ARGININE DECARBOXYLASE (POLYAMINE SYNTHESIS) MUTANTS OF ARABIDOPSIS-THALIANA EXHIBIT ALTERED ROOT-GROWTH

Putrescine and polyamines are produced by two alternative pathways in plants. One pathway starts with the enzyme arginine decarboxylase; the other with ornithine decarboxylase. The authors developed an in vivo screening strategy to identify mutants with low levels of arginine dec arboxylase activity. The screen requires both a primary screen of the M2 generation and a secondary screen of the M3 generation. The method used was to screen 15 000 EMS-mutagenized M2 seedlings for low levels of arginine decarboxylase (ADC) activity and identified seven mutants that fall into two complementation groups. These mutants have from 20% to 50% of wild-type enzyme activity. Morphological alterations common among the mutants include increased levels of lateral root branching. The authors obtained a double mutant combining the alleles with the l owest activities from the two complementation groups; this has lower A DC enzyme activity and putrescine levels than either of the single mut ants. The double mutant has highly kinked roots that form a tight clus ter; it also has narrower leaves, sepals, and petals than either singl e mutant or wild-type, and delayed flowering. These results suggest th ere may be more than one ADC gene in Arabidopsis, and that ADC and pol yamine levels play roles in root meristem function and in lateral grow th of leaf-homolog organs.