Osmotic pretreatment promotes axillary shooting from cauliflower curd pieces by acting through internal cytokinin level modifications

In vitro propagation of cauliflower has generally been achieved through axillary shoot proliferation of curd explants on Murashige and Skoog (MS) medium supplemented with an auxin and a cytokinin. Recently, it has been shown (Vandemoortele 1999) that a soaking in sucrose (-2 MPa for 24 h) of cauliflower curd explants, before culture without any growth regulator, also induced axillary branching. The later procedure avoids the phenomenon of hyperhydricity in the shoots formed. Axillary shooting obtained by the two methods appears to be mediated by modifications of internal cytokinin levels. The osmotic pretreatment did not influence auxin levels, but induced a zeatin and a zeatin riboside levels increase. Curd explants cultured with the usual procedure (on MS medium supplemented with 5 mu mol/L BA and 0.5 mu mol/L NAA) showed a zeatin and zeatin riboside levels increase of the same magnitude and a higher one for isopentenyl adenine and isopentenyl adenosine. The modification of the cytokinin status in the curd explants subjected to a short osmotic pretreatment thus should be less favourable for hyperhydricity.