Oligogalacturonides enhance cytokinin-induced vegetative shoot formation in tobacco explants, inhibit polyamine biosynthetic gene expression, and promote long-term remobilisation of cell calcium

Abstract Long-sized oligogalacturonides (OGs) are cell wall fragments that induce defence and developmental responses. The Ca2+-dependent “egg-box” conformation is required for their activity, and polyamines may prevent them from adopting this conformation. Although OGs are known to inhibit auxin-induced growth processes, their eVect on cytokinin-induced ones requires investigation. In the present work OGs were shown to promote cytokinin (benzyladenine, BA)-induced vegetative shoot formation from tobacco leaf explants, independent of the presence of CaCl2 in the medium and of auxin (indoleacetic acid, IAA) supply. The eVect of polyamines, putrescine (PU) and spermidine (SD) supplied with/without their biosynthetic inhibitors (DFMO, CHA) was also investigated, and showed that spermidine enhanced adventitious vegetative shoot formation, but only on medium containing Ca2+ and IAA. Treatments with inhibitors blocked this promotive eVect. OGs did not alter free polyamine concentrations, but caused a moderate increase of conjugated ones, and exhibited an early inhibitory eVect on polyamine biosynthetic gene expression. OGs, but not SD, caused long-term changes in calcium-associated epiXuorescent signals in the cell walls, and, later, inside the cells of speciWc tissues. Electron microscopy analysis (ESI system) demonstrated that calcium accumulated in the cell walls and vacuoles of OGcultured explants.