Tsukuba Botanical Garden, National Museum of Nature and Science, Tsukuba, Japan.
Am J Bot. 2012 Jul;99(7):1158-76. doi: 10.3732/ajb.1100464. Epub 2012 Jul 4.
Mycoheterotrophic plants, which completely depend upon mycorrhizal fungi for their nutrient supply, have unusual associations with fungal partners. The processes involved in shifts in fungal associations during cladogenesis of plant partners from autotrophy to mycoheterotrophy have not been demonstrated using a robust phylogenetic framework.
Consequences of a mycorrhizal shift were examined in Cymbidium (Orchidaceae) using achlorophyllous and sister chlorophyllous species. Fungal associates of the two achlorophyllous mycoheterotrophs (C. macrorhizon and C. aberrans), their close relatives, the chlorophyllous mixotrophs (C. goeringii and C. lancifolium) and an outgroup, the chlorophyllous autotroph C. dayanum, were identified by internal transcribed spacers of the nuclear ribosomal DNA sequences.
Molecular identification of mycorrhizal fungi revealed: (1) the outgroup autotroph is predominantly dependent on saprobic Tulasnellaceae, (2) the mixotrophs associate with the Tulasnellaceae and ectomycorrhizal groups including the Sebacinales, Russulaceae, Thelephoraceae and Clavulinaceae, and (3) the two mycoheterotrophs are mostly specialized with ectomycorrhizal Sebacinales.
Fungal partners in Cymbidium have shifted from saprobic to ectomycorrhizal fungi via a phase of coexistence of both nutritional types of fungi. These three phases correspond to the evolution from autotrophy to mycoheterotrophy via mixotrophy in Cymbidium. We demonstrate that shifts in mycorrhizal fungi correlate with the evolution of nutritional modes in plants. Furthermore, gradual shifts in fungal partners through a phase of coexistence of different types of mycobionts may play a crucial role in the evolution of mycoheterotrophic plants.
完全依赖菌根真菌获取营养的菌根异养植物与真菌伙伴之间存在着特殊的共生关系。然而，在植物从自养到菌根异养的系统发育过程中，真菌共生关系转变的过程尚未通过稳健的系统发育框架得到证明。
本研究以兰科杓兰属（Cymbidium）为研究对象，利用无叶绿素和具叶绿素的姐妹种来检验菌根转变的后果。利用核核糖体 DNA 序列的内部转录间隔区（ITS）对两个无叶绿素菌根异养种（杓兰 C. macrorhizon 和 C. aberrans）及其近缘种、具叶绿素兼性异养种（杓兰 C. goeringii 和 C. lancifolium）和一个外类群（具叶绿素自养种杓兰 C. dayanum）的真菌共生体进行了鉴定。
真菌的分子鉴定结果表明：（1）外类群自养种主要依赖于腐生的 Tulasnellaceae；（2）兼性异养种与 Tulasnellaceae 和外生菌根共生群（包括 Sebacinales、Russulaceae、Thelephoraceae 和 Clavulinaceae）共生；（3）两个菌根异养种主要与外生菌根 Sebacinales 共生。
在杓兰属中，真菌共生体从腐生真菌转变为外生菌根真菌，经历了一个两种营养类型真菌共存的阶段。这三个阶段对应着杓兰属植物从自养到菌根异养再到兼性异养的进化过程。我们证明了菌根真菌的转变与植物营养模式的进化密切相关。此外，不同类型菌根共生体共存阶段中真菌共生体的逐渐转变可能在菌根异养植物的进化中发挥关键作用。
