Species-Specific Interactions of Bacillus Innocula and Arbuscular Mycorrhizal Fungi Symbiosis with Winter Wheat

Abstract

Arbuscular mycorrhizal (AM) fungi establish close interactions with host plants, an estimated 80% of vascular plant species. The host plant receives additional soil bound nutrients that would otherwise not be available. Other components of the microbiome, such as rhizobacteria, may influence interactions between AM fungi and the host plant. Within a commercial arable crop selected rhizobacteria in combination with AM fungi may benefit crop yields. The precise nature of interactions between rhizobacteria and AM fungi in a symbiotic relationship overall requires greater understanding. The present study aims to assess this relationship by quantifying: (1) AM fungal intracellular root structures (arbuscules) and soil glomalin as an indicator of AM fungal growth; and (2) root length and tiller number as a measure of crop growth, in response to inoculation with one of three species of Bacillus: B. amyloliquefaciences, B. pumilis, or B. subtilis. The influence of soil management, conventional (CT) or zero tillage (ZT) was a further variable evaluated. A significant (p < 0.0001) species-specific impact on the number of quantifiable AM fungal arbuscules was observed. The inoculation of winter wheat (Triticum aestivum) with B. amyloliquefaciences had a positive impact on AM fungal symbiosis, as indicated by an average of 3226 arbuscules per centimetre of root tissue. Bacillus subtilis increased root length significantly (p < 0.01) but decreased fungal symbiosis (p < 0.01). The inoculation of field soils altered the concentration of glomalin, an indicator of AM fungal growth, significantly (p < 0.00001) for each tillage treatment. The greatest increase was associated with B. amyloliquefaciences for both CT (p < 0.0001) and ZT (p < 0.00001). Bacillus subtilis reduced measured glomalin significantly in both tillage treatments (p < 0.0001 and p < 0.00001 for CT and ZT respectively). The interaction between rhizobacteria and AM fungi is variable, being beneficial or detrimental depending on species. This relationship was evident in both tillage treatments and has important implications for maximizing symbiosis in the crop plant-microbiome present in agricultural systems.

The Tripartite Rhizobacteria-AM Fungal-Host Plant Relationship in Winter Wheat: Impact of Multi-Species Inoculation, Tillage Regime and Naturally Occurring Rhizobacteria Species

“…Tillage is detrimental to AMF [s] that facilitate nutrient transfer to the crop plant and enhance growth, due to the destruction of hyphal networks [ 2 , 20 , 32 ]. The aerobic soil conditions created by soil inversion, however, benefit the gram positive PGPR [s] B. amyloliquefaciens [ 33 ], noted to benefit plant growth at earlier growth stages by [ 17 ] and the later growth stages in this study. Wilkes et al [ 34 ] also report the detrimental impact of the herbicide glyphosate on AM fungi although they do not quantify the impact on PGPR.…”

“…A decrease in hyphal length reduces the capacity of the AM fungi to acquire nutrients for the host plant [57], decreasing the efficiency of the symbiotic relationship. Wilkes et al [17] observe an increase in root length indicative of the need by the plant to assimilate nutrients mainly via its own rooting system rather than through the increased acquisition efficiency provided by the AM fungi hyphae. This relationship may be site specific, with different PGPR interacting positively or negatively with a given species of AM fungi depending on the profile of the naturally occurring soil microorganisms.…”

“…Following the method described in Wilkes et al [ 17 ], rhizobacteria were derived from a commercially available (Tribus ® original, Impello Biosciences, Fort Collins, Colorado, USA) mixture consisting of three Gram-positive Bacilli species: B. subtilis , B. pumilis and B. amyloliquefaciens at a combined cfu count of 10 billion mL −1 . The identity of each of the three species was confirmed in the laboratory by API ® (bioMerieux ® Ltd., Marcy l’Etoile, ARA, France) colour guides.…”

“…The nature of the tripartite relationship is multi-faceted and consequently may not always be beneficial. Bacillus subtilis for example has been shown to inhibit the AM fungi Rhizophagus intraradices when applied as an inoculant of winter wheat [ 17 ] despite being reported as having a positive interaction with AM fungi by Alam et al [ 18 ]. It is evident that a multitrophic community exists with complex interactions between the plant, PGPR and AM fungi.…”

“…This benefit however may not be realised until later crop growth development stages. Further, this benefit may be amplified in the presence of co-inoculation with a known MHB such as B. amyloliquefaciens [ 17 ] in addition to an AM fungus. A key question this study aims to address is, can the benefits of the MHB B. amyloliquefaciens be enhanced further in the presence of additional AM fungi through co-inoculation?…”