Selinene Volatiles Are Essential Precursors for Maize Defense Promoting Fungal Pathogen Resistance

Abstract

To ensure food security, maize (<i>Zea mays</i>) is a model crop for understanding useful traits underlying stress resistance. In contrast to foliar biochemicals, root defenses limiting the spread of disease remain poorly described. To better understand belowground defenses in the field, we performed root metabolomic profiling and uncovered unexpectedly high levels of the sesquiterpene volatile β-selinene and the corresponding nonvolatile antibiotic derivative β-costic acid. The application of metabolite-based quantitative trait locus mapping using biparental populations, genome-wide association studies, and near-isogenic lines enabled the identification of terpene synthase21 (<i>ZmTps21</i>) on chromosome 9 as a β-costic acid pathway candidate gene. Numerous closely examined β-costic acid-deficient inbred lines were found to harbor <i>Zmtps21</i> pseudogenes lacking conserved motifs required for farnesyl diphosphate cyclase activity. For biochemical validation, a full-length <i>ZmTps21</i> was cloned, heterologously expressed in <i>Escherichia coli</i>, and demonstrated to cyclize farnesyl diphosphate, yielding β-selinene as the dominant product. Consistent with microbial defense pathways, <i>ZmTps21</i> transcripts strongly accumulate following fungal elicitation. Challenged field roots containing functional <i>ZmTps21</i> alleles displayed β-costic acid levels over 100 μg g^-1 fresh weight, greatly exceeding in vitro concentrations required to inhibit the growth of five different fungal pathogens and rootworm larvae (<i>Diabrotica balteata</i>). In vivo disease resistance assays, using <i>ZmTps21</i> and <i>Zmtps21</i> near-isogenic lines, further support the endogenous antifungal role of selinene-derived metabolites. Involved in the biosynthesis of nonvolatile antibiotics, <i>ZmTps21</i> exists as a useful gene for germplasm improvement programs targeting optimized biotic stress resistance.