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.

Light strongly promotes gene transfer from Agrobacterium tumefaciens to plant cells.

Light conditions during Agrobacterium-based plant transformation, the most routinely used method in plant genetic engineering, differ widely and, to our knowledge, have not been studied systematically in relation to transformation efficiency. Here, light effects were examined in two already optimized transformation procedures: coculture of Agrobacterium tumefaciens with callus from two genotypes of the crop plant Phaseolus acutifolius (tepary bean) and coculture of root segments from two ecotypes of Arabidopsis thaliana. Except for the light conditions during coculture, all steps followed established procedures. Coculture was done either under continuous darkness, under a commonly used photoperiod of 16 h light/8 h darkness or under continuous light. beta-glucuronidase (GUS) production due to the transient expression of an intron-containing uidA gene in the binary vector was used to evaluate T-DNA transfer. In all situations, uidA expression correlated highly and positively with the light period used during coculture; it was inhibited severely by darkness and enhanced more under continuous light than under a 16 h light/8 h dark photoperiod. The promotive effect of light was observed with Agrobacterium strains harboring either a nopaline-, an octopine- or an agropine/succinamopine-type non-oncogenic helper Ti plasmid. The observed positive effect of light has obvious implications for developing and improving transient and stable transformation protocols, specifically those involving dark coculture conditions.