Endo‐1,4‐β‐glucanase gene expression and cell wall hydrolase activities during abscission in Valencia orange

Abstract

The physiological and molecular events of ethylene‐induced abscission in mature fruit calyx, laminar and floral abscission zones of cv. Valencia orange were examined. Continuous exposure of fruit explants to 5 µl 1−1 ethylene for 2 to 40 h resulted in marked increases in endo‐1,4‐β‐glucanase (cellulase) and polygalacturonase (PG) activities in calyx abscission zones. Two abscission‐related cellulases and one PG were found. The major peak of cellulase activity corresponded to a pI of 8.0 and molecular weight of 51 kDa, whereas the minor cellulase peak 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 cellulase, and cloned. The two partial cellulase cDNA clones were 59% identical at the nucleotide level. Genomic Southern analysis suggested that Valencia orange contained two groups of cellulase genes. A full‐length cDNA clone from each group was isolated from a cDNA library prepared from ethylene‐induced calyx abscission zone mRNA. Both genes were expressed in ethylene‐induced calyx, laminar and floral abscission zones, but were not expressed in non‐induced abscission zones or mature leaves treated with or without ethylene, young bark or young fruit of Valencia.