White lupin (Lupinus albus L.)

A classic view of the evolution of mutualism is that it derives from a pathogenic relationship that attenuated over time to a situation in which both partners can benefit. If this is the case for rhizobia, then one might uncover features of the symbiosis that reflect this earlier pathogenic state. For example, as with plant pathogens, it is now generally assumed that rhizobia actively suppress the host immune response to allow infection and symbiosis establishment. Likewise, the host has retained mechanisms to control the nutrient supply to the symbionts and the number of nodules so that they do not become too burdensome. The open question is whether such events are strictly ancillary to the central symbiotic nodulation factor signaling pathway or are essential for rhizobial host infection. Subsequent to these early infection events, plant immune responses can also be induced inside nodules and likely play a role in, for example, nodule senescence. Thus, a balanced regulation of innate immunity is likely ...

The Role of Plant Innate Immunity in the Legume-Rhizobium Symbiosis

The roles of flavonoids in roots of legumes in the symbiotic relationships with nitrogen-fixing bacteria and arbuscular mycorrhizal fungi are compared with defense functions, using examples from three legume genera,Lotus, Medicago, andGlycine. Pathways leading to proanthocyanidins and isoflavonoids are emphasized. The localization of flavonoids in nodules involved in nitrogen fixation and in the apoplastic compartment of mycorrhizal associations is briefly described, with emphasis on the need for more information concerning their precise localization. Also emphasized are the limits of our knowledge about the regulatory genes of the flavonoid pathway involved in both exogenous and endogenous regulation of these complex interrelationships.

Roles of flavonoids in symbiotic and defense functions in legume roots

Medicarpin 3-O-glucoside-6″-O-malonate (MGM), formononetin 7-O-glucoside-6″-O-malonate (FGM) and two glycosides of coumestrol (CG), one of which was characterised as a malonylated glucoside, have been identified as major isoflavonoid metabolites in the roots of healthy alfalfa (Medicago sativa) plants. Foliage contained a conjugate of the flavonoid apigenin, very low levels of medicarpin and MGM, and depending on the cultivar studied, formononetin and FGM. In the foliage the isoflavonoids were restricted to the stems. Seeds contained conjugates of quercetin, luteolin and 7,4′-dihydroxyflavone but no isoflavonoid conjugates. When alfalfa plants were grown under controlled conditions, isoflavonoid conjugates were first observed in the roots 24 h after emerging, and continued to accumulate as the plants matured over a 57 day period. Inoculating the plants with a commercial formulation of Rhizobium meliloti (Nodulaid) reduced FGM accumulation in the roots but did not affect MGM or CG content. Under field conditions plants accumulated higher concentrations of the isoflavonoid conjugates in both leaves and roots than plants grown under controlled conditions. Field-grown plants which were not inoculated with Nodulaid contained higher levels of both MGM and FGM in the roots than nodulated plants. The conjugation of isoflavonoids in alfalfa cell cultures resembled that of the roots in that they accumulated FGM and MGM as major metabolites. However, cultures accumulated a higher proportion of their conjugated isoflavonoids as MGM and formononetin glucoside than did the roots and did not synthesise coumestrol. Comparisons with clover species revealed that FGM and MGM were common metabolites in forage legumes but alfalfa was unusual in having negligible levels of isoflavonoid conjugates in the foliage. Our results clarify the nature and distribution of isoflavonoid conjugates in alfalfa and demonstrate that their accumulation is regulated in response to both plant age and nodulation.

Changes in the accumulation of flavonoid and isoflavonoid conjugates associated with plant age and nodulation in alfalfa (Medicago sativa)

Classic observations describe distinctive relationships that have evolved between roots and their associated microbes:1 (1) The rhizosphere, an imprecisely defined zone near, on, and within the root, contains much higher numbers of total bacteria than root-free soil; and (2) healthy plant roots growing in soil normally are colonized by a limited number of bacterial species, which include few pathogens. Detailed field studies with mangel2 and soybean3 showed that physiological traits of heterotrophic bacteria isolated from the rhizosphere differ markedly from those in root-free soil in terms of amino acid nutrition, carbon metabolism, extracellular enzymatic activities, and resistance to antimicrobial compounds. Despite such indications that striking, potentially beneficial microbial communities develop around plant roots, our knowledge of the genetic and physiological traits in microbes that confer rhizosphere competence4 is limited. A better understanding of factors controlling microbial communities in the rhizosphere could help establish superior strains of microsymbionts, such as Rhizobium and Bradyrhizobiwn, might suppress growth of microbial pathogens, and could make it possible to establish populations of genetically altered bacteria that degrade crop pesticides or fulfill other agriculturally useful roles.

Legume Signals to Rhizobial Symbionts: A New Approach for Defining Rhizosphere Colonization

Nitrogen fixation in root nodules formed by strain 2143 ofBradyrhizobium japonicum andGlycine max (L.) Merr. cv Williams 82 reaches a maximum at 21 to 28 days postinoculation and then begins to decline. The phytoalexin, glyceollin, accumulates in nodules coincident with the decline in nitrogen fixation. Nodules formed by strain 3122, which are unable to fix nitrogen, accumulate even higher levels of glyceollin and do so beginning 21 days postinoculation even though these nodules contain no recoverable bacteria. The typical phytoalexin response occurs within days of infection. The mechanism by which this response in theBradyrhizobium japonicum-soybean combination is delayed 2 to 3 weeks after infection is presently unknown but phytoalexin accumulation could contribute to the inability of the soybean-Bradyrhizobium japonicum combination to maintain high levels of nitrogen fixation throughout the growing season.

Accumulation of the phytoalexin, glyceollin, in root nodules of soybean formed by effective and ineffective strains of Bradyrhizobium japonicum

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Accumulation of the phytoalexin, glyceollin, in root nodules of soybean formed by effective and ineffective strains of Bradyrhizobium japonicum