Specialized metabolites constitute key layers of immunity that underlie disease resistance in crops; however, challenges in resolving pathways limit our understanding of the functions and applications of these metabolites. In maize (Zea mays), the inducible accumulation of acidic terpenoids is increasingly considered to be a defence mechanism that contributes to disease resistance. Here, to understand maize antibiotic biosynthesis, we integrated association mapping, pan-genome multi-omic correlations, enzyme structure–function studies and targeted mutagenesis. We define ten genes in three zealexin (Zx) gene clusters that encode four sesquiterpene synthases and six cytochrome P450 proteins that collectively drive the production of diverse antibiotic cocktails. Quadruple mutants in which the ability to produce zealexins (ZXs) is blocked exhibit a broad-spectrum loss of disease resistance. Genetic redundancies ensuring pathway resiliency to single null mutations are combined with enzyme substrate promiscuity, creating a biosynthetic hourglass pathway that uses diverse substrates and in vivo combinatorial chemistry to yield complex antibiotic blends. The elucidated genetic basis of biochemical phenotypes that underlie disease resistance demonstrates a predominant maize defence pathway and informs innovative strategies for transferring chemical immunity between crops.
Bibliographical noteFunding Information:
We thank A. Steinbrenner, J. Chan, K. O’Leary, M. Broemmer, H. Riggleman, S. Reyes and S. Delgado for help with planting, treatments and sampling (UCSD); L. Smith (UCSD) for shared UCSD Biology Field Station management; B. Hamberger (Michigan State University) for the ElHMGR gene. This work was partially supported by the USDA-ARS National Programs for Food Safety and Plant Genetic Resources, Genomics and Genetic Improvement (to M.M.V., M.G.B. and S.A.C.). Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the USDA. Research was supported by grants from the National Science Foundation, Division of Integrative Organismal Systems (NSF-IOS) (grant no. 1936492 to B.Y. and grant no. 1546899 to S.P.B.), USDA NIFA AFRI (grant no. 2018-67013-28125 to A.H. and E.S.) for sesquiterpenoids, NSF Plant-Biotic Interactions Program (grant no. 1758976 to E.S. and P.Z.) for diterpenoids, NSF Faculty Early Career Development Program (grant no. 1943591 to A.H.), the DOE Joint Genome Institute Community Science Program (JGI-CSP) (grant nos. CSP 2568 (to P.Z., J.B., E.S. and A.H.) and CSP 503420 (to A.H. and E.S.)) and fellowships provided by the NSF Graduate Research Fellowship Program (to K.M.M.), the U.C. Davis Innovation Institute for Food and Health (IIFH) Fellowship Program (to K.M.M. and P.Z.), the USDA NIFA Predoctoral Fellowship Program (award no. 2019-67011-29544, to K.M.M.) and a Fulbright Research Grant (E0581299, to M.B.).
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