锰对外生菌根真菌生长、养分吸收、有机酸分泌和菌丝体中锰分布的影响

外生菌根真菌对于酸性和锰污染土壤的植树造林和生态恢复有重要作用。采用液体培养方法,以大白菇Rd Fr(Russula delica Fr.)、彩色豆马勃Pt 715(Pisolithus tinctorius 715)、土生空团菌Cg Fr(Cenococcum geophilum Fr.)和厚环粘盖牛肝菌Sg Kl S(Suillus grevillei(Kl.)Sing)为供试对象,研究了Mn2+对外生菌根真菌生长、养分吸收、有机酸和氢离子分泌的影响,以及锰在菌丝细胞内外的分布情况。结果表明:在0—800 mg Mn2+/L的培养液中,Mn2+对Rd Fr生长无显著影响;低浓度的Mn2+刺激Sg Kl S生长,中、高浓度无抑制作用;但大幅度降低Pt 715和Cg Fr的生物量,说明Rd Fr和Sg Kl S抗(耐)锰的能力较强。在Mn2+胁迫下,供试菌株的氮、钾含量和吸收量显著降低;含磷量和吸收量,以及草酸和柠檬酸的分泌速率因菌株不同而表现出多样性,说明在减轻Mn2+毒的过程中,磷酸盐(或聚磷酸盐)对Mn2+固定作用和有机酸的络合作用因菌株不同而异。但是,Mn2+显著降低Rd Fr和Sg Kl S的氢离子分泌速率,菌丝和原生质中的含Mn量显著低于敏感性菌株,说明降低Mn2+的活性和减少吸收可能是外生菌根真菌抗(耐)Mn2+的重要机制。此外,菌丝吸收的Mn2+绝大部份存在于质外体,少量进入细胞,前者是后者的5.23—9.21倍,说明原生质膜是外生菌根真菌防御Mn2+进入细胞的重要屏障。

Influence of manganese on growth, nutrient uptake, and organic acid efflux by ectomycorrhizal fungi and manganese distribution in hyphae

Acid soils are widespread in tropical and subtropical areas in the world and contain a high concentration of active manganese (Mn) at pH lower than 5.5. Ectomycorrhizal (ECM) fungi are important for artificial forestation in acid soils and ecological restoration in Mn-polluted regions. A better understanding of resistance mechanisms to Mn and selection of proper ECM fungi are vital prerequisites for producing ECM seedlings in nursery beds. Therefore, Russula delica Fr. (Rd Fr), Pisolithus tinctorius 715 (Pt 715), Cenococcum geophilum Fr. (Cg Fr), and Suillus grevillei (Kl.) Sing (Sg KlS) were grown in liquid culture media with various Mn²⁺ concentrations, in order to study the growth, nutrient uptake (nitrogen, phosphorus, and potassium), organic acid efflux, and Mn distribution in fungal hyphae. The results showed no significant influence of Mn²⁺ on the growth of Rd Fr at concentrations between 0 and 800 mg Mn²⁺/L. Low Mn²⁺ concentrations simulated the growth of Sg KlS and no growth inhibition was observed at high Mn²⁺ concentrations. In contrast, the growth rate of Pt 715 and Cg Fr was significantly reduced by Mn²⁺, and the biomass was decreased by 22.75% for Pt 715 and 33.33% for Cg Fr at high Mn²⁺ concentrations, compared to the control (0 mg Mn²⁺/L). These results suggested that Rd Fr and Sg KlS were highly resistant to Mn²⁺, probably because of their environmental adaptation and biological evolution, as a result of their long-term habitation in acid soils with high concentrations of active Mn²⁺. A significant decrease was observed in both the content and absorption of nitrogen and potassium by ECM fungi in liquid media with Mn²⁺ added, which could be unbeneficial to host nutrient uptake after the formation of ectomycorrhizas under Mn stress. The content and uptake of phosphorus and efflux of oxalate and citrate by ECM fungi showed diverse changes among fungi grown in culture media with added Mn²⁺. These results indicated that Mn²⁺ fixation by phosphate or polyphosphates and Mn²⁺ complexation by organic acids in the process of alleviating Mn²⁺ toxicity varied among ECM fungal strains. Under Mn²⁺ stress, proton efflux rate, and Mn content in hyphae, particularly in protoplasm, of Rd Fr and Sg KlS were consistently lower than those of Cg Fr and Pt 715, the sensitive fungi to Mn²⁺. The decreased availability and absorption of Mn²⁺ could thus be considered as an important mechanism of ECM fungi to alleviate Mn²⁺ toxicity. In addition, most of Mn²⁺ absorbed by fungal hyphae located in apoplasts and only few entered into protoplasm. Mn²⁺ in apoplast ranged between 5.23 and 9.21 folds of protoplasm. Therefore, plasma membrane could be considered as an important barrier of ECM fungi against Mn²⁺ entry into the cells. Since lab culture and field conditions are different, further studies are necessary on the influence of Mn stress on tree roots and ECM fungi in the field.