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

Abstract

To ensure food security, maize (Zea mays) 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 b-selinene and the corresponding nonvolatile antibiotic derivative b-costic acid. The application of metabolite-based quantitative trait locus mapping using biparental populations, genome-wide association studies, and nearisogenic lines enabled the identification of terpene synthase21 (ZmTps21) on chromosome 9 as a b-costic acid pathway candidate gene. Numerous closely examined b-costic acid-deficient inbred lines were found to harbor Zmtps21 pseudogenes lacking conserved motifs required for farnesyl diphosphate cyclase activity. For biochemical validation, a full-length ZmTps21 was cloned, heterologously expressed in Escherichia coli, and demonstrated to cyclize farnesyl diphosphate, yielding b-selinene as the dominant product. Consistent with microbial defense pathways, ZmTps21 transcripts strongly accumulate following fungal elicitation. Challenged field roots containing functional ZmTps21 alleles displayed b-costic acid levels over 100 mg g 21 fresh weight, greatly exceeding in vitro concentrations required to inhibit the growth of five different fungal pathogens and rootworm larvae (Diabrotica balteata). In vivo disease resistance assays, using ZmTps21 and Zmtps21 near-isogenic lines, further support the endogenous antifungal role of selinene-derived metabolites. Involved in the biosynthesis of nonvolatile antibiotics, ZmTps21 exists as a useful gene for germplasm improvement programs targeting optimized biotic stress resistance.

Ethylene signaling regulates natural variation in the abundance of antifungal acetylated diferuloylsucroses and Fusarium graminearum resistance in maize seedling roots

“…The damage treatments spanned the upper nodes, internodes and the most basal portion of unexpanded leaves. Aspergillus flavus, Rhizopus microspores, Fusarium verticilloides and Cochliobolus heterostrophus fungal spore inoculations were conducted with 100 ll of water per plant at a concentration of 1 9 10 7 spores ml À1 (Ding et al, 2017). Damage plus water alone was used for a mock inoculation.…”

“…Maize stem tissues were ground to a fine powder in liquid nitrogen and weighed out in 50 mg aliquots. Smilaside A and smiglaside C were analyzed as described previously (Ding et al, 2017). Negative ionization [M-H] À mode scans (0.1-atomic mass unit steps, 2.25 cycles s À1 ) from m/z 100-1000 were acquired.…”

Discovery, Biosynthesis and Stress-Related Accumulation of Dolabradiene-Derived Defenses in Maize

“…LC-MS analyses of diterpenoids were performed as described elsewhere (Ding et al, 2017). In brief, tissue samples were sequentially and additively bead homogenized in (1) 100 mL of 1-propanol:acetonitrile (ACN):formic acid (1:1:0.01), (2) 250 mL of ACN:ethyl acetate (1:1), and (3) 100 mL of water.…”

The maize heterotrimeric G-protein β subunit controls shoot meristem development and immune responses

“…The volatiles were collected with an inert filter containing 50 mg of HayeSep Q (80-to 100-mm mesh) polymer adsorbent (Sigma-Aldrich) and eluted in 200 µL methylene chloride. GC-MS analysis was conducted using an Agilent 6890 series gas chromatograph coupled to an Agilent 5973 mass selective detector (interface temperature, 250°C; mass temperature, 150°C; source temperature, 230°C; electron energy, 70 eV) as described previously (8). The gas chromatograph was operated with a DB-35MS chromatographic column (Agilent; 30 m × 0.25 mm × 0.25 µm).…”