Systemin modulates defense responses in roots of tomato plants (Solanum lycopersicum L.) during the pre-colonization stage of the mycorrhizal symbiosis

Aim of study: The symbiotic association with arbuscular mycorrhizal fungi (AMF) enhances the uptake of soil minerals by the plant, predominantly phosphorus, in return for plant photosynthates. This study was performed to support the premise that the suppression of root defense responses during the pre-colonization stage is required for the subsequent colonization of tomato roots by AMF. Area of study: This study was performed in the Plant Defense Laboratory of Cinvestav, at Irapuato, Guanajuato, Mexico. Material and methods: SYS was added, together with spore suspensions of three different AMF species, to young tomato plantlets. The roots were subsequently sampled, 0.5 to 12 h post-application, in order to quantify degree of mycorrhizal colonization, in vitro β-glucanase (GLN) and chitinase (CHI) enzyme activity and wound-responsive gene expression levels. Main results: The sole application of exogenous SYS induced the rapid expression of a battery of early wound-responsive genes, together with a swift and transient activation of CHI, but not GLN. However, when added together with AMF spores, SYS differentially modulated the activity of these enzymes in an AMF species-dependent manner. Modified lytic activity was preceded or accompanied by the rapid and sustained induction of the RbohD, LOXD and PLA2 genes shortly after contact with AMF spores. Research highlights: The findings of this study suggest a role for oxylipins and reactive oxygen species in the initial AMF recognition process. They also indicate that exogenous SYS is perceived by the roots, where it modulates the local root defense response to facilitate AMF colonization.