ENDO-1,4-BETA-GLUCANASE GENE-EXPRESSION AND CELL-WALL HYDROLASE ACTIVITIES DURING ABSCISSION IN VALENCIA ORANGE

The physiological and molecular events of ethylene-induced abscission in mature fruit calyx, laminar and floral abscission zones of cv. Vale ncia orange were examined. Continuous exposure of fruit explants to 5 mu l 1(-1) ethylene for 2 to 40 h resulted in marked increases in endo -1,4-beta-glucanase (cellulase) and polygalacturonase (PG) activities in calyx abscission zones. Two abscission-related cellulases and one P G were found. The major peak of cellulase activity corresponded to a p I of 8.0 and molecular weight of 51 kDa, whereas the minor cellulase p eak had a pI of 5.5. The abscission polygalacturonase had a pI of 5.5. Calyx abscission zone RNA was amplified with degenerate primers based on sequence of the purified Valencia orange calyx abscission cellulas e, and cloned. The two partial cellulase cDNA clones were 59% identica l at the nucleotide level. Genomic Southern analysis suggested that Va lencia orange contained two groups of cellulase genes. A full-length c DNA clone from each group was isolated from a cDNA library prepared fr om ethylene-induced calyx abscission zcne mRNA. Both genes were expres sed in ethylene-induced calyx, laminar and floral abscission zones, bu t were not expressed in non-induced abscission zones or mature leaves treated with or without ethylene, young bark or young fruit of Valenci a.