1. Wageningen University and Research Centre, Plant Research International B.V., P.O. Box 16, 6700 AA, Wageningen, the Netherlands
2. Department of Biology, Faculty of Science, Okayama University, Tsushima-naka, Okayama 700-8530, Japan
3. Plant Research International B.V., P.O. Box 16, 6700 AA, Wageningen, the Netherlands