Effect of Clay Mineralogy on Iron Bioavailability and Rhizosphere Transcription of 2,4-Diacetylphloroglucinol Biosynthetic Genes in Biocontrol Pseudomonas protegens

Abstract

Pseudomonas strains producing 2,4-diacetylphloroglucinol (DAPG) can protect plants from soilborne phytopathogens and are considered the primary reason for suppressiveness of morainic Swiss soils to Thielaviopsis basicola-mediated black root-rot disease of tobacco, even though they also occur nearby in conducive sandstone soils. The underlying molecular mechanisms accounting for this discrepancy are not understood. In this study, we assessed the hypothesis that the presence of iron-rich vermiculite clay (dominant in suppressive soils) instead of illite (dominant in neighboring conducive soils) translates into higher levels of iron bioavailability and transcription of Pseudomonas DAPG synthetic genes in the tobacco rhizosphere. Rhizosphere monitoring of reporter gene systems pvd-inaZ and phlA-gfp in Pseudomonas protegens indicated that the level of iron bioavailability and the number of cells expressing phl genes (DAPG synthesis), respectively, were higher in vermiculitic than in illitic artificial soils. This was in accordance with the effect of iron on phlA-gfp expression in vitro and, indeed, iron addition to the illitic soil increased the number of cells expressing phlA-gfp. Similar findings were made in the presence of the pathogen T. basicola. Altogether, results substantiate the hypothesis that iron-releasing minerals may confer disease suppressiveness by modulating iron bioavailability in the rhizosphere and expression of biocontrol-relevant genes in antagonistic P. protegens.

Alleviation of Abiotic and Biotic Stresses in Plants by Azospirillum

“…Fluorescent Pseudomonas is one of the most widely studied bacterial lineages for antibiotic-based biocontrol activities. They can produce 2,4-diacetylphloroglucinol (DAPG), a well-known antibiotic involved in soil disease suppressiveness (Almario et al 2013(Almario et al , 2014. Other Pseudomonas antimicrobial compounds are pyoluteorin active against Pythium (Maurhofer et al 1994 ), phenazines against Gaeumannomyces graminis (Thomashow and Weller 1988 ), pyrrolnitrin against Rhizoctonia solani (Ligon et al 2000 ), and cyclic lipopeptides (Clp) (Raaijmakers et al 2006 ).…”

Relationships between Root Pathogen Resistance, Abundance and Expression of Pseudomonas Antimicrobial Genes, and Soil Properties in Representative Swiss Agricultural Soils

“…Moreover, to our best knowledge, there were no reports so far on phenazine gene expression in soil or on plant roots. To date, only a handful of studies attempted to determine the expression of antimicrobial genes of pseudomonads in soil (DeCoste et al, 2011; Novinscak and Filion, 2011; Almario et al, 2013b), mainly because of practical challenges. In our study, our initial attempts of tracking fluorescence of reporter strains extracted from roots and soil with microscopy revealed impracticable because of the strong autofluorescence of the investigated soil and root material.…”

“…An alternative method of measuring gene expression in soils involves the extraction of total RNA followed by quantitative reverse transcription PCR of bacterial transcripts of interest. This method was used to quantify the expression of DAPG and HCN biosynthetic genes in natural and artificial soils (DeCoste et al, 2011; Novinscak and Filion, 2011; Almario et al, 2013b). However, the main problem with this approach is the inefficiency of RNA extraction from a complex material such as soil along with the limited abundance of transcripts for antimicrobial genes.…”

“…Indeed, DAPG and HCN producing pseudomonads could be isolated not only from suppressive but also from conducive soils, leading to the hypothesis that differential environmental factors prevailing in the two types of soils may shape the expression of relevant biocontrol genes in Pseudomonas bacteria and thus disease suppression (Ramette et al, 2006). Moreover, the abundance of the DAPG biosynthesis gene phlD was not indicative of the resistance of soils to black root rot of tobacco (Almario et al, 2013b; Kyselkova et al, 2014). Our results support the hypothesis that the abundance of pseudomonads producing antimicrobial metabolites may, in fact, be less important for disease suppression in the rhizosphere than previously hypothesized, at least for certain soils.…”

“…To date, little is known about the biotic and abiotic factors affecting the expression of biosynthetic genes for these metabolites in soil. Studies conducted under gnotobiotic conditions indicate that the expression of DAPG, HCN, and PRN biosynthesis genes is influenced by the crop species and variety (Rochat et al, 2010; de Werra et al, 2011; Latz et al, 2015) and for DAPG also by the interaction with other microorganisms and the iron availability in the rhizosphere (Notz et al, 2001, 2002; Maurhofer et al, 2004; Jousset et al, 2010, 2011; Almario et al, 2013b). How biotic and abiotic soil factors affect antimicrobial gene expression under natural conditions in agricultural soils remains, however, unexplored.…”