Imazethapyr inhibition of acetolactate synthase in Rhizobium and its symbiosis with pea

Acetolactate synthase (ALS) activity extracted from Rhizobium leguminosarum biovar. viciae has been characterized. The optimum pH for extraction was 7·6 and for the assay 7·0. The K_m for pyruvate was 7·2 mM , and the enzyme was saturated at 40 mM . An obligatory requirement of TPP and Mg²⁺ for full ALS activity was observed. Valine was the only branched-chain amino acid that caused ALS feedback inhibition. The specific activity of Rhizobium ALS was nearly 20 times the activity found in pea (Pisum sativum) leaves. Bacteroids from pea nodules also showed high ALS activity, and the nodule plant fraction had higher ALS activity than other plant tissues. ALS sensitivity to imazethapyr was also dependent on the source: ALS activity of free-living Rhizobium and bacteroids was slightly more tolerant than that of other pea tissues, but the differences were less than those found in rates of specific activity. It is proposed that the high ALS activity expressed by Rhizobium, both as free-living bacteria and as bacteroids, is related to the growth tolerance of rhizobia to imazethapyr and is also related to the relative tolerance of symbiotic pea plants. © 1998 SCI