Importance of mycorrhization helper bacteria cell density and metabolite localization for the Pinus sylvestris–Lactarius rufus symbiosis

Abstract

Mycorrhization helper bacteria, Paenibacillus sp. EJP73 and Burkholderia sp. EJP67, were used to study the importance of bacterial inoculum dose and bacterial derived soluble and volatile metabolites localization for enhancing mycorrhiza formation in the Pinus sylvestris-Lactarius rufus symbiosis, using a laboratory based microcosm. EJP73 and EJP67 produced different responses in relation to the inoculum dose; EJP73 significantly enhanced mycorrhiza formation to the same degree at all doses tested (10(5), 10(7), 10(9) and 10(10) CFU mL(-1)), whereas, EJP67 only stimulated mycorrhiza formation within a narrow range of inoculum densities (10(7) and 10(9) CFU mL(-1)). The importance of soluble bacterial metabolites was assessed by applying spent broth derived from exponential and stationary phase bacterial cultures to microcosms. No spent broth enhanced mycorrhiza formation over the control. As EJP73 produced the helper effect over a wide range of inoculum doses, this bacterium was chosen for further study. Physical separation of EJP73 from the fungal and plant symbiosis partners was carried out, in order to determine the contribution of constitutively produced bacterial volatile metabolites to the mycorrhization helper bacteria effect. When EJP73 was physically separated from the symbiosis, it had a significant negative effect on mycorrhiza formation. These results suggest that close proximity, or indeed cell contact, is required for the helper effect. Therefore, fluorescent in situ hybridization in conjunction with cryosectioning was used to determine the localization of EJP73 in mycorrhizal tissue. The cells were found to occur as rows or clusters ( approximately 10 cells) within the mycorrhizal mantle, both at the root tip and along the length of the mycorrhizal short roots.

Mycorrhizal responses to biochar in soil – concepts and mechanisms

“…flavonoids (AMF) and furans (ECM), that facilitate the growth of fungal hyphae and the subsequent colonization of plant roots by ECM (Founoune et al 2002;Duponnois and Plenchette 2003;Aspray et al 2006;Riedlinger et al 2006) and AM fungi (Duponnois and Plenchette 2003;Hildebrandt et al 2002Hildebrandt et al , 2006. Hildebrandt et al (2002Hildebrandt et al ( , 2006 have demonstrated that certain compounds (including raffinose and other unidentified metabolites) produced by strains of Paenibacillus can directly enhance the growth of AMF extraradical mycelium.…”

The Application of PCR Reaction for Identification of MHB Bacteria Species

“…The membership of the Burkholderia genus to MHB is reported by several authors [20,1,2,16]. In particular, attention deserves Poole et al (2001), who isolated this type of bacteria from symbiotic connection of Pinus sylvestris -Lactarius rufus.…”

“…A similar situation occurs in the layer of soil directly associated with mycorrhizal roots for which Linderman (1988) [19] suggested the name "mycorrhizosphere". Organisms inhabiting enriched rhizosphere are plant pathogens, opportunistic commensals, saprophytes, and associated with roots "co-workers" as well as endosymbiotes [19,1].…”

Differential effects of Paenibacillus spp. on cucumber mycorrhizas

“…Ravnskov et al (2002) observed similar differential effects of five strains of the biocontrol bacterium Burkholderia cepacia on the growth of the external mycelium of G. intraradices. Aspray et al (2006b) proposed that the expression of activities of helper bacteria depend on a variety of interacting factors, including cell density. In the present experiment the different Paenibacillus isolates were not quantified, but a food base was added in terms of TSB why the bacteria most likely were actively growing at least for a period during the experiment.…”