Differential response of tomato and tobacco to Agrobacterium mediated transformation with cytokinin independent -1 (CKI-1) gene as influenced by cytokinin levels.

Cytokinin independent-1 (CKI-1) gene was identified through its ability to confer cytokinin independent growth in Arabidopsis which has led to this gene being advocated as a selectable marker in plant transformation. Keeping this in view, CKI-1 gene was assessed as a selectable marker by transformation of tobacco (Nicotiana tabacum) and tomato (Lycopersicon esculentum L.). Overexpression of CKI-1 gene through Agrobacterium mediated transformation in tobacco and tomato conferred cytokinin independent shoot regeneration (in media devoid of cytokinin/plant growth regulators) in tobacco, but not in tomato wherein this ability (cytokinin independence) was conferred to T1 explants of CKI-1 transgenic tomato plant (T0) regenerated on cytokinin medium. Analysis of cytokinin levels revealed that cytokinin independent growth upon transformation with CKI-1 gene in tobacco (T0) and tomato (T1) was achieved through maintaining/regulating higher endogenous cytokinin levels and CKI-1 gene expression. Levels of CKI-1 transcripts assayed through quantitative RT-PCR suggested that there seemed to be a threshold level of endogenous cytokinin level, regulated due to external or internal supply via CKI-1 gene upto which CKI-1 gene expression correlated with endogenous cytokinin content and beyond that, either the gene expression was not induced or it remains same. With the incorporation of CKI-1 gene, it appeared that this threshold level of endogenous cytokinin might be reduced in crops like tomato to support shoot regeneration at lower concentration of cytokinin, but could not be made independent of external supply of cytokinin as in tobacco suggesting that use of CKI-1 gene as an effective alternate selection marker could not be universally applicable across the species. The results of the present study revealed that CKI-1 gene in addition to enhancing cytokinin levels, was also involved in contributing to the sensitivity to cytokinin and thus served as a positive regulator of cytokinin signal transduction.