CELLULASE ACTIVITY AND GENE-EXPRESSION IN CITRUS-FRUIT ABSCISSION ZONES DURING AND AFTER ETHYLENE TREATMENT

Mature and immature 'Valencia' orange [Citrus sinensis (L.) Osbeck] an d immature 'Valencia' orange and 'Tahiti' lime (Citrus latifolia Tan.) fruit with attached pedicels were treated with 8 mu L.L-1 ethylene fo r periods up to 24 hours. Endo-beta-1,4-glucanase (cellulase) activity and gene expression were determined in fruit abscission zones during and after ethylene exposure. Cellulase activities were not detected in mature 'Valencia' orange and immature 'Tahiti' lime fruit abscission zones immediately following harvest and after 6 hours of ethylene trea tment. After 12 hours of ethylene treatment, cellulase activity increa sed and was highest after 24 hours. Cellulase gene expression preceded the rise in cellulase activity and was detectable after 6 hours of et hylene treatment, but then declined after 12 hours. Following transfer to air storage, abscission zone cellulase activity in mature 'Valenci a' fruit remained high, whereas activity in immature 'Tahiti' fruit de clined. After 168 hours air storage, activity in abscission zones of m ature 'Valencia' fruit decreased slightly, but activity in abscission zones of immature 'Tahiti' lime fruit increased to the highest level. Expression of abscission zone cellulase gene Cel-a1 in abscission zone s of mature 'Valencia' fruit markedly increased after transfer to air and was highest after 48 hours air storage. Cel-a1 expression returned to low levels after 168 hours of air storage, but expression of cellu lase gene Cel-b1 remained at low levels throughout the air storage per iod. Expression of Cel-a1 and Cel-b1 declined in fruit abscission zone s of immature 'Valencia' and 'Tahiti' lime fruit upon transfer to air. After 168 hours of air storage, expression of Cel-a1 again rose to hi gh levels but Cel-b1 remained low. The results suggest that difference s in cellulase activity and gene expression measured in mature and imm ature fruit abscission zones during ethylene treatment and subsequent air storage may, in part, explain the differential response of mature and immature fruit to abscission agents.