Spatial separation of mycobionts in the giant, differentiated root system of Cyrtosia septentrionalis, a fully myco-heterotrophic orchid

The rare achlorophyllous orchid Cyrtosia septentrionalis is thought to switch mycobiont from a Physisporinus species (‘TK-10’) to an Armillaria species as it develops. We conducted morphological, chemical and molecular analyses of the root system of three adult plants growing sympatrically in Japan to assess reliance on different mycorrhizae. The root systems were found to comprise several long primary roots that radiate outwards from the emergent stem and many shorter lateral ones. Long roots branched most vigorously where they encountered A. gallica in the soil, and the resulting lateral roots contained the highest density of intra-radical rhizomorphs. One plant was associated with TK-10 and the other two with A. gallica, but only the apical portion of long roots and a few lateral roots were colonized. Mycobionts isolated from the base of long roots proved incapable of germinating C. septentrionalis seeds. Whereas glucose was the dominant monosaccharide in non-mycorrhizal roots, galactose was in mycorrhizal ones. Lateral roots are specialized in mycorrhization and their velamen and mucilage may play a key role in preventing desiccation. We hypothesize that C. septentrionalis can associate with either fungus from germination to adulthood, but that detection of A. gallica in the soil can prompt replacement of TK-10.