1. Imidazolinone Tolerance<br>Imidazolinones comprise a class of herbicide which target and bind to the acetolactate synthase (ALS) enzyme thereby inhibiting the biosynthesis of branched chain amino acids, resulting in a decrease in protein synthesis and eventual death of the plant. ALS catalyses the first reaction in the biosynthesis of the essential branched chain amino acids valine, leucine and isoleucine and is active in the glycolytic pathway of plant metabolism. <br>Imidazolinone tolerance in 3751IR corn results from a mutation in the ALS enzyme. The mutant form of ALS in 3751IR prevents the binding of imidazolinone herbicides to the enzyme. Imidazolinones applied at rates recommended for effective weed control are phytotoxic to currently cultivated corn varieties. <br>The levels of valine, leucine and isoleucine are also regulated by feedback inhibition. Pioneer has provided literature references to show that the mutant ALS of the inbred line XA17, from which the hybrid 3751IR is derived, does not affect feedback inhibition and hence, the regulation of levels of these amino acids. <br>The levels of valine, leucine and isoleucine in whole plant extracts of 3751IR and its counterpart 3751, were not statistically different, showing that the activity of the ALS enzyme in 3751IR has not been affected by the mutation.<br>2. Development Method <br>Somatic embryos grown on imidazolinone enriched media were selected, and from these, the somaclonal variant cell line XA17 was isolated. This cell line was regenerated to a whole plant and crossed to the inbred line B73. <br>The XA17/B73 line was backcrossed into each of two proprietary Pioneer inbreds which were then crossed to produce the hybrid 3751IR.<br>3. Stability of Trait <br>The imidazolinone tolerance trait was shown to be stable and to display regular Mendelian segregation in progeny from XA17 crosses with several corn inbreds. <br>3751IR is homozygous for the imidazolinone tolerance trait. Homozygous progeny were found to be tolerant to imidazolinone at up to four times the recommended rate for soybeans. Heterozygous progeny, while expressing the herbicide tolerance trait, exhibited pronounced leaf chlorosis and stunted growth at these herbicide rates.