1.？ order
The relationship between plants and arbuscular mycorrhizal fungi (AMF) is characterized by two-way material exchange, that is, AMF delivers soil nutrients to plants, while plants in turn provide carbohydrates and lipids, which may be symmetrical. Abiotic factors, including atmospheric carbon dioxide (CO2) concentration, will affect the material exchange between living organisms, thus affecting the biological function of * * *. Whether biological factors such as plant-eating insects affect the function of mycorrhiza has not been studied. There is evidence that the carbon content transferred from plants to AMF may be strictly controlled by nutrients and plant assimilates provided by AMF. More experiments show that plants can distinguish mutually beneficial mycorrhizal fungi partners and give priority to allocating carbon to more cooperative AMF. Conversely, mycorrhiza will also be stimulated by plant carbon, resulting in closer ties.
Using 33P-labeled AMF( Rhizophagus irregularis) and 14CO2-labeled wheat (Triticum aestivum), the effects of increasing carbon sink intensity (i.e. aphids feeding on plants) and increasing carbon source intensity (i.e. increasing CO2 concentration) on substance exchange between plants and AMF * * * were studied. The results showed that the amount of carbon allocated by plants to AMF decreased significantly after the aphid feeding on plant leaves, but increasing CO2 concentration could not alleviate this result. However, no matter whether aphids feed or CO2 concentration increases, the 33P of plants to mycorrhizal pathway remains unchanged, which means that insect feeding leads to the asymmetry of carbon and phosphorus exchange between organisms. It is concluded that the increase of carbon sink strength (aphids feed on plants) can limit the carbon distribution of plants to AMF, without hindering the access to nutrients provided by mycorrhiza. The results of this study emphasize the environmental dependence of resource exchange between plants and AMF, and reveal that independent biological factors or abiotic factors are effective factors to regulate the function of * * *.
2.？ Materials and methods
2. 1.？ test controler
When planting wheat, a group of PVC pipes are inserted into the flowerpot (Figure 1A and Figure 2A). The matrix is filled in the middle as the capture culture of AMF spores. After the plant grows for 8 weeks, the aphid cage with or without aphids (+/-) is clamped on the third leaf of the plant.
Note: A. Each flowerpot is inserted in the tube. The pipeline consists of three cylinders. The outermost two closely contacted pipes (1 No.2 and No.2) contain a hollow pipe wall of 15 mm × 40 mm, and the outermost pipe (1 No.2) is equipped with a 35 micron nylon net (only hyphae are allowed to pass through). 1 pipe and 2 pipes can be rotated and dislocated. Right in the center is a silica capillary (No.3 tube) for adding 33P aqueous solution (Figure 2A). B. aphid cage. Interior decoration foam: cushioning foam (to avoid blade damage).
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2.2.？ 33P isotope tracer
After 24 hours in the aphid cage, add 100 mL 1 MBq? 33P orthophosphate solution (Figure 2A). Add a 33P flowerpot (n? = 6, "rotation" treatment) can rotate, destroying the connectivity between plants and hyphae in the tube. Before adding 33P, rotate the pipe wall and then rotate it every 48 hours. The remaining 33P flowerpots (n? = 6, "static" treatment) does not rotate, so the plant keeps the mycelium connected to the inner tube. The 33P(AM-33P) obtained by mycorrhiza was calculated by using the difference between "static" treatment and "rotating" treatment.
Note: A. Add 33P-labeled orthophosphate into the test tube that only AMF mycelium can reach. B the device is in a closed space, and 14C marks the release of sodium bicarbonate 14CO2. 14CO2 is fixed by plants and distributed on hyphae outside AMF roots, or assimilated by aphids in aphid cages.
2.3.？ 14C logo
After marking with 33P 12 days, the tops of the two pipes were sealed and the device was placed in a closed space (Figure 2B). At the beginning of the photoperiod of 16 hours, 14CO2 was released into the plant headspace. Using a syringe, the top air of plants was collected after 1ml, 1.5 and 4.5 hours, and the absorption of 14CO2 by plants was recorded.
2.4. Plant harvesting and sample preparation
Take off the aphid cage and aphids and store them at -20℃. Plant and soil samples are divided into aboveground parts, roots, non-pipe soil and pipe soil (dynamic treatment matrix and static treatment matrix). The roots were washed with tap water, some roots were stored at 5℃ to determine the infection rate of AMF in roots, some soils were stored at 5℃ to determine AM hyphae, and others were stored at -20℃.
3.？ result
3. 1.CO2 concentration and aphid feeding changed the carbon availability of host plants.
Different CO2 concentrations (AC02: 440 ppm and EC02: 800 ppm) and aphid feeding (+/-) were used to control the carbon source and sink strength of wheat. The aboveground biomass of wheat plants using ECCO2 is significantly higher than that of wheat plants using ECCO2 (Figure 3a; P < 0.001), the aboveground biomass decreased by 14% and1%respectively (fig. 3a; p & lt0.00 1)。
CO2 concentration had no effect on underground biomass of wheat (Figure 3B), but aphid treatment significantly reduced root biomass (P
3.2. Response of AMF to CO2 concentration and aphids
With or without aphid treatment, the infection rate of AMF was lower than that of aCO2 (Figure 4A, P
3.3. Aphid eating reduces the carbon allocated by plants to AMF.
In order to directly test the influence of dynamic carbon allocation of different plant carbon pools and sources on AMF, 14C was marked in a closed space, and the plant carbon recently fixed and allocated to mycorrhiza was quantified (Figure 2B). Compared with plants not exposed to aphids, the carbon content of plants exposed to aphids transferred to AMF decreased significantly (Figure 5A), and decreased by 97% and 73% on aCO2 and eCO2, respectively. When expressed as the percentage of fixed carbon in plants, aphid feeding also reduced the carbon allocation of plants to AMF (Figure 5B, P
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Note: a.14c; Fixed and assigned to AMF network; B the proportion of fixed plants 14C allocated to static root soil.
3.4. AM-33P absorbed by plants has nothing to do with plant carbon allocation.
Finally, the effect of dynamic intensity of carbon source on AM-33P absorption by plants was evaluated. Compared with aCO2, the total phosphorus concentration and mycorrhizal pathway of wheat plants in eCO2 decreased (Fig.6a, P
In order to quantify how plant carbon supply affects plant phosphorus assimilation through AMF alone, the researchers added 33P-labeled orthophosphate to the substrate that only AMF hyphae can reach (Figure 1A and Figure 2A), and measured its plant assimilation by liquid scintillation measurement. When treated with eCO2, the concentration of 33P from the aerial parts of AMF treated by aphids was higher (fig. 6B). The contents of total phosphorus and 33P showed similar results (Figures 6C and 6D). However, there is no correlation between AMF infection rate and AMF-mediated 33P absorption, or between carbon content distributed by plants and AMF-mediated 33P absorption. This shows that the function and abundance of AMF have nothing to do with the recent fixed plant carbon allocation.
Note: a. P concentration above the ground; B. ground 33P concentration transferred by AMF; C. p content in bud; D. the aboveground 33P content transferred by AMF.
4.？ conclusion
The increase of carbon sink strength outside plants (aphids eat leaves) will reduce the carbon allocated to AMF by plants, but the increase of plant carbon source intensity does not necessarily lead to the increase of carbon allocated to AMF. The transfer of phosphorus from AMF to plants has nothing to do with plant carbon distribution, and there is asymmetry of nutrient exchange carbon between them.
Editor: Feng Zengwei
Audit Yao Qing
Joint Team of Strain Group of Institute of Microbiology, Guangdong Academy of Sciences-Soil Microbiology Group of Horticulture College, South China Agricultural University