1-AMINOCYCLOPROPANE-1-CARBOXYLIC ACID-INDUCED ETHYLENE STIMULATES CALLUS FORMATION BY CELL ENLARGEMENT IN THE CAMBIAL REGION OF INTERNODAL EXPLANTS OF CITRUS

摘要

The effect of 1-aminocyclopropane-1-carboxylic acid (ACC) treatments on callus formation from internodal explants of Troyer citrange (Citrus sinensis (L.) Osb. x Poncirus trifoliata) was investigated and the cellular types responsible of callus formation were identified. The in vitro culture of explants in media supplemented with 0.05, 0.5 and 2 mM ACC resulted in rapid growth of whitish soft friable calli, whereas no calli were observed in the absence of ACC. The calli were evident 3-5 days after culture and callus growth reached a stationary phase 11 days after culture. A rapid and transitory increase in the rate of ethylene production occurred at the highest ACC concentrations used. Callus formation correlated with amounts of ethylene released by the explants. The lowest ACC concentration (0.05 mM) stimulated callus growth in only 10-20% of the explants, with callus weighing 100 mg, and the explants produced low ethylene levels at day 3 of culture (33 nl g(-1) FW h(-1)). The highest ACC concentrations (0.5 and 2 mM) stimulated callus growth in 60-75% of the explants, with callus weights of 155 and 210 mg, respectively. The explants produced high levels of ethylene at day 3 of culture (330-430 nl g(-1) FW h(-1)). The inhibition of ethylene biosynthesis or action with cobalt chloride and silver thiosulfate, respectively, reduced callus formation in the ACC-treated explants in a concentration-dependent manner. Taken together, these observations indicate that ethylene synthesized from ACC is responsible for callus formation. Light-microscopy observations of the explants cultured in ACC showed that ethylene promoted enlargement of the procambial and parenchymatous cells located between the primary phloem fibers and the metaxylem. The examination of the micrographs also suggested that ethylene-induced cell growth is mostly due to radial expansion. Thus, ACC-induced ethylene is responsible for the enlargement of the procambial and parenchymatous cells from Citrus internodal explants that, in turn, results in the stimulation of callus formation. [References: 20]