Oligogalacturonides inhibit the formation of roots on tobacco explants

α‐1,4‐Oligogalacturonides with degrees of polymerization (DPs) ranging from 6 to 18 or 2 to 8 were added to tobacco leaf explants and root formation was evaluated after 15 days of incubation. Auxin‐induced formation of roots was inhibited by oligogalacturonides with DPs 6–18 but not by the oligogalacturonides with DPs 2–8. The inhibition of root formation by the larger oligogalacturonides was prevented by increasing the amount of auxin present in the medium. Oligogalacturonides (DPs 6–18) also inhibited root formation when added to tobacco thin cell‐layer (TCL) explants in a medium that is known to induce the formation of roots. The addition of size‐homogeneous oligogalacturonides, to either tobacco leaf explants or TCLs, established that oligogalacturonides with DPs between 10 and 14 were most active in inhibiting the formation of roots. These data suggest that oligogalacturonides of the same size as those known to elicit plant defense responses, and to affect floral development and membrane functions, also inhibit the induction of root morphogenesis in tobacco.