The Rg-1 encoded regeneration capacity of tomato is not related to an altered cytokinin homeostasis

Cytokinin (CK) metabolism was analyzed in tomato (Lycopersicon esculentum) Rg-1 hybrids during in vitro shoot organogenesis from root explants. Data were obtained by combining physicochemical analysis with quantification and in situ detection methods. Although exogenous zeatin is added in all classical regeneration protocols, we show here that regenerating (Rg(+)) tomato explants did not require an exogenous CK source for regeneration. Irrespective of the presence or absence of exogenous zeatin, the endogenous CK levels were not affected by Rg-1 in the initial explants or in the early callus phase. In a later stage, and related to the presence of numerous shoots, the Rg(+) explants showed much lower endogenous CK concentrations than the nonregenerating (rg(-)) explants. Cells of rg(-) explants were not able to differentiate, despite their high endogenous CK content, and did not respond to exogenously applied CKs. We show that the insensitivity of rg(-) explants to a hormonal signal, normally initiating regeneration, is not related to an altered endogenous CK metabolism. We therefore postulate that Rg-1 action involves a regeneration-specific CK receptor or a regeneration-specific CK signal transduction pathway.