Arbuscular mycorrhizal fungi alter phosphorus relations of broomsedge (Andropogon virginicus L.) plants

Rice ACID PHOSPHATASE 1 regulates Pi stress adaptation by maintaining intracellular Pi homeostasis

The concentration and homeostasis of intracellular phosphate (Pi) are crucial for sustaining cell metabolism and growth. During short-term Pi starvation, intracellular Pi is maintained relatively constant at the expense of vacuolar Pi. After the vacuolar stored Pi is exhausted, the plant cells induce the synthesis of intracellular acid phosphatase (APase) to recycle Pi from expendable organic phosphate (Po). In this study, the expression, enzymatic activity and subcellular localization of ACID PHOSPHATASE 1 (OsACP1) were determined. OsACP1 expression is specifically induced in almost all cell types of leaves and roots under Pi stress conditions. OsACP1 encodes an acid phosphatase with broad Po substrates and localizes in the endoplasmic reticulum (ER) and Golgi apparatus (GA). The phylogenic analysis demonstrates that OsACP1 has a similar structure with human acid phosphatase PHOSPHO1. Overexpression or mutation of OsACP1 affected Po degradation and utilization, which further influenced plant growth and productivity under both Pi-sufficient and Pi-deficient conditions. Moreover, overexpression of OsACP1 significantly affected intracellular Pi homeostasis and Pi starvation signalling. We concluded that OsACP1 is an active acid phosphatase that regulates rice growth under Pi stress conditions by recycling Pi from Po in the ER and GA.

The effects of soil phosphorus and zinc availability on plant responses to mycorrhizal fungi: a physiological and molecular assessment

The positive effects of arbuscular mycorrhizal fungi (AMF) have been demonstrated for plant biomass, and zinc (Zn) and phosphorus (P) uptake, under soil nutrient deficiency. Additionally, a number of Zn and P transporter genes are affected by mycorrhizal colonisation or implicated in the mycorrhizal pathway of uptake. However, a comprehensive study of plant physiology and gene expression simultaneously, remains to be undertaken. Medicago truncatula was grown at different soil P and Zn availabilities, with or without inoculation of Rhizophagus irregularis. Measures of biomass, shoot elemental concentrations, mycorrhizal colonisation, and expression of Zn transporter (ZIP) and phosphate transporter (PT) genes in the roots, were taken. Mycorrhizal plants had a greater tolerance of both P and Zn soil deficiency; there was also evidence of AMF protecting plants against excessive Zn accumulation at high soil Zn. The expression of all PT genes was interactive with both P availability and mycorrhizal colonisation. MtZIP5 expression was induced both by AMF and soil Zn deficiency, while MtZIP2 was down-regulated in mycorrhizal plants, and up-regulated with increasing soil Zn concentration. These findings provide the first comprehensive physiological and molecular picture of plant-mycorrhizal fungal symbiosis with regard to soil P and Zn availability. Mycorrhizal fungi conferred tolerance to soil Zn and P deficiency and this could be linked to the induction of the ZIP transporter gene MtZIP5, and the PT gene MtPT4.

Phosphorus Stress-Induced Changes in Plant Root Exudation Could Potentially Facilitate Uranium Mobilization from Stable Mineral Forms

Apparent deficiency of soil mineral nutrients often triggers specific physio-morphological changes in plants, and some of these changes could also inadvertently increase the ability of plants to mobilize radionuclides from stable mineral forms. This work, through a series of sand-culture, hydroponics, and batch-equilibration experiments, investigated the differential ability of root exudates of Andropogon virginicus grown under conditions with variable phosphorus (P) availability (KH₂PO₄, FePO₄, Ca₃(PO₄)₂, and no P) to solubilize uranium (U) from the uranyl phosphate mineral Chernikovite. The mineral form of P, and hence the bioavailability of P, affected the overall composition of the root exudates. The lower bioavailable forms of P (FePO₄ and Ca₃(PO₄)₂), but not the complete absence of P, resulted in a higher abundance of root metabolites with chelating capacity at 72 hrs after treatment application. In treatments with lower P-bioavailability, the physiological amino acid concentration inside of the roots increased, whereas the concentration of organic acids in the roots decreased due to the active exudation. In batch dissolution experiments, the organic acids, but not amino acids, increase the dissolution U from Chernikovite. The root exudate matrix of plants exposed to low available forms of P induced a >60% increase in U dissolution from Chernikovite due to 5-16 times greater abundance of organic acids in these treatments. However, this was ca. 70% of the theoretical dissolution achievable by this exudate matrix. These results highlight the potential of using active management of soil P as an effective tool to alter the plant-mediated mobilization of U in contaminated soil.

Nitrogen and carbon/nitrogen dynamics in arbuscular mycorrhiza: the great unknown

Many studies have established that arbuscular mycorrhizal fungi transfer N to the host plant. However, the role and importance of arbuscular mycorrhiza (AM) in plant N nutrition is still uncertain, as are the C/N interactions within the symbiosis. Published reports provide differing, and often contradictory, results that are difficult to combine in a coherent framework. This review explores questions such as: What makes the difference between a positive and a negative effect of AM on plant N nutrition? Is the mycorrhizal N response (MNR) correlated to the mycorrhizal growth response (MGR), and how or under which conditions? Is the MNR effect on plant growth C mediated? Is plant C investment on fungal growth related to N needs or N benefit? How is the N for C trade between symbionts regulated? The patternless nature of current knowledge is made evident, and possible reasons for this are discussed.

Effect of aluminium toxicity on growth responses and antioxidant activities in Gmelina arborea Roxb. inoculated with AM fungi

Arbuscular mycorrhizal fungi alleviating the adverse Aluminium effects on growth and antioxidant activity was tested in Gmelina plants. Under greenhouse and aluminium stress condition, the mycorrhizal Gmelina plants showed good growth as compared to non mycorrhizal Gmelina plants. Mycorrhizal colonization in Gmelina was found not to be significantly influenced by aluminium concentrations. Results also indicate that symbiotic association was successfully established between Glomus intraradices and Gmelina plants and mycorrhizal colonization consequently increased the biomass of Gmelina. The root proline accumulation was found to increase in mycorrhizal Gmelina plants for osmotic adjustment of stress tissues under first and second level of Aluminium stress. It was observed that Mycorrhizal colonization increased the shoot root Peroxidase and Superoxide dismutase activities in mycorrhizal Gmelina under second level of Aluminium stress. Mycorrhizal fungi play a major role in phytostabilization by secreting one of the glycoprotein, i.e., Glomalin, which stabilizes the Aluminium in soil as well as in the roots of Gmelina plants.

Role of mycorrhizal colonization in plant establishment on an alkaline gold mine tailing

The potential role of arbuscular mycorrhizal fungi (AMF) in the revegetation of an alkaline gold mine tailing was studied in Barberton, South Africa. The tailing, characterized by a slow spontaneous plant succession, is colonized by the shrub Dodonaea viscosa and the grasses, Andropogon eucomus and Imperata cylindrica, all colonized by AMF. The effectiveness of mycorrhizal colonization in grasses was tested under laboratory conditions using fungal isolates of various origins. Both grasses were highly mycorrhiza dependent, and the presence of mycorrhizal colonization significantly increased their biomass and survival rates. The fungi originating from the gold tailing were better adapted to the special conditions of the tailing than the control isolate. Although the total colonization rate found for native fungi was lower than for fungi from non-polluted sites, they were more vital and more effective in promoting plant growth. The results obtained might serve as a practical approach to the phytostabilization of alkaline gold tailings.

Metabolite profiling of mycorrhizal roots of Medicago truncatula

Metabolite profiling of soluble primary and secondary metabolites, as well as cell wall-bound phenolic compounds from roots of barrel medic (Medicago truncatula) was carried out by GC-MS, HPLC and LC-MS. These analyses revealed a number of metabolic characteristics over 56 days of symbiotic interaction with the arbuscular mycorrhizal (AM) fungus Glomus intraradices, when compared to the controls, i.e. nonmycorrhizal roots supplied with low and high amounts of phosphate. During the most active stages of overall root mycorrhization, elevated levels of certain amino acids (Glu, Asp, Asn) were observed accompanied by increases in amounts of some fatty acids (palmitic and oleic acids), indicating a mycorrhiza-specific activation of plastidial metabolism. In addition, some accumulating fungus-specific fatty acids (palmitvaccenic and vaccenic acids) were assigned that may be used as markers of fungal root colonization. Stimulation of the biosynthesis of some constitutive isoflavonoids (daidzein, ononin and malonylononin) occurred, however, only at late stages of root mycorrhization. Increase of the levels of saponins correlated AM-independently with plant growth. Only in AM roots was the accumulation of apocarotenoids (cyclohexenone and mycorradicin derivatives) observed. The structures of the unknown cyclohexenone derivatives were identified by spectroscopic methods as glucosides of blumenol C and 13-hydroxyblumenol C and their corresponding malonyl conjugates. During mycorrhization, the levels of typical cell wall-bound phenolics (e.g. 4-hydroxybenzaldehyde, vanillin, ferulic acid) did not change; however, high amounts of cell wall-bound tyrosol were exclusively detected in AM roots. Principal component analyses of nonpolar primary and secondary metabolites clearly separated AM roots from those of the controls, which was confirmed by an hierarchical cluster analysis. Circular networks of primary nonpolar metabolites showed stronger and more frequent correlations between metabolites in the mycorrhizal roots. The same trend, but to a lesser extent, was observed in nonmycorrhizal roots supplied with high amounts of phosphate. These results indicate a tighter control of primary metabolism in AM roots compared to control plants. Network correlation analyses revealed distinct clusters of amino acids and sugars/aliphatic acids with strong metabolic correlations among one another in all plants analyzed; however, mycorrhizal symbiosis reduced the cluster separation and enlarged the sugar cluster size. The amino acid clusters represent groups of metabolites with strong correlations among one another (cliques) that are differently composed in mycorrhizal and nonmycorrhizal roots. In conclusion, the present work shows for the first time that there are clear differences in development- and symbiosis-dependent primary and secondary metabolism of M. truncatula roots.

OsPTF1, a novel transcription factor involved in tolerance to phosphate starvation in rice

We report here on a novel transcription factor with a basic helix-loop-helix domain for tolerance to inorganic phosphate (Pi) starvation in rice (Oryza sativa). The gene is designated OsPTF1. The expression of OsPTF1 is Pi starvation induced in roots while constitutively expressed in shoots, as shown by northern-blot analysis. Overexpression of OsPTF1 enhanced tolerance to Pi starvation in transgenic rice. Tillering ability, root and shoot biomass, and phosphorus content of transgenic rice plants were about 30% higher than those of the wild-type plants in Pi-deficient conditions in hydroponic experiments. In soil pot and field experiments, more than 20% increase in tiller number, panicle weight, and phosphorus content was observed in transgenic plants compared to wild-type plants at low-Pi levels. In Pi-deficient conditions, transgenic rice plants showed significantly higher total root length and root surface area, which results in a higher instantaneous Pi uptake rate over their wild-type counterparts. Microarray analysis for transgenic plants overexpressing OsPTF1 has been performed to investigate the downstream regulation of OsPTF1.

Variation in aluminum resistance among arbuscular mycorrhizal fungi

Arbuscular mycorrhizal (AM) fungi mediate interactions between plants and soils, and are important where nutrient or metal concentrations limit plant growth. Variation in fungal response to edaphic conditions may influence the effectiveness of the plant-mycorrhizal association in some soil environments. Andropogon virginicus (broomsedge) colonizes disturbed sites in the eastern United States, including acidic mine soils where aluminum (Al) is phytotoxic, and Al resistance in broomsedge has been associated with colonization by the AM fungus Glomus clarum. In the present study, inter- and intra-specific variation to confer Al resistance to broomsedge was assessed among selected species of AM fungi. Broomsedge seeds were grown in sand culture inoculated with one of five isolates of three species of fungi (G. clarum, Acaulospora morrowiae, and Scutellospora heterogama). Plants were exposed to 0 or 400 microM Al in nutrient solution and harvested after 4 or 9 weeks of growth. Mean infection percentage, plant biomass, and plant tissue Al and phosphorus (P) concentrations were measured. G. clarum conferred the greatest Al resistance to broomsedge, with the lowest variability among isolates for colonization and growth inhibition by Al [tolerance indices (TI) between 22.4 and 92.7%]. Broomsedge plants colonized by A. morrowiae were consistently the most sensitive to Al, with little variation among isolates (TI between 1.6 and 12.1%). Al resistance by S. heterogama isolates was intermediate and wide-ranging (TI between 3.9 and 40.0%). Across all AM fungal isolates, resistance was associated with high rates of colonization and low tissue Al concentrations of broomsedge plants. The functional diversity in Al resistance displayed by these AM fungi reflect variation in acclimation mechanisms operating in the mycorrhizal symbiosis under environmental stress.