Ethylene involvement in vegetative bud formation in tobacco thin layers

The role of ethylene in vegetative bud regeneration was studied in cultured tobacco (Nicotiana tabacum L. cv Samsun) thin-layer explants. The experimental approach consisted in supplementing the bud-inducing medium with an inhibitor of ethylene biosynthesis, aminoethoxyvinylglycine (AVG), an ethylene antagonist, silver thiosulphate (STS), or an ethylene-releasing compound, 2-chloroethylphosphonic acid (CEPA), at various concentrations. The organogenic response was assessed both macroscopically (percentage of bud-forming explants, final number of buds per explant) and cytohistologically (number, characteristics, and localisation of meristemoids and bud primordia). The time course of ethylene production during culture was also evaluated. At the end of culture (day 27) all the explants treated with these compounds had a lower number of buds compared to controls. STS was detrimental to meristemoid initiation at all the concentrations tested. In contrast, 0.5 mu M AVG, which strongly inhibited ethylene production, provoked a large increase in the formation of meristemoids early in culture and the appearance of anomalous ("twin") buds. CEPA reduced meristemoid formation but, at the lower concentrations (1 and 10 mu M) speeded up bud emergence. On the whole it mainly favoured disorganised growth and xylogenesis. The results of this work highlight the contrasting effects of ethylene in relation to the two critical stages of the organogenic process, i.e., meristemoid formation and bud primordium development.