Characterization of a Non-abscission Mutant in Lupinus angustifolius L.: Physiological Aspects

A single-gene recessive mutant (Abs − ) of Lupinus angustifolius L. ‘Danja’ that does not abscise any organs was compared with its parent during continuous exposure of explants from 14 d old seedlings to 10 μl l −1 ethylene. Both endo- (1,4)-β- D -glucanase (cellulase) and polygalacturonase (PGA) activities increased significantly and progressively in petiole-stem abscission zones of the parent before the onset of abscission, and were reflected in a rapid decline in breakstrength from 300 to 70 g within 32 h. In the mutant there was negligible increase in hydrolytic enzyme activity, breakstrength declined slowly (to 180–200 g by 72 h) and there was no abscission. Isoelectric focusing showed two cellulase isoforms (pI 5.0 and pI 8.5) expressed in abscission zones of the parent; these were expressed at much lower levels in the mutant. These data are interpreted to indicate that expression of at least two forms of cellulase activity is enhanced by ethylene in normal petiole abscission zones of lupin. PGA activity also increased in the abscission zone tissue of the parent but to a lesser extent in that of the mutant. We attribute the Abs − phenotype to mutation of a gene regulating ethylene-responsive expression of abscission-specific hydrolytic enzymes.