Involvement of ethylene in the rooting of seedling shoot cultures of Bixa orellana L.

Abstract

Using seedlings derived from the shoot apex of annatto (Bixa orellana L. cv. Bico-de-Pato) we observed the rooting frequency of B. orellana, the number and length of roots and the rate of ethylene production during 30 d in culture. The rhizogenesis response was affected by auxins (NAA or IBA) and by both the ethylene biosynthesis precursor 1-aminocyclopropane-1-carboxylic acid (ACC) and the inhibitor 2-aminoethoxyvinylglycine (AVG). Auxin supplementation to the medium resulted in root induction, ethylene production, and an undesirable callusing in the epidermal and cortical tissues. Irrespective of the presence of auxins, supplementing the medium with ACC promoted ethylene biosynthesis and callusing, which resulted in increased cell proliferation mainly in the cortical and vascular tissues, while the epidermis was mostly unaltered. In both ACC and auxin-supplemented medium, increased ethylene production and callusing occurred, suggesting a synergistic effect between these two responses. ACC was capable of inducing adventitious root formation, but the roots produced had a wrinkled appearance when compared to normal roots. Conversely, AVG reduced ethylene production and callusing, while the epidermis, cortex, and inner tissues remained unaltered, regardless of the presence of auxins. AVG was beneficial in these aspects, although its application led to a reduction in the number of roots and in the average root length. In conclusion, it was not possible to establish a direct relation between ethylene and rooting, but we hypothesize that, under the experimental conditions described, ethylene may enhance tissue sensitivity to auxin. However, ethylene did not seem essential to the rhizogenesis process in annatto.