An extensin peroxidase is associated with white-light inhibition of lupin (Lupinus albus) hypocotyl growth

The spatial distribution of the major basic (B2; pI 8.8) peroxidase of the intercellular fluid has an inverse relation with extension rate in etiolate d hypocotyls of Lupinus albus L., suggesting its possible role in the contr ol of cell expansion. White-light irradiation of etiolated hypocotyls resul ted in growth inhibition and the induction of B2 and acidic (A2, pI 4.7-5.2 ) isoperoxidases (EC 1.1.11.7) to higher physiological activities. However, only the activities of the B2 isoperoxidases underwent quantitative change s in both space and time which suggested their role in growth-retardation. We have purified the B2 and A2 (pI 5.2) peroxidases to apparent electrophor etic homogeneity. To corroborate evidence obtained elsewhere that growth ce ssation coincides with cell wall structural changes and cell wall rigidific ation, we have shown that the B2 peroxidase, and not A2 peroxidase, cross-l inks tomato extensin in vitro. The B2 peroxidase may therefore catalyse the developmentally and light regulated formation of a covalently cross-linked cell wall extensin matrix in lupin hypocotyls. The cell wall would be more rigid or more recalcitrant to wall-loosening and subsequently contribute t o the control of cell expansion.