Influence of ethylene and Ag+ on hypocotyl growth in etiolated lupin seedlings. Effects on cell growth and division

Lupin seeds treated with 1-amino-cyclopropane-1-carboxylic acid (ACC) or 2-chloroethylphosphonic acid (CEPA) produced hypocotyls showing a typical ethylene growth response (reduced elongation and increased thickness), which could be efficiently counteracted by the presence of silver thiosulfate (STS). The fact that ACC and CEPA stimulated the ethylene produced in different zones along the hypocotyls suggests that these compounds, which are stored in the seeds during treatment, were transported to and along the hypocotyl. The same is true in hypocotyls from STS-treated seeds, which indicates that stress ethylene is induced by metal toxicity. CEPA was more effective than ACC in both producing ethylene and influencing growth due to the high capacity of the hypocotyl to conjugate ACC. At the same time that CEPA inhibited hypocotyl elongation, the hypocotyl diameter increased and ethylene production exceeded the maximum value of the control. The subsequent recovery of hypocotyl elongation coincided with a decrease in ethylene production and involved cell elongation. The final cell length was similar (in ACC-) or higher (in CEPA-treated plants) than in the control, although the hypocotyls were shorter in both cases, while the number of cells per column was reduced to half that observed in the control. This inhibition of cell division caused by ethylene was selective since the number of cell layers did not change. The variations in cell diameter in the epidermis and, especially, in the cortex and pith were correlated with the variations in hypocotyl diameter produced by ACC, CEPA and STS. The results show that the ethylene-induced hypocotyl thickening was irreversible and mainly due to an increase in cell diameter, while the inhibition of hypocotyl elongation was reversible and involved irreversible inhibition of cell division and, paradoxically, stimulation of cell elongation to produce cells longer than those of the control.