Structural and Functional Biodiversity of Microbial Communities in the Rhizosphere of Plants Infected with Broomrapes (Orobanchaceae)

ABSTRACT

Broomrapes are obligate root parasites from the family Orobanchaceae. Several of them parasitize important crops and have major economical impact. Important features of broomrapes are their great adaptability and flexibility, which hampers the understanding of the factors affecting the host-parasite system. The role of the soil microorganisms as a biotic factor in this system is not well understood yet. A number of recent studies demonstrated that parasitic weeds use a signaling mechanism between host plants and symbiotic arbuscular microbes to establish parasitic associations. Formation of arbuscular mycorrhizal (AM) associations however, significantly suppresses seed germination of parasitic weeds. The aim of this study was to determine whether the established host—parasite associations can also influence the major trophic groups of soil microorganisms in the rhizosphere. The study was conducted on 11 host plant—parasite systems with nine broomrape species. For comparison, similar non-infected host plants were used. The quantities of the autochthonous and oligotrophic microorganisms, streptomycetes, soil fungi, ammonifying microorganisms, assimilating mineral (NH₄⁺) nitrogen and aerobic nitrogen-fixing bacteria of the genus Azotobacter were determined. Microbiological indexes, characterizing the structure of microbial communities were determined. The population density of all microbial communities formed in the presence of broomrapes was considerably decreased, three-to five-fold. Regardless of the exact numbers, in all involved host—parasite systems the trend remained the same. These findings demonstrated that not only can arbuscular flora suppress parasitic weeds, but also, when host-parasitic associations have already been established, parasitic weeds can suppress and influence the level of biodiversity in major trophic groups of soil microorganisms in the rhizosphere. The statistical analysis demonstrated that the observed changes are an adaptive response of the microbial communities precisely to the presence of broomrape. The research in this area should at a next step further characterize the specifics of the host—parasite—AM interactions at different stages of the broomrape life cycle.

Keywords:

• microbial structural indexes
• Orobanchaceae
• parasitic weeds
• rhizosphere microflora