The effect of an elevated cytokinin level using the ipt gene and N⁶-benzyladenine on single node and intact potato plant tuberization in vitro

Two models of potato (Solanum tuberosum L.) tuberization in vitro (intact plants and single nodes) were used to study the role of cytokinins in this process. We applied hormone in two different ways. The exogenous addition of 10 mg · L^-1N⁶-benzyladenine (BA) into the tuberization medium resulted in advanced tuber formation in intact plants, and microtubers appeared 10-20 days earlier than in the experiments in which no cytokinin was supplied. Transformation with the Agrobacterium tumefaciens ipt gene provided potato clones with endogenously elevated cytokinin levels (3-20 times higher zeatin riboside content in different clones). The onset of tuberization in intact ipt-transformed plants with low transgene expression was advanced in comparison with control material, and exogenously applied BA further promoted the tuberization process. On the contrary, tuberization was strongly inhibited in ipt-transformed nodes, and an external increase of the cytokinin level caused complete inhibition of expiant growth. In untransformed (control) nodes cytokinin application resulted in primary and secondary tuber formation, which depended on the BA concentration in cultivation media.