Metabolite Profiles of Nodulated Alfalfa Plants Indicate That Distinct Stages of Nodule Organogenesis Are Accompanied by Global Physiological Adaptations

Abstract

An effective symbiosis between Sinorhizobium meliloti and its host plant Medicago sativa is dependent on a balanced physiological interaction enabling the microsymbiont to fix atmospheric nitrogen. Maintenance of the symbiotic interaction is regulated by still poorly understood control mechanisms. A first step toward a better understanding of nodule metabolism was the determination of characteristic metabolites for alfalfa root nodules. Furthermore, nodules arrested at different developmental stages were analyzed in order to address metabolic changes induced during the progression of nodule formation. Metabolite profiles of bacteroid-free pseudonodule extracts indicated that early nodule developmental processes are accompanied by photosynthate translocation but no massive organic acid formation. To determine metabolic adaptations induced by the presence of nonfixing bacteroids, nodules induced by mutant S. meliloti strains lacking the nitrogenase protein were analyzed. The bacteroids are unable to provide ammonium to the host plant, which is metabolically reflected by reduced levels of characteristic amino acids involved in ammonium fixation. Elevated levels of starch and sugars in Fix(-) nodules provide strong evidence that plant sanctions preventing a transformation from a symbiotic to a potentially parasitic interaction are not strictly realized via photosynthate supply. Instead, metabolic and gene expression data indicate that alfalfa plants react to nitrogen-fixation-deficient bacteroids with a decreased organic acid synthesis and an early induction of senescence. Noneffective symbiotic interactions resulting from plants nodulated by mutant rhizobia also are reflected in characteristic metabolic changes in leaves. These are typical for nitrogen deficiency, but also highlight metabolites potentially involved in sensing the N status.

Medicago truncatula DNF2 is a PI‐PLC‐XD‐containing protein required for bacteroid persistence and prevention of nodule early senescence and defense‐like reactions

“…4a). Furthermore, defense-like reactions were not described in the L. japonicus sen1 mutant (Suganuma et al, 2004) or in nodules induced by the nifH or nifA Fix À mutants (Barsch et al, 2006;Maunoury et al, 2010). This supports the idea that, during aborted symbiosis, early senescence is not necessarily associated with defense-like reactions.…”

Pathway of γ-Aminobutyrate Metabolism in Rhizobium leguminosarum 3841 and Its Role in Symbiosis

“…GABA is an abundant amino acid in nodules (2,10,28). Furthermore, 15 N 2 labeling showed that GABA is rapidly labeled in whole pea nodules (41).…”

Soybean Metabolites Regulated in Root Hairs in Response to the Symbiotic BacteriumBradyrhizobium japonicum

“…S2; Supplemental Table S2). Lactic acid was previously reported to be 4.8 times more abundant in alfalfa nodules than in roots and accumulated in nodules during salt stress (Swaraj and Bishnoi, 1999;Barsch et al, 2006). Polymers of lactic acid were shown to stimulate plant growth in corn (Zea mays) and soybean (Kinnersley et al, 1990;Chang et al, 1996).…”

“…However, few studies have examined the plant metabolome in response to rhizobial infection. Various amino acids (Asn, Gln, Pro), organic acids (threonic acid), sugars (Rib, maltose), and polyols (mannitol) were shown to be more abundant in L. japonicus and M. sativa nodules than in the corresponding roots (Colebatch et al, 2004;Desbrosses et al, 2005;Barsch et al, 2006). However, to our knowledge, no previous studies have sought to characterize metabolites responding during the earliest stages of the rhizobial infection process (e.g.…”