Salicylic acid positively regulates maize defenses against lepidopteran insects

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 metabolites benzoxazinoids(Bxs)and chlorogenic acid(CA).Quantification of simulated S.litura feedinginduced 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.Transcriptome analysis revealed that the bHLH,ERF,and WRKY transcription factors might be involved in SAregulated defenses.This study uncovers a novel and important phytohormone pathway in maize defense against lepidopterous larvae.

Current understanding of maize and rice defense against insect herbivores

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 rnays） 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 herbivores. R gene-mediated defenses against herbivores are also discussed.

ZmMYC2s play important roles in maize responses to simulated herbivory and jasmonate

Both herbivory and jasmonic acid(JA)activate the biosynthesis of defensive metabolites in maize,but the mechanism underlying this remains unclear.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 biochemical analyses were used to elucidate the functions of ZmMYC2 proteins in the maize response 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 transcripts,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 TagmentationSequencing(CUT&Tag-Seq)analysis indicated that ZmMYC2s physically targeted 60%of the JAresponsive 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.Furthermore,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.

Bacterial Effectors Induce Oligomerization of Immune Receptor ZAR1 In Vivo

Plants utilize nucleotide-binding,leucine-rich repeat receptors(NLRs)to detect pathogen effectors,leading to effector-triggered immunity.The NLR ZAR1 indirectly recognizes the Xanthomonas campestris pv.campestris effector AvrAC and Pseudomonas syringae effector HopZIa by associating with closely related receptor-like cytoplasmic kinase subfamily XII-2(RLCK XII-2)members RKS1 and ZED1,respectively.ZAR1,RKS1,and the AvrAC-modified decoy PBL2ump form a pentameric resistosome in vitro,and the ability of resistosome formation is required for AvrAC-triggered cell death and disease resistance.However,it remains unknown whether the effectors induce ZAR1 oligomerization in the plant cell.In this study,we show that both AvrAC and HopZ1 a can induce oligomerization of ZAR1 in Arabidopsis protoplasts.Residues mediating ZAR1-ZED1 interaction are indispensable for HopZIa-induced ZAR1 oligomerization in vivo and disease resistance.In addition,ZAR1 residues required for the assembly of ZAR1 resistosome in vitro are also essential for HopZIa-induced ZAR1 oligomerization in vivo and disease resistance.Our study provides evidence that pathogen effectors induce ZAR1 resistosome formation in the plant cell and that the resistosome formation triggers disease resistance.

Extensive Inter-plant Protein Transfer between Cuscuta Parasites and Their Host Plants

Cuscuta species(dodders)are holoparasites that totally rely on host plants to survive.Although various mobile proteins have been identified to travel within a plant,whether and to what extent protein transfer between Cuscuta and host plants remain unclear.We found that hundreds to more than 1500 proteins were transferred between Cuscuta and the host plants Arabidopsis and soybean,and hundreds of interplant mobile proteins were even detected in the seeds of Cuscuta and the host soybean.Different hosts bridge-connected by dodder were also found to exchange hundreds of proteins.Quantitatively,the mobile proteins represent a few to more than 10%of the proteomes of foreign plants.Using Arabidopsis plants expressing different reporter proteins,we further showed that these reporter proteins could travel between plants and,importantly,retained their activity in the foreign plants.Comparative analysis between the interplant mobile proteins and mRNAs indicated that the majority of mobile proteins were not de novo synthesized from the translocated mRNAs,but bona fide mobile proteins.We propose that large-scale inter-plant protein translocation may play an important role in the interactions between host plants and dodder and even among the dodder bridge-connected hosts.

Elevated CO2 differentially affects tobacco and rice defense against lepidopteran larvae via the jasmonic acid signaling pathway

Atmospheric CO2 levels are rapidly increasing due to human activities. However, the effects of elevated CO2 （ECO2） on plant defense against insects and the underlying mechanisms remain poorly understood. Here we show that ECO2 increased the photosynthetic rates and the biomass of tobacco and rice plants, and the chewing lepidopteran insects Spodoptera litura and Mythimna separata gained less and more mass on tobacco and rice plants, respectively. Consistently, under ECO2, the levels of jasmonic acid （JA）, the main phytohormone controlling plant defense against these lepidopteran insects, as well as the main defense-related metabolites, were increased and decreased in insectdamaged tobacco and rice plants. Importantly, bioassays and quantification of defense-related metabolites in tobacco and rice silenced in JA biosynthesis and perception indicate that ECO2 changes plant resistance mainly by affecting the JA pathway. We further demonstrate that the defensive metabolites, but not total N or protein, are the main factors contributing to the altered defense levels under ECO2. This study illustrates that ECO2 changes the interplay between plants and insects, and we propose that crops should be studied for their resistance to the major pests under ECO2 to predict the impact of ECO2 on future agroecosystems.

Jasmonic acid carboxyl methyltransferase regulates development and herbivory-induced defense response in rice

Jasmonic acid（JA） and related metabolites play a key role in plant defense and growth. JA carboxyl methyltransferase（JMT） may be involved in plant defense and development by methylating JA to methyl jasmonate（Me JA） and thus influencing the concentrations of JA and related metabolites. However, no JMT gene has been well characterized in monocotyledon defense and development at the molecular level. After we cloned a rice JMT gene,Os JMT1, whose encoding protein was localized in the cytosol, we found that the recombinant Os JMT1 protein catalyzed JA to Me JA. Os JMT1 is up-regulated in response to infestation with the brown planthopper（BPH; Nilaparvata lugens）. Plants in which Os JMT1 had been overexpressed（oeJMT plants） showed reduced height and yield. These oe-JMT plants also exhibited increased Me JA levels but reduced levels of herbivore-induced JA and jasmonoyl-isoleucine（JAIle）. The oe-JMT plants were more attractive to BPH female adults but showed increased resistance to BPH nymphs,probably owing to the different responses of BPH female adults and nymphs to the changes in levels of H_2O_2 and Me JA in oe-JMT plants. These results indicate that Os JMT1,by altering levels of JA and related metabolites, plays a role in regulating plant development and herbivore-induced defense responses in rice.