Abstract
The symbiosis established between arbuscular mycorrhizal fungi (AMF) and roots of most land plants plays a key role in plant nutrient acquisition and alleviation of environmental stresses. Despite the ubiquity of the symbiosis, AMF and host species display significant specificity in their interactions. To clarify preferential associations between wheat (Triticum aestivum) and AMF, we characterized root AMF communities in the transition from two first host species, ryegrass (Lolium rigidum) and yellow-serradella (Ornithopus compressus), grown separately or together, to a second host (wheat), by sequencing the large subunit ribosomal DNA (LSU rDNA) gene. The response of AMF communities in wheat to prior soil disturbance – and consequently of the mycelial network [intact extraradical mycelium (ERM) vs. disrupted mycelium] established with either of the first hosts – was also investigated. Since the outcome of a specific host–symbiont interaction depends on the molecular responses of the host plant upon microbial colonization, we studied the expression of six key symbiosis-related genes in wheat roots. AMF communities on L. rigidum and O. compressus roots were clearly distinct. Within an undisturbed ERM, wheat AMF communities were similar to that of previous host, and O. compressus-wheat-AMF interactions supported a greater growth of wheat than L. rigidum-wheat-AMF interactions. This effect declined when ERM was disrupted, but generated a greater activation of symbiotic genes in wheat, indicating that plant symbiotic program depends on some extent on the colonizing symbiont propagule type. When a mixture of L. rigidum and O. compressus was planted, the wheat colonization pattern resembled that of O. compressus, although this was not reflected in a greater growth. These results show a lasting effect of previous hosts in shaping wheat AMF communities through an efficient use of the established ERM, although not completely obliterating host–symbiont specificity.
Highlights
It is strongly held that the symbiosis with arbuscular mycorrhizal fungi (AMF) was a major driver of colonization of land by plants and fundamentally altered environmental conditions on earth (Humphreys et al, 2010)
Biological material was obtained through a two-phase greenhouse experiment designed to evaluate the AMF symbiotic community associated with two first host plant species, and after the transition to a second host species
We found that the expression levels of POLLUX and calcium and calmodulin-dependent kinase (CCaMK) within roots from the undisturbed soil were significantly greater in wheat after L. rigidum than after O. compressus, which might relate to differential abundance of specific taxa
Summary
It is strongly held that the symbiosis with arbuscular mycorrhizal fungi (AMF) was a major driver of colonization of land by plants and fundamentally altered environmental conditions on earth (Humphreys et al, 2010). The symbionts are horizontally transmitted (through mycelial networks) and individual plants can simultaneously associate with multiple species of AMF that may differ in provision and delivery of benefits. Despite their ubiquity and ability to colonize most plants, AMF can exhibit considerable selectivity in their association with different host species, and vice-versa (Helgason and Fitter, 2009; Öpik et al, 2009). Coexisting plant species can harbor distinct AMF communities (Öpik et al, 2009; Hazard et al, 2013; Davison et al, 2015); this is not surprising since in common with their hosts, arbuscular mycorrhiza differ in life-history traits and some host–symbiont combinations can be more beneficial than others (Bever, 2002; Zhang et al, 2010; Walder et al, 2012). The closeness of different plant species harboring distinct AMF communities can increase both species richness and induce shifts in the AMF root community composition of a particular host (Zhang et al, 2010; Meadow and Zabinski, 2012)
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