IncP-1β Plasmids Are Important Carriers of Fitness Traits for Variovorax Species in the Mycosphere—Two Novel Plasmids, pHB44 and pBS64, with Differential Effects Unveiled

Abstract

The Laccaria proxima mycosphere strongly selects Variovorax paradoxus cells. Fifteen independent V. paradoxus strains, isolated from mycospheres sampled at two occasions, were investigated with respect to the occurrence of plasmids of sizes <60-100 kb. Two V. paradoxus strains, HB44 and BS64, were found to contain such plasmids, which were coined pHB44 and pBS64. Replicon typing using a suite of plasmid-specific PCR systems indicated that both plasmids belong to the IncP-1β group. Also, both were able to mobilize selectable IncQ group plasmids into Escherichia coli as well as Pseudomonas fluorescens. Moreover, they showed stable replication in these organisms, confirming their broad host range. Strain BS64 was cured of pBS64 and plasmid pHB44 was subsequently moved into this cured strain by making use of the IncQ group tracer plasmid pSUP104, which was then removed at elevated temperature. Thus, both plasmids could be screened for their ability to confer a phenotype upon strain BS64. No evidence for the presence of genes for xenobiotic degradation and/or antibiotic or heavy metal resistances was found for either of the two plasmids. Remarkably, both could stimulate the production of biofilm material by strain BS64. Also, the population densities of pBS64-containing strain BS64 were temporarily raised in liquid as well as soil systems (versus the plasmid-cured strain), both in the presence of the fungal host Lyophyllum sp. strain Karsten. Strikingly, plasmid pHB44 significantly enhanced the fitness of strain BS64 in soil containing Lyophyllum sp. strain Karsten, but decreased its fitness in soil supplemented with extra FeCl3. The effect was noted both in separate (no inter-strain competition) and joint (competition) inoculations.

Gene mobility in microbiomes of the mycosphere and mycorrhizosphere –role of plasmids and bacteriophages

“…Whereas this finding can be taken as evidence for an IncP-1β selective effect of plant roots, a possible role for fungi of the rhizosphere (and so mycospheres and/or mycorrhizospheres) cannot be excluded. Indeed, as described in Zhang et al (2015), IncP-1β plasmids were typically present in several Variovorax paradoxus isolates obtained from the mycosphere of L. proxima. In other work, they were also encountered in the microbiomes of various other mycospheres, such as those of Russula, Inocybe and Ampulloclitocybe spp.…”

“…Selected plasmids, i.e. pHB44 and pBS64, were found to stimulate the production of biofilms in their V. paradoxus BS64 hosts as compared to plasmid-less variants (Zhang et al 2015). Moreover, a presumed iron uptake system present on the IncP-1β plasmid pHB44 was found to assist the fitness of the host organism, P. terrae strain BS001, in the mycosphere of Lyophyllum sp.…”

“…Roles of two groups of broad-host-range plasmids in the mycosphere. (A) The IncP-1β plasmid pHB44 enhances Paraburkholderia terrae BS001 fitness under low-iron conditions in the mycosphere and also transfers to Variovorax paradoxus HB44 under these conditions (Zhang et al 2014a(Zhang et al , 2014b(Zhang et al , 2015. (B) The broad-hostrange PromA group plasmids act as agents of highly promiscuous and frequent HGT, including insertions of accessory genes into the parA locus (van Elsas et al 1998;Zhang et al 2015;Dias et al 2018).…”

The Complete Sequences and Ecological Roles of Two IncP-1β Plasmids, pHB44 and pBS64, Isolated from the Mycosphere of Laccaria proxima

“…In a recent study, we reported experiments with two plasmids, pHB44 and pBS64, of Variovorax paradoxus obtained from the mycosphere of Laccaria proxima . Replicon typing provided preliminary evidence that both plasmids belong to the IncP-1β group (Zhang et al, 2015 ). Also, both were able to mobilize selectable IncQ group plasmids into other bacteria.…”

“…Remarkably, plasmid pHB44 enhanced the fitness of strain BS64 in soil microcosms with the fungus Lyophyllum sp. strain Karsten under iron-limiting conditions, indicating a fitness-enhancing role for its host in this setting (Zhang et al, 2015 ). As pHB44 and pBS64 thus constitute the first plasmids that foster host fitness in the mycosphere, we here broaden the scope of the analyses.…”