Taylor D L, Bruns T D
Division of Plant and Microbial Biology, University of California, Berkeley, CA 94720, USA.
Proc Natl Acad Sci U S A. 1997 Apr 29;94(9):4510-5. doi: 10.1073/pnas.94.9.4510.
We have investigated the mycorrhizal associations of two nonphotosynthetic orchids from distant tribes within the Orchidaceae. The two orchids were found to associate exclusively with two distinct clades of ectomycorrhizal basidiomycetous fungi over wide geographic ranges. Yet both orchids retained the internal mycorrhizal structure typical of photosynthetic orchids that do not associate with ectomycorrhizal fungi. Restriction fragment length polymorphism and sequence analysis of two ribosomal regions along with fungal isolation provided congruent, independent evidence for the identities of the fungal symbionts. All 14 fungal entities that were associated with the orchid Cephalanthera austinae belonged to a clade within the Thelephoraceae, and all 18 fungal entities that were associated with the orchid Corallorhiza maculata fell within the Russulaceae. Restriction fragment length polymorphism and single-strand conformational polymorphism analysis of ectomycorrhizal tree roots collected adjacent to Cephalanthera showed that (i) the fungi associated internally with Cephalanthera also form typical external ectomycorrhizae and that (ii) ectomycorrhizae formed by other Basidiomycetes were abundant where the orchid grows but these fungi did not associate with the orchid. This is the first proof of ectomycorrhizal epiparasitism in nature by an orchid. We argue that these orchids are cheaters because they do not provide fixed carbon to associated fungi. This view suggests that mycorrhizae, like other ancient mutualisms, are susceptible to cheating. The extreme specificity in these orchids relative to other ectomycorrhizal plants agrees with trends seen in more conventional parasites.
我们研究了来自兰科不同族的两种非光合兰花的菌根共生关系。发现这两种兰花在广泛的地理范围内仅与外生菌根担子菌的两个不同分支形成共生关系。然而，这两种兰花都保留了不与外生菌根真菌共生的光合兰花典型的内生菌根结构。对两个核糖体区域的限制性片段长度多态性和序列分析以及真菌分离为真菌共生体的身份提供了一致的、独立的证据。与秀丽头蕊兰相关的所有14个真菌实体都属于革菌科的一个分支，与珊瑚兰相关的所有18个真菌实体都属于红菇科。对在秀丽头蕊兰附近采集的外生菌根树根进行限制性片段长度多态性和单链构象多态性分析表明：（i）在内部与秀丽头蕊兰相关的真菌也形成典型的外部外生菌根，并且（ii）在兰花生长的地方，由其他担子菌形成的外生菌根很丰富，但这些真菌不与兰花共生。这是自然界中兰花外生菌根寄生现象的首个证据。我们认为这些兰花是欺骗者，因为它们不向相关真菌提供固定碳。这种观点表明，菌根与其他古老的共生关系一样，容易受到欺骗。相对于其他外生菌根植物，这些兰花的极端特异性与在更传统的寄生植物中看到的趋势一致。
