Heme Synthesis in Soybean Root Nodules

Abstract

During nodulation of soybean (Glycine max) by Rhizobium japonicum, variations in the activities of two enzymes of heme biosynthesis, δ-aminolevulinic acid synthase (ALAS) and δ-aminolevulinic acid dehydrase (ALAD) are described. δ-Aminolevulinic acid synthase activity is found in the bacteroid fraction of nodules, but is not detected in the plant fraction. Bacteroid ALAS activity parallels heme accumulation during nodule development. δ-Aminolevulinic acid dehydrase activity is found in both bacteroid and plant cytosol fractions. Bacteroid ALAD activity is constant or increases during nodulation while plant ALAD activity falls.

Bacteroid ALAD activity is found in effective, not in inefficient nodules. Plant ALAD activity is found in both effective and inefficient nodules. Plant ALAD activity falls during development of both types of root nodules.

These results support the contention that it is the bacteroid ALAS and ALAD activities, not those of the plant, that are directly involved in formation of leghemoglobin heme, suggesting that the bacteroid may be solely responsible for formation of leghemoglobin heme in the nodule symbiosis.