1 Plant Symbiosis Research Unit, National Institute of Agrobiological Sciences, 2-1-2 Kannondai, Tsukuba, Ibaraki 305-8602, Japan. Electronic address: makotoh@affrc.go.jp.
2 Faculty of Biology, Genetics, University of Munich (LMU), Martinsried, Germany. Electronic address: parniske@lmu.de.