mycorrhizal (AM) fungi, are increasingly subjected to various environmental stressors due to climate change, including drought. As a response to drought, plants generally allocate more biomass to roots over shoots, thereby facilitating water uptake. However, whether this biomass allocation shift is modulated by AM fungi remains unknown. Based on 5691 paired observations from 154 plant species, we conducted a meta-analysis to evaluate how AM fungi modulate the responses of plant growth and biomass allocation (e.g., root-to-shoot ratio, R/S) to drought. We found that AM fungi attenuate the negative impact of drought on plant growth, including biomass production, photosynthetic performance and resource (e.g. nutrient and water) uptake. Accordingly, drought significantly increased R/S in non-inoculated plants, but not in plants symbiotic with established AM fungal symbioses. These results suggest that AM fungi promote plant growth and stabilize their R/S through facilitating nutrient and water uptake in plants under drought. Our findings highlight the crucial role of AM fungi in enhancing plant resilience to drought by optimizing resource allocation. This knowledge opens avenues for sustainable agricultural practices that leverage symbiotic relationships for climate adaptation.

Enhancing plant resilience: arbuscular mycorrhizal fungi's role in alleviating drought stress in vegetation concrete

Introduction

Drought stress usually inhibits plant growth, which may increase the difficulty of greening slopes.

Methods

In this study, we systematically investigated the effects of arbuscular mycorrhizal (AM) fungi on the growth and drought tolerance of two plant species, Festuca elata and Cassia glauca, in a vegetation concrete environment by exogenously inoculating AM fungi and setting three drought levels: well water, moderate drought and severe drought. The results showed that plant growth was significantly inhibited under drought stress; however, AM fungi inoculation significantly promoted plant height, root length, and above- and belowground biomass in these two plant species.

Results

Compared with, those in the CK treatment, the greatest increases in the net photosynthesis rate, stomatal conductance and transpiration rate in the AM treatment group were 36.72%, 210.08%, and 66.41%, respectively. Moreover, inoculation with AM fungi increased plant superoxide dismutase and catalase activities by 4.70-150.73% and 9.10-95.70%, respectively, and reduced leaf malondialdehyde content by 2.79-55.01%, which alleviated the damage caused by oxidative stress. These effects alleviated the damage caused by oxidative stress and increased the content of soluble sugars and soluble proteins in plant leaves by 1.52-65.44% and 4.67-97.54%, respectively, which further increased the drought adaptability of plants. However, inoculation with AM fungi had different effects on different plants.

Conclusion

In summary, this study demonstrated that the inoculation of AM fungi in vegetation concrete environments can significantly increase plant growth and drought tolerance. The plants that formed a symbiotic structure with AM fungi had a larger root uptake area, greater water uptake capacity, and greater photosynthesis and gas exchange efficiency. In addition, AM fungi inoculation further increased the drought adaptability of the plants by increasing their antioxidant enzyme activity and regulating their metabolite content. These findings are highly important for promoting plant growth and increasing drought tolerance under drought conditions, especially for potential practical applications in areas such as slope protection, and provide useful references for future ecological engineering and sustainable development.

Tomato plant growth promotion and drought tolerance conferred by three arbuscular mycorrhizal fungi is mediated by lipid metabolism

Arbuscular mycorrhizal fungi (AMF) can promote plant growth and enhance plant drought tolerance with varying effect size among different fungal species. However, the linkage between the variation and the lipid metabolism, which is exclusively derived from plants, has been little explored thus far. Here, we established AM symbiosis between tomato (Solanum lycopersicum) plants and three AMF species (Rhizophagus intraradices, Funneliformis mosseae, Rhizophagus irregularis) under well watered (WW) or drought stressed (DS) conditions in pot experiment. The plant biomass, chlorophyll fluorescence Fv/Fm, shoot P content and mycorrhizal colonization were determined. Meanwhile, fatty acid (FA) profiles and relative expression of genes encoding for nutrition exchange (SlPT4, SlPT5, RAM2, STR/STR2) in roots were also monitored. DS significantly decreased plant biomass while AMF significantly increased it, with three fungal species varying in their growth promoting capacity and drought tolerance capacity. The growth promoting effect of R. irregularis was lower than those of R. intraradices and F. mosseae, and was associated with higher mycorrhizal colonization and more consumption of lipids. However, the drought tolerance capacity of R. irregularis was greater than those of R. intraradices and F. mosseae, and was associated with less decrease in mycorrhizal colonization and lipid content. We also found that AMF mediated plant drought tolerance via regulating both AM specific FAs and non-AM specific FAs in a complementary manner. These data suggest that lipid metabolism in AM plays a crucial role in plant drought tolerance mediated by AMF.

Arbuscular mycorrhizal fungal contribution towards plant resilience to drought conditions

Climate changes cause altering rainfall patterns resulting in an increase in drought occurrences globally. These events are disrupting plants and agricultural productivity. To evade droughts, plants try to adapt and modify in the best capacities possible. The plants have adapted by structurally modifying roots, stems, and leaves, as well as modifying functions. Lately, the association of microbial communities with plants has also been proven to be an important factor in aiding resilience. The fungal representatives of the microbial community also help safeguard the plants against drought. We discuss how these fungi associate with plants and contribute to evading drought stress. We specifically focus on Arbuscular mycorrhizal fungi (AMF) mediated mechanisms involving antioxidant defenses, phytohormone mediations, osmotic adjustments, proline expressions, fungal water absorption and transport, morphological modifications, and photosynthesis. We believe understanding the mechanisms would help us to optimize the use of fungi in agricultural practices. That way we could better prepare the plants for the anticipated future drought events.

Resistance to Plant Parasites in Tomato Is Induced by Soil Enrichment with Specific Bacterial and Fungal Rhizosphere Microbiome

Commercial formulations of beneficial microbes have been used to enrich the rhizosphere microbiome of tomato plants grown in pots located in a glasshouse. These plants have been subjected to attacks by soil-borne parasites, such as root-knot nematodes (RKNs), and herbivores, such as the miner insect Tuta absoluta. The development of both parasites and the symptoms of their parasitism were restricted in these plants with respect to plants left untreated. A mixture, named in the text as Myco, containing plant growth-promoting rhizobacteria (PGPR), opportunistic biocontrol fungi (BCF), and arbuscular mycorrhizal fungi (AMF) was more effective in limiting pest damage than a formulation containing the sole AMF (Ozor). Therefore, Myco-treated plants inoculated with RKNs were taken as a model for further studies. The PGPR contained in Myco were not able to reduce nematode infection; rather, they worsened symptoms in plants compared with those observed in untreated plants. Therefore, it was argued that both BCF and AMF were the microorganisms that colonized roots and stimulated the plant immune system against RKNs. Beneficial fungi colonized the roots by lowering the activities of the defense supporting enzymes endochitinases and β-1,3-glucanase. However, as early as three days after nematode inoculation, these enzyme activities and the expression of the encoding pathogenesis-related genes (PR-2, PR-3) were found to be enhanced in roots with respect to non-inoculated plants, thus indicating that plants had been primed against RKNs. The addition of paclobutrazol, which reduces salicylic acid (SA) levels in cells, and diphenyliodonium chloride, which inhibits superoxide generation, completely abolished the repressive effect of Myco on nematode infection. Inhibitors of copper enzymes and the alternative cyanide-resistant respiration did not significantly alter resistance induction by Myco. When Myco-treated plants were subjected to moderate water stress and inoculated with nematodes, they retained numbers of developed individuals in the roots similar to those present in regularly watered plants, in contrast to what occurred in roots of untreated stressed plants that hosted very few individuals because of poor nutrient availability.