Effect of pO₂ on the Formation and Status of Leghemoglobin in Nodules of Cowpea and Soybean

Abstract

Nodulated cowpea (Vigna unguiculata [L.] Walp. cv Vita 3:Bradyrhizobium strain CB756) and soybean (Glycine max [L.] Merr. cv White Eye: Bradyrhizobium strain CB1809) were grown with their root systems maintained in a flowing gas stream containing a range of P02 (1-80%, v/v) in N2 for up to 28 days after planting. At the extremes of sub-and supra-ambient P02, the levels of leghemoglobin (Lb) There is considerable evidence that synthesis of the protoheme moiety of Lb occurs principally in the Rhizobium symbiont and is stimulated greatly by microaerobic conditions (16). Although this may not be the case for the Lb polypeptides, which are synthesized in the plant host cytosol (23), the rapid assembly of functional Lb in the cytosol of the infected cells appears to require conditions of increasing O2 demand and consequent lowered O2 levels (2).The proportional composition of Lb component proteins has been found to vary with nodule development (14, 27) and in relation to other environmental factors (2, 4, 5). Although adaptation of nitrogenase activity to altered PO2 (7, 9) is now believed to be due partly to operation of a variable diffusion barrier in nodules, Fuchsman et al. (13) suggested that changes in the proportions of Lb components might be involved.Detopping, which has been shown to reduce nitrogenase activity (18) in legumes through increased diffusion resistance in nodules (7), was accompanied by decreased oxygenation of ferrous Lb (18). Although King et al. (17) found that the degree of oxygenation of Lb in vivo may change in response to PO2 without alteration in nodule respiration, it seems possible that O2 supply could also affect the redox status of Lb and, in this way, modify its function as a nodule 02 carrier. Ferric Lb does not combine reversibly with 02 (2). There is experimental evidence for the occurrence of Lb only in its reduced (ferrous) form in unstressed, functioning nodules (1, 22). However, the demonstration of the presence of a nodule cytosolic reductase, able to reduce ferric Lb to ferrous Lb ( 19,24), coupled with a recent report of in vitro nonenzymatic reduction of Lb in soybeans (26), together suggest the operation of reactions which maintain Lb in its functional, ferrous state.This investigation reports on two major questions relating to the effect of 02 on the nodule's O2 buffering system. First, do the proportions of Lb components change in relation to external gas phase P02; and second, does the total amount of Plant Physiol. Vol. 95, 1991 plants were harvested 18, 24, and 28 DAP and soybean 20 and 28 DAP. The nodules were removed rapidly and stored immediately in liquid N2. In some cases, the atmospheres surrounding roots were changed, at 25 DAP, from sub-ambient PO2 (1-10% 02, v/v, in N2) to air, for 7 d before harvesting nodules. Similarly, plants were transferred after 20 or 21 d culture with their nodulated roots in air to rooting atmospheres containing a range of PO2 for 3 to 6 d. In each case where the p02 of the rooting atmosphere was changed, controls ...

A functional relationship between leghaemoglobin and nitrogenase based on novel measurements of the two proteins in legume root nodules

“…a reduction in nodule Lb concentration from oxidation reactions often leads to a dec1inein nitrogenase activity due to Lb's inability to support high levels of bacteroid respiration (Appleby, 1984). In fact, it has been demonstrated that with high Lb concentration in nodules, a high level of nitrogenase activity was maintained; however as Lb concentration dec1ined 0305-7364/95/010049+06 $08.00/0 from oxidation by excess 02' nitrogenase activity also dec1ined (Dakora, Appleby and Atkins, 1991).…”

Identification of cultivar‐specific leghaemoglobin components in Pisum sativum

“…4), and of the other minor components, remained more or less constant during plant ontogenesis. Although it is speculated that the different Lb components play an important role in maintaining an optimal supply of Oo and in adaptation to different environmental conditions (Dakora, Appleby & Atkins, 1991: Becana & Sprent, 1989Hunt & Layzell, 1993), no specific functions have so far been attributed to the different Lb components. The identification of cultivar-specific Lb components, as demonstrated in this work, offers new opportunities for investigation of their physiological functions and for explaining the importance of Lb heterogeneity in P. sativum.…”

Diffusion and reaction of oxygen in the central tissue of ureide‐producing legume nodules

“…Higher concentrations of Lb (0·7 mM), such as those occurring in nodules grown at subambient pO 2 (Dakora, Appleby & Atkins 1991), led to slightly better cell oxygenation but with cellular homeostatic control of Oi maintained.…”