Leghemoglobins with Nitrated Hemes in Legume Root Nodules

Abstract

Globins are widespread in all kingdoms of life, where they fulfil multiple functions such as efficient O 2 transport and modulation of nitric oxide bioactivity. Plants contain nonsymbiotic (class 1 and 2) and truncated (class 3) hemoglobins in virtually all tissues. In legume nodules, however, the most abundant hemoglobins are the symbiotic leghemoglobins (Lbs) that facilitate O 2 diffusion to the N 2-8 fixing bacteroids. Virtually nothing is known about the green 9 derivatives of Lbs generated during nodule senescence. Here we 10 characterize green modified forms of Lbs, termed Lba m , Lbc m , 11 and Lbd m , of soybean nodules. They have identical globins to the parent red Lbs but their hemes are nitrated. By combining diverse spectroscopies with reconstitution of the apoprotein with protoheme or mesoheme, we show that the nitro (NO 2) group is on the 4-vinyl. In vitro nitration of Lba with excess nitrite produced several isomers of nitrated heme, one of which is identical to those found in vivo. We show that nitration requires binding of nitrite to the heme iron and that the reactive nitrogen species involved derives from nitrous acid (HNO 2)and is most probably the nitronium cation (NO 2 +). The identification of these green Lbs provides conclusive evidence that highly oxidizing and nitrating species are increasingly produced in senescent nodules, leading to nitrosative stress.

Impact of Pea necrotic yellow dwarf virus (PNYDV) on nodulation, N2 fixation and yield in faba bean (Vicia faba, L.)

“…V. faba has indeterminate nodules with distal a persistent meristem and proximal an older senescent zone (Hirsch 1992). Getting senescent, leghemoglobin is degraded by heme oxygenase (Baudouin et al 2004;Kikuchi et al 2005) and nitration (Becana et al 2015) to green derived pigments. Consequentially, green nodules have previously been red and contained leghemoglobin.…”

In Vivo Metabolic Regulation of Alternative Oxidase under Nutrient Deficiency—Interaction with Arbuscular Mycorrhizal Fungi and Rhizobium Bacteria

“…To increase O 2 diffusion to bacteroids, the infected cells contain leghemoglobin that acts as an O 2 carrier. This is an iron protein with high affinity for O 2 , which regulates O 2 diffusion from the cytosol to the bacteroid in adequate concentrations to fuel its respiration, preventing inhibition of nitrogenase [146,147]. Thus, the ability of the nodule to respond to sudden increases of O 2 in infected cells is very important because of their repercussions on biological nitrogen fixation.…”