Shooting Control in Eucalyptus Grandis × E. urophylla Hybrid: Comparative Effects of 28-Homocastasterone and a 5α-Monofluoro Derivative

28-Homocastasterone (28-HCTS), a brassinosteroid, was used to treat in vitro-grown shoots of a hybrid between Eucalyptus grandis and E. urophylla. Treated shoots showed enhanced elongation and formation of new main shoots (the shoots originating directly from the initial explant) at low doses. Coincidently, there was reduced elongation and formation of primary lateral shoots (shoots originating from the main shoot). However, a 5α-monofluoro derivative of 28-HCTS (5F-HCTS) was unable to either stimulate elongation or formation of new main shoots, although it did stimulate elongation of primary lateral shoots. In conclusion, it is quite apparent that exogenously supplied brassinosteroids are able to change shooting patterns in Eucalyptus. These findings have practical biotechnological applications, for example on the improvement of micropropagation techniques for clonal propagation of woody angiosperms.