We thank Dr. Jeff Young, Dr. Pat Krysan and the Arabidopsis Knockout Facility (University of Wisconsin – Madison, USA) for assistance in identifying Arabidopsis knockout mutants. We are grateful to Dr. Ling Zhang (Soil Analysis Lab, University of Wiscon- sin) and to Armand Krueger (Arlington Agricultural Station, University of Wisconsin) for technical assistance. We thank Dr. Jamie Lasswell and Dr. Yoo-Sun Noh for critical reading of this manuscript. This work was supported by the College of Agriculture and Life Sciences of the University of Wisconsin – Madison, the Consortium for Plant Biotech- nology Research (grant no. DE-FG02-97ER20280) and the United States – Israel Binational Agricultural Research and Development Fund (BARD, Grant No. IS-2894-97).