Suppression of arbuscular mycorrhizal colonization and nodulation in split-root systems of alfalfa after pre-inoculation and treatment with Nod factors

Abstract

Roots of legumes establish symbiosis with arbuscular mycorrhizal fungi (AMF) and nodule-inducing rhizobia. The existing nodules systemically suppress subsequent nodule formation in other parts of the root, a phenomenon termed autoregulation. Similarly, mycorrhizal roots reduce further AMF colonization on other parts of the root system. In this work, split- root systems of alfalfa (Medicago sativa) were used to study the autoregulation of symbiosis with Sinorhizobium meliloti and the mycorrhizal fungus Glomus mosseae. It is shown that nodulation systemically influences AMF root colonization and vice versa. Nodules on one half of the split-root system suppressed subsequent AMF colonization on the other half. Conversely, root systems pre-colonized on one side by AMF exhibited reduced nodule formation on the other side. An inhibition effect was also observed with Nod factors (lipo-chito-oligosaccharides). NodSm-IV(C16:2, S) purified from S. meliloti systemically suppressed both nodule formation and AMF colonization. The application of Nod factors, however, did not influence the allocation of (14)C within the split-root system, excluding competition for carbohydrates as the regulatory mechanism. These results indicate a systemic regulatory mechanism in the rhizobial and the arbuscular mycorrhizal association, which is similar in both symbioses.

Flavonoids exclusively present in mycorrhizal roots of white clover exhibit a different effect on arbuscular mycorrhizal fungi than flavonoids exclusively present in non-mycorrhizal roots of white clover

“…These two flavonoids are widely distributed in plants (Santos et al 1995, Santos & Salatino 2000 and exhibit antifungal and bacteriostatic activities (Serra-Bonvehi et al 1994, Fawe et al 1998). Interestingly, both newly synthesized compounds exhibit an inhibitory effect on nearly all fungal parameters of both AM genera (Gigaspora and Glomus ) tested, pointing towards a possible implication in the autoregulation of mycorrhization (Pinior et al 1999, Vierheilig et al 2000a, Vierheilig et al 2000b, Catford et al 2003, Vierheilig 2004b). This could mean that once the AM symbiosis is well established the observed suppression of further root colonization by AM fungi is at least partially due to newly synthesized compounds exhibiting an inhibitory effect on AM fungi such as acacetin and rhamnetin in white clover.…”

Flavonoids and Strigolactones in Root Exudates as Signals in Symbiotic and Pathogenic Plant-Fungus Interactions

“…In several studies it has been demonstrated that once plants are mycorrhizal, further root colonization by AMF is reduced compared to colonization of nonmycorrhizal plants [24, [81][82][83][84][85][86]. Looking at these observations it is tempting to speculate that due to altered levels of strigolactones in root exudates of mycorrhizal plants further root colonization by AMF is affected.…”

Programming good relations — development of the arbuscular mycorrhizal symbiosis

“…Interestingly, negative regulation of the AMS and the RNS influence each other [62]. Whether this reflects regulation at the level of the common sym pathway, or whether the two developmental pathways converge at a second point remains to be seen.…”