Role of extensin peroxidase in tomato (Lycopersicon esculentum Mill.) seedling growth

It is proposed that inhibition of extensin peroxidase activity leads to a l ess rigid cell wall and thus promotes cell expansion and plant growth. A lo w-molecular-weight inhibitor derived from the cell walls of suspension-cult ured tomato cells was found to completely inhibit extensin peroxidase-media ted extensin cross-linking in vitro at a concentration of 260 mu g/ml. The inhibitor had no effect upon,guaiacol oxidation catalyzed by extensin perox idase or horseradish peroxidase. We have demonstrated that the light-irradi ated inhibition of plant growth may be partially offset by inhibition of en dogenous extensin peroxidase activity. Overall plant growth was enhanced by up to 15% in the presence of inhibitor relative to control plants. Inhibit or-treated and illuminated tomato hypocotyls grew up to 15% taller than unt reated controls. The inhibitor had no effect upon etiolated plants over a 1 5-d period, suggesting that only low levels of peroxidase-mediated cross-li nking can be found in the eel walls of etiolated plants. SDS-PAGE/Western b lots of ionically bound protein from both etiolated and illuminated hypocot yls identified a doublet at 57/58.5 kDa which is immunoreactive with antibo dies raised to tomato extensin peroxidase. Levels of the 58.5-kDa protein, determined by SDS-PAGE, were at least threefold higher in illuminated tomat o hypocotyls than in etiolated hypocotyls. Three fold higher levels of exte nsin peroxidase, elevated in-vitro extensin cross-linking activity and 15% higher levels of cross-linked, non-extractable extensin were observed in il luminated tomato hypocotyls compared with etiolated tomato hypocotyls. This suggests that white-light inhibition of tomato hypocotyl growth appears to be mediated, at least partially, by deposition of cell wall extensin, a pr ocess regulated by M-r-58,500 extensin peroxidase. Our results indicate tha t the contribution of peroxidase-mediated extensin deposition to plant cell wall architecture may have an important role in plant growth.