Oligogalacturonides inhibit the induction of late but not of early auxin-responsive genes in tobacco.

Oligogalacturonides (OGs) released from the plant cell wall regulate several defense responses, as well as various aspects of plant growth and development. In these latter effects, OGs exhibit auxin-antagonist activity. To shed light on the mechanism by which OGs antagonise auxin, we analysed the ability of these oligosaccharides to inhibit the activity of four auxin-up-regulated promoters [pGm-GH3 of soybean (Glycine max L. Merr.), pNt114 of tobacco (Nicotiana tabacum L.), and prolB and prolD of Agrobacterium rhizogenes] driving the expression of the beta-glucuronidase reporter gene (GUS) in transgenic tobacco seedlings. Our results indicate that OGs at submicromolar concentrations inhibit the activation by auxin of pNt114, prolB and prolD, but not that of pGm-GH3. Comparative analysis of the kinetics of activation of the four promoters in response to the hormone shows that, while pGm-GH3 is rapidly activated, the other three promoters exhibit a delayed activation, with a lag of at least 4 h before the appearance of GUS activity. The lack of effect of the OGs on early auxin-responsive genes was confirmed by RNA gel blot analysis of the tobacco genes Nt-GH3 and Nt-iaa2.3/2.5. Our results suggest that the auxin-antagonist action of OGs affects the expression of late but not of early auxin-responsive genes.