Evidence that the Plant Host Synthesizes the Heme Moiety of Leghemoglobin in Root Nodules1

Abstract

Although it is well established that the plant host encodes and synthesizes the apoprotein for leghemoglobin in root nodules, the source of the heme moiety has been uncertain. We recently found that the transcript for coproporphyrinogen III oxidase, one of the later enzymes of heme synthesis, is highly elevated in soybean (Glycine max L.) nodules compared with roots. In this study we measured enzyme activity and carried out western-blot analysis and in situ hybridization of mRNA to investigate the levels during nodulation of the plant-specific coproporphyrinogen 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 these enzymes are elevated in the infected cells of nodules. Because these are the same cells that express apoleghemoglobin, the data strongly support a role for the plant in the synthesis of the heme moiety of leghemoglobin.

Comparative Proteomic Analysis of Soybean Nodulation Using a Supernodulation Mutant, SS2-2

“…S3A). The specific activity of this enzyme in nodule was 27-fold higher than that in the uninfected roots, 40) due to the high demand for heme required for the formation of hemoproteins in the nodule. 41) Although Lb is an essential protein in nitrogen fixation, it is also the main source of reactive oxygen species (ROS) in the nodule.…”

TonB-Dependent Receptors in Nitrogen-Fixing Nodulating Bacteria

“…B. japonicum can also synthesize heme (13) and export the heme to periplasm by the cycVWX complex for cytochrome c biogenesis (14,39). It has been concluded that the heme of leghemeglobin was mainly derived from plants during nodulation (34,42). It is unlikely that leghemoglobin is a source of heme or Fe 3+ for B. japonicum during symbiosis.…”

“…In M. loti MAFF303099, the only TBDR sequence is a putative heme/hemoprotein-specific TBDR, implying that the TBDR for Fe 3+ -siderophores is not essential in this bacteria. The synthesis of heme involves multiple steps (19) and both rhizobacteria and plants have this ability (29,42). In plants, a large amount of heme is required for functional a Transcriptomic analyses were carried out on free-living cells growing in PSY medium (OD 0.4-0.5) and on 21-day-old bacteroids.…”

Nodule Formation and Function

“…There is now strong evidence that the synthesis of the haem group that is required for the fully functional lehaemoglobin protein is also under the control of plant genes (O'Brian 1996). Five enzymes involved in haem synthesis were shown to increase during the development of soybean nodules (Santana et al 1998). For coprogen oxidase (Madsen et al 1993), the increases in enzyme activity also correlated with mRNA and enzyme protein.…”