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.