Disruption of the rice 4-DEOXYOROBANCHOL HYDROXYLASE unravels specific functions of canonical strigolactones.

Strigolactones (SLs) regulate many developmental processes, including shoot-branching/tillering, and mediate rhizospheric interactions. SLs originate from carlactone (CL) and are structurally diverse, divided into a canonical and a noncanonical subfamily. Rice contains two canonical SLs, 4-deoxyorobanchol (4DO) and orobanchol (Oro), which are common in different plant species. The cytochrome P450 OsMAX1-900 forms 4DO from CL through repeated oxygenation and ring closure, while the homologous enzyme OsMAX1-1400 hydroxylates 4DO into Oro. To better understand the biological function of 4DO and Oro, we generated CRISPR/Cas9 mutants disrupted in OsMAX1-1400 or in both OsMAX1-900 and OsMAX1-1400. The loss of OsMAX1-1400 activity led to a complete lack of Oro and an accumulation of its precursor 4DO. Moreover, Os1400 mutants showed shorter plant height, panicle and panicle base length, but no tillering phenotype. Hormone quantification and transcriptome analysis of Os1400 mutants revealed elevated auxin levels and changes in the expression of auxin-related, as well as of SL biosynthetic genes. Interestingly, the Os900/1400 double mutant lacking both Oro and 4DO did not show the observed Os1400 architectural phenotypes, indicating their being a result of 4DO accumulation. Treatment of wild-type plants with 4DO confirmed this assumption. A comparison of the Striga seed germinating activity and the mycorrhization of Os900, Os900/1400, and Os1400 loss-of-function mutants demonstrated that the germination activity positively correlates with 4DO content while disrupting OsMAX1-1400 has a negative impact on mycorrhizal symbiosis. Taken together, our paper deciphers the biological function of canonical SLs in rice and reveals their particular contributions to establishing architecture and rhizospheric communications.

Evolution of unexpected diversity in a putative mating type locus and its correlation with genome variability reveals likely asexuality in the model mycorrhizal fungus Rhizophagus irregularis.

BACKGROUND: Arbuscular mycorrhizal fungi (AMF) form mutualistic partnerships with approximately 80% of plant species. AMF, and their diversity, play a fundamental role in plant growth, driving plant diversity, and global carbon cycles. Knowing whether AMF are sexual or asexual has fundamental consequences for how they can be used in agricultural applications. Evidence for and against sexuality in the model AMF, Rhizophagus irregularis, has been proposed. The discovery of a putative mating-type locus (MAT locus) in R. irregularis, and the previously suggested recombination among nuclei of a dikaryon R. irregularis isolate, potentially suggested sexuality. Unless undergoing frequent sexual reproduction, evolution of MAT-locus diversity is expected to be very low. Additionally, in sexual species, MAT-locus evolution is decoupled from the evolution of arbitrary genome-wide loci. RESULTS: We studied MAT-locus diversity of R. irregularis. This was then compared to diversification in a phosphate transporter gene (PTG), that is not involved in sex, and to genome-wide divergence, defined by 47,378 single nucleotide polymorphisms. Strikingly, we found unexpectedly high MAT-locus diversity indicating that either it is not involved in sex, or that AMF are highly active in sex. However, a strongly congruent evolutionary history of the MAT-locus, PTG and genome-wide arbitrary loci allows us to reject both the hypothesis that the MAT-locus is involved in mating and that the R. irregularis lineage is sexual. CONCLUSION: Our finding shapes the approach to developing more effective AMF strains and is highly informative as it suggests that introduced strains applied in agriculture will not exchange DNA with native populations.

Niche types and community assembly.

Studies of niche differentiation and biodiversity often focus on a few niche dimensions due to the methodological challenge of describing hyperdimensional niche space. However, this may limit our understanding of community assembly processes. We used the full spectrum of realized niche types to study arbuscular mycorrhizal fungal communities: distinguishing abiotic and biotic, and condition and resource, axes. Estimates of differentiation in relation to different niche types were only moderately correlated. However, coexisting taxon niches were consistently less differentiated than expected, based on a regional null model, indicating the importance of habitat filtering at that scale. Nonetheless, resource niches were relatively more differentiated than condition niches, which is consistent with the effect of a resource niche-based coexistence mechanism. Considering niche types, and in particular distinguishing resource and condition niches, provides a more complete understanding of community assembly, compared with studying individual niche axes or the full niche.

Mitigating gadolinium toxicity in guar (Cyamopsis tetragonoloba L.) through the symbiotic associations with arbuscular mycorrhizal fungi: physiological and biochemical insights.

BACKGROUND: Gadolinium (Gd) is an increasingly found lanthanide element in soil; thus, understanding its impact on plant physiology, biochemistry, and molecular responses is crucial. Here, we aimed to provide a comprehensive understanding of Gd (150 mg kg- 1) impacts on guar (Cyamopsis tetragonoloba L.) plant yield and metabolism and whether the symbiotic relationship with arbuscular mycorrhizal fungi (AMF) can mitigate Gd toxicity of soil contamination. RESULTS: AMF treatment improved mineral nutrient uptake and seed yield by 38-41% under Gd stress compared to non-inoculated stressed plants. Metabolic analysis unveiled the defense mechanisms adopted by AMF-treated plants, revealing carbon and nitrogen metabolism adaptations to withstand Gd contamination. This included an increase in the synthesis of primary metabolites, such as total sugar (+ 39% compared to control), soluble sugars (+ 29%), starch (+ 30%), and some main amino acids like proline (+ 57%) and phenylalanine (+ 87%) in the seeds of AMF-treated plants grown under Gd contamination. Furthermore, fatty acid and organic acid profile changes were accompanied by the production of secondary metabolites, including tocopherols, polyamines, phenolic acids, flavones, and anthocyanins. CONCLUSIONS: Overall, the coordinated synthesis of these compounds underscores the intricate regulatory mechanisms underlying plant-AMF interactions and highlights the potential of AMF to modulate plant secondary metabolism for enhanced Gd stress tolerance.

Combined application of arbuscular mycorrhizal fungi and selenium fertilizer increased wheat biomass under cadmium stress and shapes rhizosphere soil microbial communities.

BACKGROUND: Selenium (Se) fertilizer and arbuscular mycorrhizal fungi (AMF) are known to modulate cadmium (Cd) toxicity in plants. However, the effects of their co-application on wheat growth and soil microbial communities in Cd-contaminated soil are unclear. RESULTS: A pot experiment inoculation with two types of AMF and the application of Se fertilizer under Cd stress in wheat showed that inoculation AMF alone or combined with Se fertilizer significantly increased wheat biomass. Se and AMF alone or in combination significantly reduced available Cd concentration in wheat and soil, especially in the Se combined with Ri treatment. High throughput sequencing of soil samples indicated that Se and AMF application had stronger influence on bacterial community compared to fungal community and the bacterial network seemed to have more complex interconnections than the fungal network, and finally shaped the formation of specific microflora to affect Cd availability. CONCLUSION: These results indicate that the application of Se and AMF, particularly in combination, could successfully decrease soil Cd availability and relieve the harm of Cd in wheat by modifying rhizosphere soil microbial communities.

Arbuscular mycorrhizal fungi by inducing watermelon roots secretion phthalates, altering soil enzyme activity and bacterial community composition to alleviate the watermelon wilt.

BACKGROUND: Long-term continuous cropping has resulted in the frequent occurrence of fusarium wilt of watermelon (Citrullus lanatus). AMF inoculation can alleviate the continuous cropping barrier and reduce the incidence of fusarium wilt of watermelon. Our previous study found that the root exudates of mycorrhizal watermelon can enhance watermelon resistance to this disorder. It is necessary to further isolate and identify the specific compounds in root exudates of mycorrhizal watermelon and explore their control effects on fusarium wilt of continuous cropping watermelon. RESULT: The results of this study showed that the root system of watermelon seedlings inoculated with AMF (Funneliformis mosseae or Glomus versiforme) secreted diisooctyl phthalate (A) and dibutyl phthalate (B). Compared with water treatment, treatment with 0.1 ml/L (A1, B1), 0.5 ml/L (A2, B2) and 1 ml/L (A3, B3) of A or B significantly increased soil enzyme activities, the numbers of bacteria and actinomycetes, and the bacteria/fungi ratio in the rhizosphere. Furthermore, the Disease indexes (DI) of A1 and B3 were 25% and 20%, respectively, while the prevention and control effects (PCE) were 68.8% and 75%, respectively. In addition, diisooctyl phthalate or dibutyl phthalate increased the proportions of Gemmatimonadetes, Chloroflexi, and Acidobacteria in the rhizosphere of continuous cropping watermelon, and decreased the proportions of Proteobacteria and Firmicutes, with Novosphingobium, Kaistobacter, Bacillus, and Acinetobacter as the predominant bacteria. Compared with the water treatment, the abundance of Neosphingosaceae, Kateybacterium and Bacillus in the A1 group was increased by 7.33, 2.14 and 2.18 times, respectively, while that in the B2 group was increased by 60.05%, 80.24% and 1 time, respectively. In addition, exogenous diisooctyl phthalate and dibutyl phthalate were shown to promote growth parameters (vine length, stem diameter, fresh weight and dry weight) and antioxidant enzyme system activities (SOD, POD and CAT) of continuous cropping watermelon. CONCLUSION: Lower watermelon fusarium wilt incidence in mycorrhizal watermelons was associated with phthalate secretion in watermelons after AMF inoculation. Exogenous diisooctyl phthalate and dibutyl phthalate could alleviate the continuous cropping disorder of watermelon, reduce the incidence of fusarium wilt, and promote the growth of watermelon by increasing the enzyme activities and the proportion of beneficial bacteria in rhizosphere soil. In addition, the low concentration of phthalate diisooctyl and high concentration of phthalic acid dibutyl works best. Therefore, a certain concentration of phthalates in the soil can help alleviate continuous cropping obstacles.

Inoculation with arbuscular mycorrhizal fungi improves plant biomass and nitrogen and phosphorus nutrients: a meta-analysis.

Arbuscular mycorrhizal fungi (AMF) have profound effects on plant growth and nitrogen (N) and phosphorus (P) nutrition. However, a comprehensive evaluation of how plant N and P respond to AMF inoculation is still unavailable. Here, we complied data from 187 original researches and carried out a meta-analysis to assess the effects of AMF inoculation on plant growth and N and P nutrition. We observe overall positive effects of AMF inoculation on plant performance. The mean increases of plant biomass, N concentration, P concentration, N and P uptake of whole plant are 47%, 16%, 27%, 67%, and 105%, respectively. AMF inoculation induces more increases in plant concentrations and storage of P than N. Plant responses to AMF inoculation are substantially higher with single AMF species than with mixed AMF species, in laboratory experiments than in field experiments, and in legumes than in non-legumes. The response ratios of plant N and P nutrition are positively correlated with AMF colonization rate, N addition, P addition, and water condition, while unvaried with experiment duration. The biggest and smallest effect sizes of AMF inoculation on plant performance are observed in the application of nitrate and ammonium, respectively. Accordingly, this meta-analysis study clearly suggests that AMF inoculation improves both plant N and P nutrients and systematically clarifies the variation patterns in AMF effects with various biotic and abiotic factors. These findings highlight the important role of AMF inoculation in enhancing plant N and P resource acquisitions and provide useful references for evaluating the AMF functions under the future global changes.

Facultative mycorrhization in a fern (Struthiopteris spicant L. Weiss) is bound to light intensity.

BACKGROUND: The establishment of mycorrhizal relationships between a fungus and a plant typically enhances nutrient and water uptake for the latter while securing a carbon source for the fungus. However, under a particular set of environmental conditions, such as low availability of light and abundant nutrients in the soil, the resources invested in the maintenance of the fungi surpass the benefits obtained by the host. In those cases, facultative mycorrhizal plants are capable of surviving without symbiosis. Facultative mycorrhization in ferns has been overlooked until now. The present study measured the response of Struthiopteris spicant L. Weiss, and its root-associated fungi to different levels of light and nutrient availability in terms of growth, mycorrhizal presence, and leaf nutrient content. This fern species exhibits a great tolerance to variable light, nutrient, and pH conditions, and it has been found with and without mycorrhizae. We conducted a greenhouse experiment with 80 specimens of S. spicant and three factors (Light, Phosphorus, and Nitrogen) resulting in eight treatments. RESULTS: We found a significant influence of the factor light on fungal community composition, plant biomass, and nutrient accumulation. Departing from a lack of colonization at the initial stage, plants showed a remarkable increment of more than 80% in the arbuscular mycorrhizal fungi (AMF) richness and abundance in their roots when grown under high light conditions, compared with the ones in low light. We also observed an upward trend of C:P and C:N ratios and the above- and belowground biomass production when AMF abundance increased. Furthermore, the compositional analysis of the whole fungal communities associated with S. spicant roots revealed clear differences among low-light and high-light treatments. CONCLUSIONS: This study is the first to investigate the importance of light and nutrient availability in determining fern-AMF relationships. We confirmed that Struthiopteris spicant is a facultative mycorrhizal plant. The composition and diversity of AMF found in the roots of this fern are strongly influenced by light and less by nutrient conditions. Our study shows that ferns respond very sensitively to changes in environmental factors, leading to shifts in the associated mycorrhizal communities.

Testing the joint effects of arbuscular mycorrhizal fungi and ants on insect herbivory on potato plants.

MAIN CONCLUSION: Ants, but not mycorrhizae, significantly affected insect leaf-chewing herbivory on potato plants. However, there was no evidence of mutualistic interactive effects on herbivory. Plants associate with both aboveground and belowground mutualists, two prominent examples being ants and arbuscular mycorrhizal fungi (AMF), respectively. While both of these mutualisms have been extensively studied, joint manipulations testing their independent and interactive (non-additive) effects on plants are rare. To address this gap, we conducted a joint test of ant and AMF effects on herbivory by leaf-chewing insects attacking potato (Solanum tuberosum) plants, and further measured plant traits likely mediating mutualist effects on herbivory. In a field experiment, we factorially manipulated the presence of AMF (two levels: control and mycorrhization) and ants (two levels: exclusion and presence) and quantified the concentration of leaf phenolic compounds acting as direct defenses, as well as plant volatile organic compound (VOC) emissions potentially mediating direct (e.g., herbivore repellents) or indirect (e.g., ant attractants) defense. Moreover, we measured ant abundance and performed a dual-choice greenhouse experiment testing for effects of VOC blends (mimicking those emitted by control vs. AMF-inoculated plants) on ant attraction as a mechanism for indirect defense. Ant presence significantly reduced herbivory whereas mycorrhization had no detectable influence on herbivory and mutualist effects operated independently. Plant trait measurements indicated that mycorrhization had no effect on leaf phenolics but significantly increased VOC emissions. However, mycorrhization did not affect ant abundance and there was no evidence of AMF effects on herbivory operating via ant-mediated defense. Consistently, the dual-choice assay showed no effect of AMF-induced volatile blends on ant attraction. Together, these results suggest that herbivory on potato plants responds mainly to top-down (ant-mediated) rather than bottom-up (AMF-mediated) control, an asymmetry in effects which could have precluded mutualist non-additive effects on herbivory. Further research on this, as well as other plant systems, is needed to examine the ecological contexts under which mutualist interactive effects are more or less likely to emerge and their impacts on plant fitness and associated communities.

Progressing beyond colonization strategies to understand arbuscular mycorrhizal fungal life history.

Knowledge of differential life-history strategies in arbuscular mycorrhizal (AM) fungi is relevant for understanding the ecology of this group and its potential role in sustainable agriculture and carbon sequestration. At present, AM fungal life-history theories often focus on differential investment into intra- vs extraradical structures among AM fungal taxa, and its implications for plant benefits. With this Viewpoint we aim to expand these theories by integrating a mycocentric economics- and resource-based life-history framework. As in plants, AM fungal carbon and nutrient demands are stoichiometrically coupled, though uptake of these elements is spatially decoupled. Consequently, investment in morphological structures for carbon vs nutrient uptake is not in competition. We argue that understanding the ecology and evolution of AM fungal life-history trade-offs requires increased focus on variation among structures foraging for the same element, that is within intra- or extraradical structures (in our view a 'horizontal' axis), not just between them ('vertical' axis). Here, we elaborate on this argument and propose a range of plausible life-history trade-offs that could lead to the evolution of strategies in AM fungi, providing testable hypotheses and creating opportunities to explain AM fungal co-existence, and the context-dependent effects of AM fungi on plant growth and soil carbon dynamics.

Manipulation of soil mycorrhizal fungi influences floral traits.

Most plants form root hyphal relationships with mycorrhizal fungi, especially arbuscular mycorrhizal fungi (AMF). These associations are known to positively impact plant biomass and competitive ability. However, less is known about how mycorrhizas impact other ecological interactions, such as those mediated by pollinators. We performed a meta-regression of studies that manipulated AMF and measured traits related to pollination, including floral display size, rewards, visitation, and reproduction, extracting 63 studies with 423 effects. On average, the presence of mycorrhizas was associated with positive effects on floral traits. Specifically, we found impacts of AMF on floral display size, pollinator visitation and reproduction, and a positive but nonsignificant impact on rewards. Studies manipulating mycorrhizas with fungicide tended to report contrasting results, possibly because fungicide destroys both beneficial and pathogenic microbes. Our study highlights the potential for relationships with mycorrhizal fungi to play an important, yet underrecognized role in plant-pollinator interactions. With heightened awareness of the need for a more sustainable agricultural industry, mycorrhizal fungi may offer the opportunity to reduce reliance on inorganic fertilizers. At the same time, fungicides are now ubiquitous in agricultural systems. Our study demonstrates indirect ways in which plant-belowground fungal partnerships could manifest in plant-pollinator interactions.

Dynamics and drivers of mycorrhizal fungi after glacier retreat.

The development of terrestrial ecosystems depends greatly on plant mutualists such as mycorrhizal fungi. The global retreat of glaciers exposes nutrient-poor substrates in extreme environments and provides a unique opportunity to study early successions of mycorrhizal fungi by assessing their dynamics and drivers. We combined environmental DNA metabarcoding and measurements of local conditions to assess the succession of mycorrhizal communities during soil development in 46 glacier forelands around the globe, testing whether dynamics and drivers differ between mycorrhizal types. Mycorrhizal fungi colonized deglaciated areas very quickly (< 10 yr), with arbuscular mycorrhizal fungi tending to become more diverse through time compared to ectomycorrhizal fungi. Both alpha- and beta-diversity of arbuscular mycorrhizal fungi were significantly related to time since glacier retreat and plant communities, while microclimate and primary productivity were more important for ectomycorrhizal fungi. The richness and composition of mycorrhizal communities were also significantly explained by soil chemistry, highlighting the importance of microhabitat for community dynamics. The acceleration of ice melt and the modifications of microclimate forecasted by climate change scenarios are expected to impact the diversity of mycorrhizal partners. These changes could alter the interactions underlying biotic colonization and belowground-aboveground linkages, with multifaceted impacts on soil development and associated ecological processes.

The mycorrhizal symbiosis: research frontiers in genomics, ecology, and agricultural application.

Mycorrhizal symbioses between plants and fungi are vital for the soil structure, nutrient cycling, plant diversity, and ecosystem sustainability. More than 250 000 plant species are associated with mycorrhizal fungi. Recent advances in genomics and related approaches have revolutionized our understanding of the biology and ecology of mycorrhizal associations. The genomes of 250+ mycorrhizal fungi have been released and hundreds of genes that play pivotal roles in regulating symbiosis development and metabolism have been characterized. rDNA metabarcoding and metatranscriptomics provide novel insights into the ecological cues driving mycorrhizal communities and functions expressed by these associations, linking genes to ecological traits such as nutrient acquisition and soil organic matter decomposition. Here, we review genomic studies that have revealed genes involved in nutrient uptake and symbiosis development, and discuss adaptations that are fundamental to the evolution of mycorrhizal lifestyles. We also evaluated the ecosystem services provided by mycorrhizal networks and discuss how mycorrhizal symbioses hold promise for sustainable agriculture and forestry by enhancing nutrient acquisition and stress tolerance. Overall, unraveling the intricate dynamics of mycorrhizal symbioses is paramount for promoting ecological sustainability and addressing current pressing environmental concerns. This review ends with major frontiers for further research.

Mycorrhizal fungi modify decomposition: a meta-analysis.

It has been proposed that ectomycorrhizal fungi can reduce decomposition while arbuscular mycorrhizal fungi may enhance it. These phenomena are known as the 'Gadgil effect' and 'priming effect', respectively. However, it is unclear which one predominates globally. We evaluated whether mycorrhizal fungi decrease or increase decomposition, and identified conditions that mediate this effect. We obtained decomposition data from 43 studies (97 trials) conducted in field or laboratory settings that controlled the access of mycorrhizal fungi to substrates colonized by saprotrophs. Across studies, mycorrhizal fungi promoted decomposition of different substrates by 6.7% overall by favoring the priming effect over the Gadgil effect. However, we observed significant variation among studies. The substrate C : N ratio and absolute latitude influenced the effect of mycorrhizal fungi on decomposition and contributed to the variation. Specifically, mycorrhizal fungi increased decomposition at low substrate C : N and absolute latitude, but there was no discernable effect at high values. Unexpectedly, the effect of mycorrhizal fungi was not influenced by the mycorrhizal type. Our findings challenge previous assumptions about the universality of the Gadgil effect but highlight the potential of mycorrhizal fungi to negatively influence soil carbon storage by promoting the priming effect.

Los hongos ectomicorrízicos puden reducir la descomposición mientras que los hongos micorrízico­arbusculares pueden potenciarla. Ambos fenómenos son conocidos como "Gadgil effect" y "priming effect", respectivamente. Sin embargo, no es claro cuál predomina mundialmente. En este trabajo evaluamos si los hongos micorrízicos disminuyen o promueven la descomposición, e identificamos las condiciones que regulan este efecto. Para ello, recopilamos datos de descomposición de 43 estudios (97 observaciones) realizados en condiciones de campo o laboratorio que controlaron el acceso de los hongos micorrízicos a sustratos colonizados por saprótrofos. Los hongos micorrízicos promovieron la descomposición de diferentes sustratos en un 6.7%. Sin embargo, observamos una variación significativa entre estudios. La relación C : N del sustrato y la latitud influyeron en el efecto de los hongos micorrícicos sobre la descomposición y contribuyeron a la variabilidad. Específicamente, los hongos micorrízicos aumentaron la descomposición a valores bajos de C : N del sustrato y latitud, pero no hubo un efecto discernible en valores altos. Inesperadamente, el tipo de micorriza no influyó en el efecto de los hongos micorrízicos. Nuestros hallazgos cuestionan la universalidad del Gadgil effect, y resaltan el potencial de los hongos micorrízicos para influir negativamente en el almacenamiento de carbono del suelo al promover el priming effect.

Mycorrhizal associations modify tree diversity-productivity relationships across experimental tree plantations.

Decades of studies have demonstrated links between biodiversity and ecosystem functioning, yet the generality of the relationships and the underlying mechanisms remain unclear, especially for forest ecosystems. Using 11 tree-diversity experiments, we tested tree species richness-community productivity relationships and the role of arbuscular (AM) or ectomycorrhizal (ECM) fungal-associated tree species in these relationships. Tree species richness had a positive effect on community productivity across experiments, modified by the diversity of tree mycorrhizal associations. In communities with both AM and ECM trees, species richness showed positive effects on community productivity, which could have resulted from complementarity between AM and ECM trees. Moreover, both AM and ECM trees were more productive in mixed communities with both AM and ECM trees than in communities assembled by their own mycorrhizal type of trees. In communities containing only ECM trees, species richness had a significant positive effect on productivity, whereas species richness did not show any significant effects on productivity in communities containing only AM trees. Our study provides novel explanations for variations in diversity-productivity relationships by suggesting that tree-mycorrhiza interactions can shape productivity in mixed-species forest ecosystems.

The root microlandscape of arbuscular mycorrhizal fungi.

Understanding the drivers of assemblages of arbuscular mycorrhizal fungi (AMF) is essential to leverage the benefits of AMF for plant growth and health. Arbuscular mycorrhizal fungi are heterogeneously distributed in space even at small scale. We review the role of plant distribution in driving AMF assemblages (the passenger hypothesis), using a transposition of the conceptual framework of landscape ecology. Because rooting systems correspond to habitat patches with limited carrying capacity that differ in quality due to host-preference effects, we suggest considering plant communities as mosaics of AMF microhabitats. We review how predictions from landscape ecology apply to plant community effects on AMF, and the existing evidence that tests these predictions. Although many studies have been conducted on the effect of plant compositional heterogeneity on AMF assemblages, they mostly focused on the effect of plant richness, while only a few investigated the effect of configurational heterogeneity, plant connectivity or plant community temporal dynamics. We propose key predictions and future prospects to fill these gaps. Considering plant communities as landscapes extends the passenger hypothesis by including a spatially explicit dimension and its associated ecological processes and may help understand and manipulate AMF assemblages at small spatial scales.

The receptor kinase RiSho1 in Rhizophagus irregularis regulates arbuscule development and drought tolerance during arbuscular mycorrhizal symbiosis.

In terrestrial ecosystems, most plant species can form beneficial associations with arbuscular mycorrhizal (AM) fungi. Arbuscular mycorrhizal fungi benefit plant nutrient acquisition and enhance plant tolerance to drought. The high osmolarity glycerol 1 mitogen-activated protein kinase (HOG1-MAPK) cascade genes have been characterized in Rhizophagus irregularis. However, the upstream receptor of the HOG1-MAPK cascade remains to be investigated. We identify the receptor kinase RiSho1 from R. irregularis, containing four transmembrane domains and one Src homology 3 (SH3) domain, corresponding to the homologue of Saccharomyces cerevisiae. Higher expression levels of RiSho1 were detected during the in planta phase in response to drought. RiSho1 protein was localized in the plasma membrane of yeast, and interacted with the HOG1-MAPK module RiPbs2 directly by protein-protein interaction. RiSho1 complemented the growth defect of the yeast mutant ∆sho1 under sorbitol conditions. Knock-down of RiSho1 led to the decreased expression of downstream HOG1-MAPK cascade (RiSte11, RiPbs2, RiHog1) and drought-resistant genes (RiAQPs, RiTPSs, RiNTH1 and Ri14-3-3), hampered arbuscule development and decreased plants antioxidation ability under drought stress. Our study reveals the role of RiSho1 in regulating arbuscule development and drought-resistant genes via the HOG1-MAPK cascade. These findings provide new perspectives on the mechanisms by which AM fungi respond to drought.

Arbuscular mycorrhiza convey significant plant carbon to a diverse hyphosphere microbial food web and mineral-associated organic matter.

Arbuscular mycorrhizal fungi (AMF) transport substantial plant carbon (C) that serves as a substrate for soil organisms, a precursor of soil organic matter (SOM), and a driver of soil microbial dynamics. Using two-chamber microcosms where an air gap isolated AMF from roots, we 13CO2-labeled Avena barbata for 6 wk and measured the C Rhizophagus intraradices transferred to SOM and hyphosphere microorganisms. NanoSIMS imaging revealed hyphae and roots had similar 13C enrichment. SOM density fractionation, 13C NMR, and IRMS showed AMF transferred 0.77 mg C g-1 of soil (increasing total C by 2% relative to non-mycorrhizal controls); 33% was found in occluded or mineral-associated pools. In the AMF hyphosphere, there was no overall change in community diversity but 36 bacterial ASVs significantly changed in relative abundance. With stable isotope probing (SIP)-enabled shotgun sequencing, we found taxa from the Solibacterales, Sphingobacteriales, Myxococcales, and Nitrososphaerales (ammonium oxidizing archaea) were highly enriched in AMF-imported 13C (> 20 atom%). Mapping sequences from 13C-SIP metagenomes to total ASVs showed at least 92 bacteria and archaea were significantly 13C-enriched. Our results illustrate the quantitative and ecological impact of hyphal C transport on the formation of potentially protective SOM pools and microbial roles in the AMF hyphosphere soil food web.

The soybean sugar transporter GmSWEET6 participates in sucrose transport towards fungi during arbuscular mycorrhizal symbiosis.

In arbuscular mycorrhizal (AM) symbiosis, sugars in root cortical cells could be exported as glucose or sucrose into peri-arbuscular space for use by AM fungi. However, no sugar transporter has been identified to be involved in sucrose export. An AM-inducible SWEET transporter, GmSWEET6, was functionally characterised in soybean, and its role in AM symbiosis was investigated via transgenic plants. The expression of GmSWEET6 was enhanced by inoculation with the cooperative fungal strain in both leaves and roots. Heterologous expression in a yeast mutant showed that GmSWEET6 mainly transported sucrose. Transgenic plants overexpressing GmSWEET6 increased sucrose concentration in root exudates. Overexpression or knockdown of GmSWEET6 decreased plant dry weight, P content, and sugar concentrations in non-mycorrhizal plants, which were partly recovered in mycorrhizal plants. Intriguingly, overexpression of GmSWEET6 increased root P content and decreased the percentage of degraded arbuscules, while knockdown of GmSWEET6 increased root sugar concentrations in RNAi2 plants and the percentage of degraded arbuscules in RNAi1 plants compared with wild-type plants when inoculated with AM fungi. These results in combination with subcellular localisation of GmSWEET6 to peri-arbuscular membranes strongly suggest that GmSWEET6 is required for AM symbiosis by mediating sucrose efflux towards fungi.

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.