Analysis of primer independent phosphorylase activity in potato plants: high levels of activity in sink organs and sucrose-dependent activity in cultured stem explants.

One isoform of potato (Solanum tuberosum L., cv. Spunta), type L phosphorylase (EC 2.4.1.1), exhibiting primer independent activity appears to be tuber-specific. However, this activity can also be modulated by exogenous sucrose in storage as well as in non-storage organs. Primer independent phosphorylase (PIPh) activity in microtubers and shoots of in vitro plantlets was found to be much higher than in tubers and shoots of soil-grown plants. Detached leaves of soil-grown plants showed an increase in PIPh activity as well when incubated in sucrose-containing Murashige-Skoog (MS) medium. This increase was always accompanied by a rise in starch content. The presence of metabolizable carbohydrates in the growth or incubation medium are likely to be responsible for the observed rise in PIPh activity. In vitro microtubers and micropropagated plantlet organs (shoots and roots) exhibited a correlation between measurable PIPh activity and presence of enzyme protein, as judged by Western blot analysis using anti-potato tuber type L phosphorylase antibody. Therefore, in addition, to be developmentally regulated (tuber-specific accumulation), PIPh activity associated with the tuber type L isoform might be under a form of metabolic regulation.