Cucumber hypocotyls were cut in half transversely, and morphological and histochemical analyses of the process of tissue-reunion in the cortex were performed. Cell division in the cortex commenced 3 days after cutting, and the cortex was nearly fully united within 7 days. DAPI- staining and BrdU-labeling experiments indicate that nuclear DNA synthesis occurred during this process. A specific accumulation of pectic substances was observed in the cell wall of attached cells in the reunion region of the cortex. Cell division in the tissue-reunion process in the cortex was strongly inhibited by removal of the cotyledon, and this inhibition was reversed by the application of gibberellin (10^<-4> M GA_3), but not auxin (10^<-4> MIAA), to the tip of the cotyledon-excised plant. Furthermore, cell division in the cortex was inhibited by treatment of the cotyledon with 10^<-4>M uniconazole-P, and this inhibition was also reversed by simultaneous application of GA. In contrast, removing the shoot apex did not inhibit tissue-reunion, and normal cortex tissue-reunion was observed when 10^<-4>M TIBA was applied to the upper hypocotyl above the position of the cut. Furthermore, similar phenomena were observed in tomato seedlings, and cell division in the tissue-reunion process did not occur in cut hypocotyls of a tomato GA-deficient mutant (gib-1). These results suggest novel functions for GA in cell division during the process of tissue-reunion in the cortex, as well as involvement of the cotyledon in the production of GA.
