THE ROLE OF POLYAMINES IN POTATO-TUBER FORMATION

The involvement of free and conjugated polyamines in tuber formation was studied in in vitro cultured node explants of Solanum tuberosum cv. Superior. Tubers developed from the axillary buds in 100% of the explants cultured in MS medium containing high sucrose levels and supplemented with kinetin (Kin) and chlorocholine chloride (CCC). The addition of growth regulators was not essential for tuber formation, although smaller tubers were formed in the medium devoid of Kin and CCC. Tuber formation was inhibited in about 75% of node explants treated with 0.5 mM difluoromethylornithine (DFMO), a specific and irreversible inhibitor of ornithine decarboxylase. The inhibitory effect of DFMO was almost completely reversed by putrescine addition. Addition of difluoromethylarginine (DFMA), the analogous inhibitor of arginine decarboxylase, had no effect on tuber formation. DFMO, but not DFMA, also inhibited the development of axillary buds into shoots in light-grown node explants. Aminooxyphenylpropionic acid (0.1 to 0.25 mM), an inhibitor of phenylalanine ammonia lyase, caused a sharp reduction in cinnamoyl putrescines, but had no effect on tuber formation. Our results suggest that hydroxycinnamic acids are not causal in tuber formation but may serve as polyamine storage pools. Our findings support the hypothesis that polyamines derived via the ornithine decarboxylase-mediated pathway are necessary for tuber formation in vitro, probably at the early phase of morphogenesis involving active cell division.