Expansins in growing tomato leaves

An expansin-like protein from growing tomato leaves was identifed by its ability to restore the 'acid-growth' response to heat-inactivated tomato walls and by its similarity to expansins from cucumber hypocotyls. Native walls from growing tomato leaves exhibit an endogenous acidinduced extension (creep) that resembles in various biochemical characteristics 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 fashion characteristic of cucumber expansins. HPLC fractionation of the crude wall protein from tomato leaves yielded an active fraction containing a major 27 kDa protein that cross-reacts with an antibody raised against cucumber expansin. The results show that tomato leaf walls possess at least one expansin that is responsible for the acid-growth property of leaves and indicate that cell wall extension in leaves shares an underlying protein mechanism common to cell wall expansion in stems.