MODIFICATION OF A SPECIFIC CLASS OF PLASMODESMATA AND LOSS OF SUCROSEEXPORT ABILITY IN THE SUCROSE EXPORT DEFECTIVE1 MAIZE MUTANT

We report on the export capability and structural and ultrastructural characteristics of leaves of the sucrose export defective1 (sed1; form erly called sut1) maize mutant. Whole-leaf autoradiography was combine d with light and transmission electron microscopy to correlate leaf st ructure with differences in export capacity in both wild-type and sed1 plants. Tips of sed1 blades had abnormal accumulations of starch and anthocyanin and distorted vascular tissues in the minor veins, and the y did not export sucrose. Bases of sed1 blades were structurally ident ical to those of the wild type and did export sucrose. Electron micros copy revealed that only the plasmodesmata at the bundle sheath-vascula r parenchyma cell interface in sed1 minor veins were structurally modi fied. Aberrant plasmodesmal structure at this critical interface resul ts in a symplastic interruption and a lack of phloem-loading capabilit y. These results clarify the pathway followed by photosynthates, the p ivotal role of the plasmodesmata at the bundle sheath-vascular parench yma cell interface, and the role of the vascular parenchyma cells in p hloem loading.