Proteolysis of ribulose‐1,5‐bisphosphate carboxylase/oxygenase in leaves of Citrus explants throughout the annual cycle and its regulation by ethylene

Abstract

of rihulose-I ,S-bisphosphate carboxylase/oxygenase in leaves o f Citrus explants throughout the annual cycle and its regulation by ethylene. -Physiol. Plant. 73: 14.Leaf senescence and ribulose-1 ,5-bisphosphate carboxylase/oxygcnase (RuBP carboxylase. EC 4.1.1.3Y) degradation in orange [Citrus sinensis (L.) Osbeck cv. Washington Navel] explants have been investigated. Explants consisted of a segment o f stem (ca 15 cm) and 5 mature leaves. I n vitro RuBP carboxylase degradation was determined by culturing the explants in water for diffcrcnt periods of time (3 days usually) and quantifying the two RuBP carboxylasc subunits in the extracts following sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). In vitro RuBP carboxylase degradation was estimated by autodigestion of leaf cxtracts and SDS-PAGE. The extent of in vivo RuBP carboxylase degradation in explants cultured under 16 h light/8 h dark photoperiod varied throughout the year and showed a cyclic behaviour correlated with the growth cycle of Citrus. The highest proteolytic activity both in vivo and in vitro was found in explants made from April to August coinciding with the maximum vegetative growth period of the tree. Leaf senescence and abscission could be retarded significantly at any time of the year by maintaining the explants continuously in the dark. Treatment of the explants in the dark with a continuous flow of ethylene enhanced both leaf abscission and rate of RuBP carboxylase degradation, proportionally to ethylene concentration ( 0 . 1 4 . 6 ppm). Ethylene-induced senescence of Citrus leaf explants in the dark appears to be a convenient model system to study the regulation of the proteolytic degradation of RuBP carboxylase.

Evidence for the involvement of ethylene in the expression of specific RNAs during maturation of the orange, a non-climacteric fruit

“…This contrasts with what happens in tomato and avocado, where only 19 and 5 different transcripts were found out of 146 and 15 ripening related clones analysed, respectively [9,14]. These resuits indicate that ethylene induced changes in the levels of a number of specific m R N A s , and that the levels of these m R N A s do not reach levels as high as those achieved in climacteric fruits.…”

“…These two transcripts were not detected until 6 h of treatment, from this stage they increased continuously reaching a maximum at 48 h. These results agree with ethylene stimulating similar processes in flavedo and leaf tissues. Thus, the reported chlorophyll and Rubisco degradation induced by ethylene both in flavedo and leaves [ 1,2,14] could reflect similar ethylene-mediated changes in gene expression in both tissues. In tomato, some ripening-related mRNAs are induced by ethylene in leaves as well as in fruits [4].…”