| Al3+is widely present in acid soils with pH<5.5. Al3+toxicity and soil acidification is one of the reasons for forest decline. Ectomycorrhizal fungi (ECF) are the important component of forest ecosystems. Some of them can promote tree growth, increase the ability resistant to stress, and stimulate nutrient absorption. Therefore, the study on the mechanisms of ECF resistant to aluminum could be of significance for artificial forestation and natural forest production. Our present experiment was carried out to study fungal Al3+distribution in hyphae, growth, nutrient uptake, organic acid efflux, and enzyme activity related to organic acid metabolisms. Three of fungal species, including Pisolithus tinctorius715,(Pt715) isolated from red soil (pH<5.9), Suillus luteus (L. Fr.) Gray,(Sl13) from yellow soil pH4.0with in Jinfushan, Chongqing, and Suillus subluteus (Peck)Snell ex Slipp&Snell,(Ss00) from soil with pH=7.1in Daqinshan, inner Mongolia were cultured in liquid solution with various Al3+concentrations. Following are the main results obtained:(1) ECMF varied significantly in response to Al3+, with Pt715showing the higher ability resistant to Al3+, followed by SI13and Ss00with lowest, implying variable sensitivity to Al3+among same fungal genus.(2) when fungal isolates were grown in solution with0.5-2.5mmol Al3+·L-1, Al3+distributed mainly in apoplast, which accounted for91-98%of total Al3+in hyphae. Among them, exchangeable Al3+reached up to53-59%of Al3+in apoplast. Al3+entered into cells was quite small. Even so,20-80%of them were bumped into vacuoles. Cell membrane as a barrier and apoplast with an ability to fix Al3+could be one of the reasons for ECF to decrease Al3+toxicity.(3) Under Al3+stress, more phosphorus uptake by Pt715could increase the ability resistant to Al3+due to the similar trend for phosphorus content and non-exchangeable Al3+in apoplast.(4) ECF varied greatly in the amount and types of organic acid efflux in Al3+stress. Oxalate, malate, and citrate were detected in all culture solutions. In addition, succinic acid, formic acid and acetic acid were also detected in the culture solution with Pt715, which was the fungal isolate to effuse largest amount and number of organic acids.(5) Fungal species and incubation time influenced pH in culture solutions with Al3+. As Al3+increased, pH in solution with Pt715and Sl13decreased more obviously than Ss00. pH decreased by1.2in solution with Pt715, a specie resistant to Al3+, during the initial time of incubation (3-6d), suggesting fast reaction of the fungus to Al3+stress by the efflux of protons and organic acids.(6) The concentration of oxalate in solution with Pt715was2-4folds of blank control during the initial incubation period (within3d) and other two fungi showed no any sign of this phenomenon, indicating that Pt715could have the ability to adapt Al3+stress.(7) The influence of Al3+stress on the activities of malate dehydrogenase and isocitrate lyase also varied among fungal isolates. There was a higher activity of malate dehydrogenase in Ss00and Pt715than Sl13. Under Al3+stress, activity of malate dehydrogenase showed a positive correlation with oxalate efflux by Ss00. the activity of isocitrate lyase increased as Al3+increased. After reached the peak, the enzyme activity behaved otherwise as Al3+increased. Activity of isocitrate lyase in Sl13changed little irrespective of Al3+variation. |
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