Stimulation of putrescine biosynthesis via the ornithine decarboxylase pathway by gibberellic acid in the in vitro rooting of globe artichoke ( Cynara scolymus )

Abstract

Rooting and growth of globe artichoke (Cynarascolymus) cuttings were improved by supplementing the rooting mediumwith 10 μM gibberellic acid(GA3). Exogenous putrescine (Put) (0.5mM) raised the percentage of rooting to thesame level as GA3, but decreased the number of roots per plant.Addition of Put to GA3 produced the same effects on growthparametersand rooting percentage as Put alone. Changes in endogenous contents of freepolyamines (PAs) were determined in plants cultured 4 weeks in a rootingmedium.Whatever the treatments, rooting caused a large decrease in Put, the largelypredominant PA, in the leaves. GA3 increased the free Put content ofthe leaves and roots. DL-α-difluoromethylarginine (DFMA), which inhibitsarginine decarboxylase (ADC), did not affect rooting while Put content wasdecreased by 12–15%. DFMA changed neither GA3 effectsnor those from DFMO when used in combination. An inhibitor of ornithinedecarboxylase (ODC), DL-α-difluoromethylornithine (DFMO), inhibited leafgrowth as well as rooting and decreased the Put content; these effects werepartially reversed by addition of Put. GA3 combined with DFMO didnotalleviate the inhibitory effects of DFMO. The GA3 treatment caused aclear stimulation of ODC activity in the leaves; it also stimulated14C-putrescine uptake (2.8-fold) by leaves and its translocationtowards roots. These results indicate that the ODC pathway mediates thehormone-induced rooting response in globe artichoke. Moreover, a release of Putfrom its conjugates could contribute to the increase of free-Put levels in theroots under GA3 treatment. Thus, Put can be considered to be a goodmarker of rhizogenesis.