Export of β-1,3-glucanase from mutant rice cells rechallenged and stressed with lysine plus threonine

Mutant rice cells (Oryza sativa L.) grown in liquid suspension cultures exported greater quantities of protein and β-glucanases than controls. These mutants were isolated from anther calli resistant to 1 mM lysine plus threonine (LT), regenerated and reestablished as cell suspension cultures from seeds. Cellular protein levels are genetically conditioned, and the levels of extracellular proteins and enzyme activities are inversely related to that of the cellular portions. The rechallenge of cells with 1 mM LT inhibited the expression of both β-1,3-glucanases and β-1,4-glucosidases but had no significant effect upon the levels of chitinase activity. Mutant cells were more sensitive than controls to stress caused by exogenous LT. In general, under exogenous LT stress the mutant/control ratio for extracellular glucanases increased as the assay conditions were changed from a basic to an acidic pH. The specific activity of βglucanases was highest in media and lowest in cells. Both the mutant and control cells exported β-glucanases into the suspension medium, but the level of activity in media was greater in that in which the mutant was suspended. The export was probably modulated by the internal protein levels which were highest in mutant cells without LT. Seedlings from mutants with enhanced lysine also had enhanced acidic β-glucanase activity.