EXPANSINS IN GROWING TOMATO LEAVES

An expansin-like protein from growing tomato leaves was identifed by i ts ability to restore the 'acid-growth' response to heat-inactivated t omato walls and by its similarity to expansins from cucumber hypocotyl s. Native walls from growing tomato leaves exhibit an endogenous acid- induced extension (creep) that resembles in various biochemical charac teristics the acid-growth activity of cucumber hypocotyls. For example , the acid-growth activity is lost when the walls of tomato leaves are briefly heated and is largely restored by addition of a crude protein extract from the walls of growing leaves. Wall proteins from growing leaves enhance the stress relaxation spectrum of tomato walls in a fas hion characteristic of cucumber expansins. HPLC fractionation of the c rude wall protein from tomato leaves yielded an active fraction contai ning a major 27 kDa protein that cross-reacts with an antibody raised against cucumber expansin. The results show that tomato leaf wa I Is p ossess at least one expansin that is responsible for the acid-growth p roperty of leaves and indicate that cell wall extension in leaves shar es an underlying protein mechanism common to cell wall expansion in st ems.