In response to insect attack,plants use intricate signaling pathways,including phytohormones,such as jasmonate(JA),ethylene(ET),and salicylic acid(SA),to activate defenses.Maize(Zea mays)is one of the most important staple food crops around the world.Previous studies have shown that the JA and ET signaling play important roles in maize defense against insects,but little is known about whether and how SA regulates maize resistance to insect herbivores.In this study,we ectopically expressed the NahG(salicylate hydroxylase)gene in maize plants(NahG maize)to block the accumulation of SA.It was found that compared with the wild-type(WT)maize,the NahG maize exhibited decreased resistance to the generalist insects Spodoptera litura and Spodoptera frugiperda and the specialist Mythimna separata,and the compromised resistance in the NahG maize was associated with decreased levels of defensive me-tabolites benzoxazinoids(Bxs)and chlorogenic acid(CA).Quantification of simulated S.litura feeding-induced JA,JA-isoleucine conjugate(JA-Ile),and ET in the WT and NahG maize indicated that SA does not regulate JA or JA-Ile,but positively controls ET.We provide evidence suggesting that the SA pathway does not crosstalk with the JA or the ET signaling in regulating the accumulation of Bxs and CA.Tran-scriptome analysis revealed that the bHLH,ERF,and WRKY transcription factors might be involved in SA-regulated defenses.This study uncovers a novel and important phytohormone pathway in maize defense against lepidopterous larvae.

Plants have sophisticated defense systems to fend off insect herbivores. How plants defend against herbivores in dicotyledonous plants, such as Arabidopsis and tobacco, have been relatively well studied, yet little is known about the defense responses in monocotyledons. Here, we review the current un-derstanding of rice (Oryza sativa) and maize (Zea mays) defense against insects. In rice and maize, elicitors derived from insect herbivore oral secretions or oviposition fluids activate phytohormone signaling, and transcriptomic changes mediated mainly by transcription factors lead to accumulation of defense-related secondary metabolites. Direct defenses, such as trypsin protein inhibitors in rice and benzoxazinoids in maize, have anti-digestive or toxic effects on insect herbivores. Herbivory-induced plant volatiles, such as terpenes, are indirect defenses, which attract the natural enemies of herbi-vores. R gene-mediated defenses against herbivores are also discussed.

To optimize fitness,plants must efficiently allocate their resources between growth and defense.Although phytohormone crosstalk has emerged as a major player in balancing growth and defense,the genetic basis by which plants manage this balance remains elusive.We previously identified a quantitative diseaseresistance locus,qRfg2,in maize (Zea mays) that protects against the fungal disease Gibberella stalk rot.Here,through map-based cloning,we demonstrate that the causal gene at qRfg2 is ZmAuxRP1,which encodes a plastid stroma-localized auxin-regulated protein.ZmAuxRP1 responded quickly to pathogen challenge with a rapid yet transient reduction in expression that led to arrested root growth but enhanced resistance to Gibberella stalk rot and Fusarium ear rot.ZmAuxRP1 was shown to promote the biosynthesis of indole-3-acetic acid (IAA),while suppressing the formation of benzoxazinoid defense compounds.ZmAuxRP1 presumably acts as a resource regulator modulating indole-3-glycerol phosphate and/or indole flux at the branch point between the IAA and benzoxazinoid biosynthetic pathways.The concerted interplay between IAA and benzoxazinoids can regulate the growth-defense balance in a timely and efficient manner to optimize plant fitness.

Both herbivory and jasmonic acid (JA) activate the biosynthesis of defensive metabolites in maize, but the mechanism underlying this remains un-clear. We generated maize mutants in which ZmMYC2a and ZmMYC2b, two transcription factor genes important in JA signaling, were individually or both knocked out. Genetic and bi-ochemical analyses were used to elucidate the functions of ZmMYC2 proteins in the maize re-sponse to simulated herbivory and JA. Compared with the wild-type (WT) maize, the double mutant myc2ab was highly susceptible to insects, and the levels of benzoxazinoids and volatile terpenes, and the levels of their biosynthesis gene tran-scripts, were much lower in the mutants than in the WT maize after simulated insect feeding or JA treatment. Moreover, ZmMYC2a and ZmMYC2b played a redundant role in maize resistance to insects and JA signaling. Transcriptome and Cleavage Under Targets and Tagmentation-Sequencing (CUT&Tag-Seq) analysis indicated that ZmMYC2s physically targeted 60％of the JA-responsive genes, even though only 33％of these genes were transcriptionally ZmMYC2-dependent. Importantly, CUT&Tag-Seq and dual luciferase assays revealed that ZmMYC2s transactivate the benzoxazinoid and volatile terpene biosynthesis genes IGPS1/3, BX10/11/12/14, and TPS10/2/3/4/5/8 by directly binding to their promoters. Fur-thermore, several transcription factors physically targeted by ZmMYC2s were identified, and these are likely to function in the regulation of benzoxazinoid biosynthesis. This work reveals the transcriptional regulatory landscapes of both JA signaling and ZmMYC2s in maize and provides comprehensive mechanistic insight into how JA signaling modulates defenses in maize responses to herbivory through ZmMYC2s.