EVIDENCE THAT THE PLANT HOST SYNTHESIZES THE HEME MOIETY OF LEGHEMOGLOBIN IN ROOT-NODULES

Although it is well established that the plant host encodes and synthe sizes the apoprotein for leghemoglobin in root nodules, the source of the heme moiety has been uncertain. We recently found that the transcr ipt for coproporphyrinogen III oxidase, one of the later enzymes of he me synthesis, is highly elevated in soybean (Glycine max L.) nodules c ompared with roots. In this study we measured enzyme activity and carr ied out western-blot analysis and in situ hybridization of mRNA to inv estigate the levels during nodulation of the plant-specific coproporph yrinogen oxidase and four other enzymes of the pathway in both soybean and pea (Pisum sativum L.). We compared them with the activity found in leaves and uninfected roots. Our results demonstrate that all of th ese enzymes are elevated in the infected cells of nodules. Because the se are the same cells that express apoleghemoglobin, the data strongly support a role for the plant in the synthesis of the heme moiety of l eghemoglobin.