Articles
PLANT DEFENCE IN ALFALFA PSEUDONODULES INDUCED BY AN EXOPOLYSACCHARIDE (EPS I) – DEFICIENT SYMBIONT (RHIZOBIUM MELILOTI)
Article number
381_31
Pages
258 – 264
Language
Abstract
Mutants of the symbiotic soil bacterium Rhizobium meliloti that fail to synthesize the acidic exopolysaccharide EPS I were unable to induce infected root nodules on the host plant Medicago sativa L. (alfalfa). The pseudonodules induced by EPS I deficient R. meliloti mutants showed a brown, necrotic area on their surface indicating a plant defence reaction.
HPLC analysis of pseudonodules revealed an increase of phenolic compounds incorporated into the nodule cell walls when compared with wildtype nodules.
An increased enzymatic activity of peroxidase (POD) and phenylalanine ammonia-lyase (PAL) was found in noninfected pseudonodules.
In contrast to pseudonodules, the activity of these enzymes was not increased in roots inoculated with EPS I deficient R. meliloti mutants, indicating a strictly localized plant response.
Hybridization with a phenylalanine ammonia-lyase homologous cDNA probe confirmed that the increased enzymatic activity in pseudonodules was the consequence of an enhanced transcription of the PAL-gene.
A plant test with more than twenty Medicago species revealed that the inability of EPS I deficient R. meliloti mutants to establish an effective symbiosis is conserved in all tested species.
Based on the mechanism of phytopathogenic interactions, we propose that EPS I or a related compound acts as a suppressor of the plant defence system, enabling R. meliloti to infect the plant.
HPLC analysis of pseudonodules revealed an increase of phenolic compounds incorporated into the nodule cell walls when compared with wildtype nodules.
An increased enzymatic activity of peroxidase (POD) and phenylalanine ammonia-lyase (PAL) was found in noninfected pseudonodules.
In contrast to pseudonodules, the activity of these enzymes was not increased in roots inoculated with EPS I deficient R. meliloti mutants, indicating a strictly localized plant response.
Hybridization with a phenylalanine ammonia-lyase homologous cDNA probe confirmed that the increased enzymatic activity in pseudonodules was the consequence of an enhanced transcription of the PAL-gene.
A plant test with more than twenty Medicago species revealed that the inability of EPS I deficient R. meliloti mutants to establish an effective symbiosis is conserved in all tested species.
Based on the mechanism of phytopathogenic interactions, we propose that EPS I or a related compound acts as a suppressor of the plant defence system, enabling R. meliloti to infect the plant.
Authors
K. Niehaus, D. Kapp, J. Lorenzen, P. Meyer-Gattermann, S. Sieben, A. Pühler
Keywords
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