汞对外生菌根真菌氮素利用酶活性的影响

在含有0、5、50和150μmol/L Hg2+的液体培养基中培养土生空团菌Cenococcum geophilum Fr.菌株Cg SIV、彩色豆马勃Pisolithus tinctorius(Pers.)Coker et Couch菌株Pt715、松乳菇Lactarius sp.菌株Ld-1和Ld-3,研究汞对外生菌根真菌生长和氮素利用酶活性的影响。结果表明,汞对外生菌根真菌生长有不同程度的抑制作用,其中Cg SIV生物量降幅最小,在高汞浓度的培养基中生物量仅比对照减少9.7%,具有较高的耐汞性。供试菌株均能合成蛋白酶、几丁质酶、脲酶和硝酸还原酶,但不同菌株之间酶活性差异显著。说明外生菌根真菌既有益于寄主植物利用氮源的多样性,又具有对不同氮源的偏嗜性。汞对外生菌根真菌氮素利用酶活性的影响因菌株、酶类和汞浓度的不同而异,原因可能是不同菌株遗传特性的差异,使其在汞胁迫条件下产酶量不同,并表达对汞敏感性不同的等位酶。此外,低浓度(5μmol Hg2+/L)~中浓度(50μmol Hg2+/L)的汞提高或不影响氮素利用酶的活性,对外生菌根真菌氮素利用能力应无抑制作用。在正常和汞胁迫条件下,Pt715和Ld-3的蛋白酶、脲酶、硝酸还原酶和几丁质酶的活性均最高,表现出较强的氮素利用能力。推断在汞污染的土壤上种植桉树和松树,接种Pt715和Ld-3可能改善寄主植物的氮素营养。

Effects of Hg on activities of nitrogen utilizing enzymes in ectomycorrhizal fungi

Mycelial biomass and protease, urease, nitrate reductase and chitinase activities of four ectomycorrhizal fungal isolates, viz. Cenococcum geophilum strain Cg SIV, Pisolithus tinctorius strain Pt715 and two Lactarius sp. strains Ld-1 and Ld-3, were evaluated in pure liquid culture containing 0, 5, 50 or 150μmol/L Hg2+. Growth of fungal hyphae was restrained by mercury, whereas Cg SIV showed tolerance to Hg in some extent, with the least decrease on biomass. All of the four strains expressed protease,chitinase, urease and nitrate reductase, and showed significant differences in enzyme activities. It implied that ectomycorrhizal fungi might not only benefit to nitrogen utilization diversity of host plants, but also favored different nitrogen sources. Effects of Hg on activities of nitrogen utilizing enzymes in ectomycorrhizal fungi varied with strains, enzyme categories and Hg2+ concentration. Since the inherited characteristics differ among strains, enzyme synthesis may vary under condition of the mercury existence, and express allozymes with different sensitivity to Hg. Moreover, low to middle Hg concentration (5-50μmol Hg2+/L) promoted or did not affect the activities of nitrogen utilizing enzymes, suggesting that nitrogen utilization by ectomycorrhizal fungi might not be restraint or even be promoted. Activities of protease, urease, nitrate reductase and chitinase were the highest in Pt715 and Ld-3 with or without Hg treatment, which showed relatively high capacity of nitrogen utilization. Inoculating Pt715 and Ld-3 may improve nitrogen nutrition of eucalyptuses and pines planting in mercury contaminated soils.