Biosynthesis of Phenolic Compounds of Medicago truncatula After Inoculation with Selected PGPR Strains

The phenylpropanoid biosynthesis pathway is involved in the response of plants to stress factors, including microorganisms. This paper presents how free-living strains of rhizobacteria Pseudomonas brassicacearum KK5, P. corrugata KK7, Paenibacillus borealis KK4, and the symbiotic strain Sinorhizobium meliloti KK13 affect the expression of genes encoding phenylalanine ammonia-lyase (PAL), the activity of this enzyme, and the production of phenolic compounds in Medicago truncatula. Seedlings were inoculated with rhizobacteria, then at T0, T24, T72, and T168 after inoculation, the leaves and roots were analyzed for gene expression, enzyme activity, and the content of phenolic compounds. All bacteria affected PAL gene expression, in particular, MtPAL2, MtPAL3, and MtPAL4. Pseudomonas strains had the greatest impact on gene expression. The inoculation affected PAL activity causing it to increase or decrease. The most stimulating effect on enzyme activity was observed 168 h after inoculation. A varied effect was also observed in the case of the content of phenolic compounds. The greatest changes were observed 24 h after inoculation, especially with the KK7 strain. The influence of the studied rhizobacteria on the biosynthesis of phenolic compounds at the molecular level (expression of MtPAL genes) and biochemical level (PAL activity and content of phenolic compounds) was confirmed. The MtPAL3 gene underwent the most significant changes after inoculation and can be used as a marker to assess the interaction between M. truncatula and rhizobacteria. The Pseudomonas strains had the greatest influence on the biosynthesis pathway of phenolic compounds.