Distinct gene expression and secondary metabolite profiles in suppressor of prosystemin-mediated responses2 (spr2) tomato mutants having impaired mycorrhizal colonization.

Kena Casarrubias-Castillo,Norma A Martinez-Gallardo,John P Délano-Frier,Robert Winkler,Hamlet Avilés-Arnaut,Nicole Dabdoub-González,Julia Zañudo-Hernández,Josaphat M Montero-Vargas

doi:10.7717/peerj.8888

Abstract

Arbuscular mycorrhizal fungi (AMF) colonization, sampled at 32–50 days post-inoculation (dpi), was significantly reduced in suppressor of prosystemin-mediated responses2 (spr2) mutant tomato plants impaired in the ω−3 FATTY ACID DESATURASE7 (FAD7) gene that limits the generation of linolenic acid and, consequently, the wound-responsive jasmonic acid (JA) burst. Contrary to wild-type (WT) plants, JA levels in root and leaves of spr2 mutants remained unchanged in response to AMF colonization, further supporting its regulatory role in the AM symbiosis. Decreased AMF colonization in spr2 plants was also linked to alterations associated with a disrupted FAD7 function, such as enhanced salicylic acid (SA) levels and SA-related defense gene expression and a reduction in fatty acid content in both mycorrhizal spr2 roots and leaves. Transcriptomic data revealed that lower mycorrhizal colonization efficiency in spr2 mutants coincided with the modified expression of key genes controlling gibberellin and ethylene signaling, brassinosteroid, ethylene, apocarotenoid and phenylpropanoid synthesis, and the wound response. Targeted metabolomic analysis, performed at 45 dpi, revealed augmented contents of L-threonic acid and DL-malic acid in colonized spr2 roots which suggested unfavorable conditions for AMF colonization. Additionally, time- and genotype-dependent changes in root steroid glycoalkaloid levels, including tomatine, suggested that these metabolites might positively regulate the AM symbiosis in tomato. Untargeted metabolomic analysis demonstrated that the tomato root metabolomes were distinctly affected by genotype, mycorrhizal colonization and colonization time. In conclusion, reduced AMF colonization efficiency in spr2 mutants is probably caused by multiple and interconnected JA-dependent and independent gene expression and metabolomic alterations.

Highlights

The roots of the majority of higher plants, and many other host plants including pteridophytes, a number of mosses, lycopods, and psilotales are associated symbiotically with biotrophic and aseptate filamentous fungi of the Glomeromycota phylum, the so-called arbuscular mycorrhizal fungi (AMF) (Redecker & Raab, 2006; Schüssler, Schwarzott & Walker, 2001; Van der Heijden et al, 2015)
AMF colonization parameters, irrespective of the AMF used as inoculum and method employed to evaluate colonization, were significantly higher in roots of WT plants compared to spr2 mutants
Root transcriptomic and metabolomic data indicated that reduced AMF colonization in spr2 roots was likely caused by jasmonic acid (JA)-dependent and JA-independent factors

Summary

Introduction

The roots of the majority of higher plants, and many other host plants including pteridophytes, a number of mosses, lycopods, and psilotales are associated symbiotically with biotrophic and aseptate filamentous fungi of the Glomeromycota phylum, the so-called arbuscular mycorrhizal fungi (AMF) (Redecker & Raab, 2006; Schüssler, Schwarzott & Walker, 2001; Van der Heijden et al, 2015). An important feature of the AM symbiosis is the nutrient exchange between both partners, which occurs within individual cortical cells, where AMF form highly branched hyphae, called arbuscules, surrounded by a plant derived periarbuscular membrane. This structure has a unique transport protein distribution designed to deliver mineral nutrients to the cortical cell in exchange for the 4–20% of the photosynthates allocated to the roots and subsequently transferred to the fungal symbiont (Gutjahr & Parniske, 2013; Bhandari & Garg, 2017; Rich et al, 2017). Several of the above effects are finely regulated by phytohormones, via a limitedly understood and highly context dependent interaction (Gutjahr, 2014; Selosse, Bessis & Pozo, 2014; Pozo et al, 2015; Lenoir, Fontaine & Sahraoui, 2016; Bedini et al, 2018; Liao et al, 2018; Cabral et al, 2019; Evelin et al, 2019)

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