Dual morphogenic role of 1,3-di(benzo[d]oxazol-5-yl)urea: xylogenesis enhancer or adventitious rooting adjuvant in carob and pine microcuttings

The asexual propagation of Ceratonia siliqua L. (carob), a Mediterranean species of economic value, is difficult because of adventitious rooting recalcitrance. In Pinus radiata adventitious rooting of hypocotyl cuttings is enhanced by the 1,3-di(benzo[d]oxazol-5-yl)urea (5-BDPU), combined with exogenous indole-3-butyric acid (IBA). In carob, 5-BDPU (10 mM) highly promoted AR-formation in combination with exogenous IBA (1 mM) when applied for 3 days, followed by a transfer onto hormone free medium (HF) up to culture end (four weeks). The research was aimed to define the role of this urea derivative in adventitious root (AR) formation of carob, and to identify morphogenic roles induced not only in carob but also in pine, a distantly-related forest species. The histological analysis showed that the cambial cells initiated the ARs, and a similar number of AR-primordia were visible at the same time (day 12), independently of the AR-inductive treatment (i.e., IBA, IBA+5-BDPU). The number of AR-forming explants increased with time in the presence of IBA+5-BDPU. By contrast, the cambial cells were stimulated to initiate deuteroxylem instead of ARs when 5-BDPU was applied alone. The histological analysis in pine microcuttings treated with IBA and/or 5-BDPU at the same concentrations confirmed that 5-BDPU applied alone enhanced xylogenesis, highlighting that this urea derivative exhibits a dual morphogenic role being involved in the switching between adventitious rooting and xylogenesis depending on the presence of exogenous auxin in both species.