Ethylene Responses and ACC oxidase Gene Expression in Lippia alba (Verbenaceae) Chemotypes with Varying Ploidy Levels

Lippia alba (Mill.) N. E. Br ex Britton & P. Wilson is a species of great economic importance due to the medicinal activity conferred by its essential oils. Ethylene (ET) is a gaseous hormone that affects several processes in plant growth and development. This hormone is synthesized through a reaction in which 1-aminocyclopropane-1-carboxylic acid (ACC) is converted to ET by the action of the enzyme ACC oxidase (ACO). The ET activity in three L. alba chemotypes (BGEN-01, triploid; BGEN-02, diploid; and BGEN-42, hexaploid) was evaluated by culturing plants in vitro in the presence of the ET precursor (ACC), scavenger mercury perchloride (MP), the inhibitors aminoethoxyvinylglycine (AVG) and silver thiosulfate (STS), and a CO2-enriched atmosphere. Leaf abscission rate, ET accumulation, and ACO gene expression (LaACO) were investigated. Leaf abscission was higher in BGEN-42, regardless of the precursor/inhibitor. ACC had the highest ET accumulation in BGEN-02 and BGEN-01, whereas BGEN-42 presented lower ET accumulation than the control. MP and AVG were effective in preventing ET production when compared to the control. LaACO was upregulated in BGEN-01 and BGEN-42 treated with ACC and in BGEN-01 treated with STS. In the CO2 enrichment experiments, LaACO expression was higher in BGEN-42 in the treatments without forced ventilation. Thus, the expression of this gene in L. alba can be altered by elevating CO2 levels and also by the addition of ACC. This is the first report of ET interactions in L. alba cultured in vitro