Abstract
Mycorrhizal plants display enhanced resistance to several pathogens. However, the molecular mechanisms regulating mycorrhiza-induced resistance (MIR) are still elusive. We aim to study the mechanisms underlying MIR against Botrytis cinerea and the role of callose accumulation during this process. Mycorrhizal tomato plants inoculated with Rhizoglomus irregularis displayed callose priming upon B. cinerea infection. The callose inhibitor 2-deoxy-d-glucose abolished MIR, confirming the relevance of callose in the bioprotection phenomena. While studying the mechanisms underlying mycorrhiza-induced callose priming, we found that mycorrhizal plants display an enhanced starch degradation rate that is correlated with increased levels of β-amylase1 transcripts following pathogen infection. Starch mobilization in mycorrhizal plants seems coordinated with the increased transcription of sugar transporter and invertase genes. Moreover, the expression levels of genes encoding the vesicular trafficking proteins ATL31 and SYP121 and callose synthase PMR4 were higher in the mycorrhizal plants and further boosted by subsequent pathogen infection. All these proteins play a key role in the priming of callose accumulation in Arabidopsis, suggesting that callose priming is an induced resistance mechanism conserved in different plant species. This evidence highlights the importance of sugar mobilization and vesicular trafficking in the priming of callose as a defence mechanism in mycorrhiza-induced resistance.
Highlights
Beneficial microorganism–plant interactions are widespread in nature
To determine whether mycorrhiza-induced resistance (MIR) against B. cinerea was mediated by enhanced callose deposition, the control and AM plants were infected with the fungus, and callose accumulation was quantified
To investigate whether this callose accumulation is an important component in MIR, the plants were infiltrated with 2-DDG, which is a competitive inhibitor of callose synthase, 24 h prior to inoculation with B. cinerea.The infiltration resulted in almost complete inhibition of callose accumulation and the impairment of MIR (Fig. 1C), whereas in the waterinfiltrated plants, MIR against B. cinerea was fully functional
Summary
Beneficial microorganism–plant interactions are widespread in nature. Among such interactions, >80% of plants are associated with arbuscular mycorrhiza fungi (AMF), which are soil-borne obligate biotrophs belonging to the Glomeromycota phylum. AMF are found in practically all agricultural and natural environments These fungi establish a mutualistic symbiosis with plant roots, forming specialized intracellular structures in the root cortex known as arbuscules, where the interchange of nutrients between the symbionts occurs (Gutjahr and Parniske, 2013). Once symbiosis is well established, the plant provides photosynthates and lipids to the fungus and, in return, the AMF improve the plant mineral nutrients and water uptake. In addition to these benefits, AM plants present enhanced tolerance to abiotic and biotic stresses (Miransari, 2010; Jung et al, 2012; Sánchez-Bel et al, 2016; Rivero et al, 2018)
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