•This study analyzed how PAs were involved in AMF-enhanced drought tolerance of plants.•AM trifoliate orange had higher Put and Cad and lower Spd and Spm under drought.•Mycorrhizal plants had higher ...activity of PA catabolic enzymes and Put synthetase.•AM plants showed overexpression of PtSPMS, PtPAO, PtSOD, and PtCAT1 under DS.•AM plants represented down-regulated expression of PtCuAO under DS.
Arbuscular mycorrhizas enhance the drought tolerance of host plants through several underlying mechanisms. Polyamines (PAs) are known to protect plant cells from the damages of drought by enhancing the antioxidant defense system, restricting ethylene synthesis, maintaining cell pH and ion homeostasis, preventing chlorophyll loss, and so on; concomitantly, the mechanism through which arbuscular mycorrhizal (AM) fungi modulate PA metabolism to enhance drought tolerance of plants remains unclear. A pot experiment was conducted to assess how an AM fungus, Funneliformis mosseae, affects root PA homeostasis, activities and gene expressions of PA-related synthesizing and degrading enzymes, hydrogen peroxide (H2O2, a PA metabolite) production, and antioxidant enzyme gene expressions in trifoliate orange (Poncirus trifoliata) exposed to drought stress (DS). AM seedlings showed higher growth traits, leaf water potential, two plasma membrane intrinsic protein aquaporin gene expressions, and chlorophyll concentrations than non-AM seedlings under well-watered (WW) and DS conditions. Mycorrhizal treatment induced higher putrescine and cadaverine but lower spermidine and spermine concentrations, with higher activity of PA catabolic enzymes (copper-containing diamine oxidase, CuAO; polyamine oxidase, PAO) and putrescine synthases (ornithine decarboxylaseby; arginine decarboxylase, ADC). Mycorrhizas up-regulated the expression of the spermine synthase gene, PtSPMS, under DS, and down-regulated the transcript levels of PA catabolic enzyme genes (PtCuAO1, PtCuAO2, PtCuAO6, and PtCuAO8) and PA synthase genes (PtADC1 and PtADC2) under DS. PtPAO1, PtPAO2, and PtPAO3 had higher expression levels in AMF-inoculated seedlings, as compared to non-AMF-inoculated seedlings, under DS, triggering reactive oxygen species-related signalling for stress responsiveness through low H2O2 levels by up-regulating the expression of PtMn-SOD, PtCu/Zn-SOD, and PtCAT1. This study demonstrated that mycorrhizas have the capacity to modulate PA metabolism to enhance the drought tolerance of plants.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Root-hair growth and development regulated by soil microbes is associated with auxin. In this background, we hypothesized that mycorrhizal fungal inoculation induces greater root-hair growth through ...stimulated auxin synthesis and transport under water stress conditions. Trifoliate orange (Poncirus trifoliata) was inoculated with an arbuscular mycorrhizal (AM) fungus (Funneliformis mosseae) under well-watered (WW) and drought stress (DS) for 9 weeks. Compared with non-AM seedlings, AM seedlings displayed significantly higher density, length, and diameter of root hairs and root indoleacetic acid (IAA) level, whereas lower total root IAA efflux, regardless of soil moisture status. Root PtYUC3 and PtYUC8 involved in IAA biosynthesis were up-regulated by mycorrhization under WW and DS, whereas AM-modulated expression in PtTAA1, PtTAR2, PtYUC4, and PtYUC6 depended on status of soil moisture. Mycorrhizal inoculation down-regulated the transcript level of root auxin efflux carriers like PtPIN1 and PtPIN3, whereas significantly up-regulated the expression of root auxin-species influx carriers like PtABCB19 and PtLAX2 under DS. These results indicated that AMF-stimulated greater root-hair growth of trifoliate orange under DS that is independent on AMF species is related with mycorrhiza-modulated auxin synthesis and transport, which benefits the host plant to enhance drought tolerance.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
•This study mainly analyzed changes of root TIPs expression under mycorrhization and drought.•AM plants had higher leaf water status and root ABA levels under WW and DS.•AMF up-regulated PtTIP1;2, ...PtTIP2;1, PtTIP4;1 and PtTIP5;1 expression under WW.•Root PtTIP1;2, PtTIP 1;3 and PtTIP 4;1 were up-regulated by AMF under DS.•Root PtTIP2;1 and PtTIP 5;1 were down-regulated by AMF under DS.
Arbuscular mycorrhizal fungi (AMF) can enhance plant tolerance to drought stress (DS), while it is unclear whether aquaporins take part in the AMF-modulated mechanisms. In this study, a potted experiment was conducted to investigate the effects of an arbuscular mycorrhizal fungus, Funneliformis mosseae, on plant growth, leaf water status, root abscisic acid (ABA), and relative expression levels of root tonoplast intrinsic protein (TIPs, one of the most abundant aquaporins on plant vacuoles and plasma membranes) genes of trifoliate orange (Poncirus trifoliata L. Raf.) seedlings under well-watered (WW) and DS. The 8-week DS significantly decreased mycorrhizal colonization by 19.5%. In both WW and DS, AMF significantly enhanced leaf relative water content, leaf water potential, and plant growth performance (plant height, stem diameter, leaf number, and biomass), as well as root ABA levels. With regards to root TIPs, seven TIP genes were identified. Meanwhile, AMF treatment up-regulated PtTIP1;2, PtTIP2;1, PtTIP4;1, and PtTIP5;1 and down-regulated PtTIP1;1 and PtTIP2;2 under WW. Under DS, AMF seedlings showed higher expression levels of root PtTIP1;2, PtTIP1;3, and PtTIP4;1 and lower expression levels of root PtTIP2;1 and PtTIP5;1. It concludes that root TIPs exhibited diverse responses to mycorrhization, indicating the multiple roles of AMF in water absorption under drought.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
•Mycorrhizal hyphae have the role in water absorption from soil.•Hyphal water absorption rate was 0.126–1.973mg H2O/h/mm in trifoliate orange.•Drought significantly elevated hyphal water absorption ...rate by 2.3–6.6 times.•Hyphal water contribution to host plants is more important under DS than under WW.
Mycorrhizal hyphae have the functioning on water absorption from soil, while the information regarding hyphal water absorption rate is not fully known. In this study, 37-μm nylon meshes were positioned in a pot bottom to allow mycorrhizal extraradical hyphae, but not roots, to pass through the mesh. The whole pot was placed in a beaker, where distilled water was supplied, reaching a 0.5-cm air gap between the pot bottom and water surface of the beaker. Citrus rootstock, trifoliate orange (Poncirus trifoliata) seedlings of pots were inoculated with Funneliformis mosseae and Paraglomus occultum and also exposed to well-watered (WW) and drought stress (DS). The 71-days soil DS significantly inhibited root mycorrhizal colonization and hyphal length in soil and mesh, regardless of mycorrhizal fungal species. The hyphal water absorption rate was 0.607–1.973mg H2O/h/mm for F. mosseae and 0.126–0.963mg H2O/h/mm for P. occultum, respectively. The DS treatment significantly elevated hyphal water absorption rate by 2.3–6.6 times. The increase of leaf water potential by mycorrhization was higher under DS than under WW. Our results provide a quantitative estimation of water absorption rate by mycorrhizal extraradical hyphae and also suggest more important water contribution of hyphae to the host plant under DS than under WW.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Citrus canker, caused by Xanthomonas axonopodis pv. citri (‘Xac’), is an important quarantine disease in citrus crops. Arbuscular mycorrhizal fungi (AMF) form symbiotic interactions with host plants ...and further affect their disease resistance, possibly by modulating the activity of salicylic acid (SA), a key phytohormone in disease resistance. Common mycorrhizal networks (CMNs) can interconnect plants, but it is not yet clear whether CMNs promote resistance to citrus canker and, if so, whether SA signaling is involved in this process. To test this possibility, we used a two‐chambered rootbox to establish CMNs between trifoliate orange (Poncirus trifoliata) seedlings in chambers inoculated (treated) or not (neighboring) with the AMF, Paraglomus occultum. A subset of the AMF‐inoculated seedlings were also inoculated with Xac (+AMF+Xac). At 2 d post‐inoculation (dpi), compared with the +AMF−Xac treatment, neighboring seedlings in +AMF+Xac treatment had lower expression levels of the SA biosynthetic genes, PtPAL, PtEPS1, and PtPBS3, but higher SA levels, which attributed to the upregulation of PtPAL and PtPBS3 in treated seedlings and the transfer of SA, via CMNs, to the neighboring seedlings. At 4 dpi, the pathogenesis‐related (PR) protein genes, PtPR1, PtPR4, and PtPR5, and the transcriptional regulatory factor gene, PtNPR1, were activated in neighboring seedlings of +AMF+Xac treatment. At 9 dpi, root phenylalanine ammonia‐lyase activity and total soluble phenol and lignin concentrations increased in neighboring seedlings of +AMF+Xac treatment, likely due to the linkage and signal transfer, via CMNs. These findings support the hypothesis that CMNs transfer the SA signal from infected to neighboring healthy seedlings, to activate defense responses and affording protection to neighboring plants against citrus canker infection.
Extraradical hyphae of arbuscular mycorrhizas simultaneously colonize two or more neighboring plants to form common mycorrhizal networks. In this study, we show that the mycorrhizal network transferred a disease‐resistant signal substance, salicylic acid, from the seedlings infected by citrus canker to neighboring healthy seedlings for activating their defense responses.
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FZAB, GIS, IJS, IZUM, KILJ, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK
Arbuscular mycorrhizal fungi (AMF) can enhance drought tolerance in plants, whereas little is known regarding AMF contribution to sucrose and proline metabolisms under drought stress (DS). In this ...study, Funneliformis mosseae and Paraglomus occultum were inoculated into trifoliate orange (Poncirus trifoliata) under well watered and DS. Although the 71-days DS notably (P < 0.05) inhibited mycorrhizal colonization, AMF seedlings showed significantly (P < 0.05) higher plant growth performance and leaf relative water content, regardless of soil water status. AMF inoculation significantly (P < 0.05) increased leaf sucrose, glucose and fructose concentration under DS, accompanied with a significant increase of leaf sucrose phosphate synthase, neutral invertase, and net activity of sucrose-metabolized enzymes and a decrease in leaf acid invertase and sucrose synthase activity. AMF inoculation produced no change in leaf ornithine-δ-aminotransferase activity, but significantly (P < 0.05) increased leaf proline dehydrogenase activity and significantly (P < 0.05) decreased leaf both Δ
-pyrroline-5-carboxylate reductase and Δ
-pyrroline-5-carboxylate synthetase activity, resulting in lower proline accumulation in AMF plants under DS. Our results therefore suggest that AMF strongly altered leaf sucrose and proline metabolism through regulating sucrose- and proline-metabolized enzyme activities, which is important for osmotic adjustment of the host plant.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
•Molecular mechanisms of AM fungi-enhanced plant drought tolerance are reviewed.•Functional (e.g., AQPs, PTs and LEA) and regulatory (e.g., 14–3-3 protein) genes are modulated by AM fungi under ...drought.•Transcription factors including GRAS, MYB and AP2/ERF are regulated by AM fungi to trigger hormone signaling.
Soil drought, an important abiotic stress, seriously inhibits plant growth and physiological activities. However, arbuscular mycorrhizal (AM) fungi in soil can enhance plant adaptability and tolerance to drought stress after establishing mycorrhizal symbiosis with plant roots. Studies on AM fungi-enhanced host drought tolerance are involved in physiological, morphological and molecular levels, with a focus on physiological mechanisms. Study on the molecular mechanisms explaining how arbuscular mycorrhizae enhance plant drought tolerance has being lagging behind that on the physilogical mechanisms, however, increasingly accumulated experiments have focused on the molecular mechanisms during the past decade. This review addresses the question of which functional and regulatory genes in host plants are involved in AM fungi-modulated drought tolerance of host plants, and highlights how these genes play the role in physiological activities such as osmoregulation, water and nutrient transport, hormones, and signal transduction under mycorrhization. The review also outlines how mycorrhizal plants modulate plant drought tolerance through the regulation of transcription factors (GRAS, MYB and AP2/ERF), and prospects an outlook for future research in exploring the interaction of signal pathways in gene regulation of mycorrhizal plants.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
This study aimed to evaluate effects of Funneliformis mosseae on plant growth and root exudate compositions and contents, soil and root phosphatase activity, soil glomalin concentrations, and thus ...soil aggregate stability and distribution in trifoliate orange under well-watered (75% of maximum water holding capacity) and drought stress (55% of maximum water holding capacity) conditions. After eight weeks of drought treatment, mycorrhizal fungal inoculation improved plant growth and exhibited altered composition of root exudates than non-inoculated treatment. Mycorrhizal fungal inoculation dramatically increased the relative abundance of phenolics (e.g., 2 H,8 H-Benzo1,2-b:3,4-b' dipyran-2-one,8,8-dimethyl), terpenoids (e.g., geijerene), and acids (n-hexadecanoic acid), while notably reduced the relative abundance of alkanes (e.g., tridecane, 2-methyl-), esters (e.g., hexanedioic acid and dimethyl ester), and amides (e.g.,13-docosenamide) in root exudates. Mycorrhizal fungal colonization profoundly increased easily extractable and total glomalin-related soil protein levels under two soil water regimes, which cemented soil macroaggregate (2-4 mm size) formation, thereby, improving soil aggregate stability. Mycorrhizal fungal-inoculated plants represented higher soil acid, alkaline and total phosphatase activities, irrespective of well-watered and drought treatment. The results suggest that mycorrhizal plants had improved root microenvironment to mitigate drought damage through changes in root exudate components along with glomalin, phosphatase, and soil aggregate stability in the mycorrhizosphere.
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BFBNIB, GIS, IJS, KISLJ, NUK, PNG, UL, UM, UPUK
A feature of arbuscular mycorrhiza is enhanced drought tolerance of host plants, although it is unclear whether host H
-ATPase activity and gene expression are involved in the physiological process. ...The present study aimed to investigate the effects of an arbuscular mycorrhizal fungus (AMF),
, on H
-ATPase activity, and gene expression of trifoliate orange (
) seedlings subjected to well-watered (WW) and drought stress (DS), together with the changes in leaf gas exchange, root morphology, soil pH value, and ammonium content. Soil drought treatment dramatically increased H
-ATPase activity of leaf and root, and AMF inoculation further strengthened the increased effect. A plasma membrane (PM) H
-ATPase gene of trifoliate orange, PtAHA2 (MW239123), was cloned. The
expression was induced by mycorrhization in leaves and roots and also up-regulated by drought treatment in leaves of AMF-inoculated seedlings and in roots of AMF- and non-AMF-inoculated seedlings. And, the induced expression of
under mycorrhization was more prominent under DS than under WW. Mycorrhizal plants also showed greater photosynthetic rate, stomatal conductance, intercellular CO
concentration, and transpiration rate and better root volume and diameter than non-mycorrhizal plants under DS. AMF inoculation significantly increased leaf and root ammonium content, especially under DS, whereas it dramatically reduced soil pH value. In addition, H
-ATPase activity was significantly positively correlated with ammonium contents in leaves and roots, and root H
-ATPase activity was significantly negatively correlated with soil pH value. Our results concluded that AMF stimulated H
-ATPase activity and
gene expression in response to DS, which resulted in great nutrient (e.g., ammonium) uptake and root growth, as well as low soil pH microenvironment.
•Citrus is often exposed to drought stress, which restricts tree growth and yield.•Citrus rhizosphere inhabits AMF, which can form arbuscular mycorrhizas with roots.•AM-inoculated citrus has shown ...greater tolerance to drought stress.•An overview of possible AM-mediated mechanisms to drought tolerance was provided.
Citrus is one of the most widely cultivated fruit crops, whose rhizosphere inhabits a class of beneficial fungi, popularly known as arbuscular mycorrhizal fungi (AMF). Different species of AMF viz., Acaulospora, Entrophospora, Gigaspora, Glomus, Pacispora, Sclerocystis, and Scutellospora have been observed to colonize citrus roots for the formation of arbuscular mycorrhizal (AM) symbiosis, where both the symbiotic partners are mutually benefited (up to 20% of photosynthetic carbohydrates from the host plant is diverted toward the growth of AM, in the exchange of water and nutrient uptake from the fungal partner to the host plant). AM symbiosis can usually confer better plant growth, higher nutrient uptake, greater tolerance to abiotic and biotic stresses, and soil structure improvement in the host plant. Meanwhile, AM-inoculated citrus plants have shown greater tolerance to drought stress (DS). Drought stress strongly restricted both the development of non-AM-citrus and the mycorrhizal development of AM-citrus, but AM colonization produced a positive effect on plant growth and photosynthesis, even under DS. This review provides an overview of possible mechanisms involved in DS tolerance through improved water and nutrient uptake (especially P nutrition) using extraradical hyphal growth; effective spatial configuration of root system; elevated concentration of tetramine spermine; osmotic adjustment through non-structural carbohydrates, K+, Ca2+, and Mg2+, but not proline; scavenging reactive oxygen species through antioxidant enzymes and antioxidants; and glomalin-bound soil structural improvements, besides, some new exciting perspectives including water transport by mycorrhizal hyphae and molecular analysis are suggested.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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