基金We thank Mary Bryk and Timothy Hall for critical suggestions to improve the manuscript,David Stelly and Keerti Rathore for assistance in GFP localization studies in onion cells,and Stanislav Vitha in the Microscopy and Imaging Center at Texas A&M University for technical support for epifluorescence microscopic image analysis in the transgenic plants.The work is supported by grants from the National Institutes of Health(GM067015)the National Science Foundation Plant Genome Research Program(DBI0077774)to Z J C.