Effects of Nitrogen Supply on Induced Defense in Maize (Zea mays) against Fall Armyworm (Spodoptera frugiperda).

How nitrogen (N) supply affects the induced defense of plants remains poorly understood. Here, we investigated the impacts of N supply on the defense induced in maize (Zea mays) against the fall armyworm (Spodoptera frugiperda). In the absence of herbivore attack or exogenous jasmonic acid (JA) application, N supply increased plant biomass and enhanced maize nutrient (soluble sugar and amino acid) contents and leaf area fed by S. frugiperda (the feeding leaf area of S. frugiperda larvae in maize supplemented with 52.2 and 156.6 mg/kg of N was 4.08 and 3.83 times that of the control, respectively). When coupled with herbivore attack or JA application, maize supplemented with 52.2 mg/kg of N showed an increased susceptibility to pests, while the maize supplemented with 156.6 mg/kg of N showed an improved defense against pests. The changes in the levels of nutrients, and the emissions of volatile organic compounds (VOCs) caused by N supply could explain the above opposite induced defense in maize. Compared with herbivore attack treatment, JA application enhanced the insect resistance in maize supplemented with 156.6 mg/kg of N more intensely, mainly reflecting a smaller feeding leaf area, which was due to indole emission and two upregulated defensive genes, MPI (maize proteinase inhibitor) and PAL (phenylalanine ammonia-lyase). Hence, the optimal N level and appropriate JA application can enhance plant-induced defense against pests.

Cabbage cultivars influence transfer and toxicity of cadmium in soil-Chinese flowering cabbage Brassica campestris-cutworm Spodoptera litura larvae.

We executed a pot experiment to examine the differences of absorption, chemical forms, subcellular distribution, and toxicity of Cd between two cultivars of Chinese flowering cabbage Brassica campestris [Lvbao701 (low-Cd cultivar) and Chicaixin No.4 (high-Cd cultivar)]. Compared to Chicaixin No.4, the presence of Lvbao701 enhanced the proportion of insoluble Cd forms in soil, Lvbao701 roots and leaves had higher proportion of Cd converted into insoluble phosphate precipitates and pectate-or protein-bound forms and lower proportion of inorganic Cd, which result in low accumulation and toxicity of Cd to Lvbao701 and cutworm Spodoptera litura fed on Lvbao701 leaves. Instead of total Cd, Cd transfer and toxicity in B. campestris-S. litura system depend on chemical Cd forms in soil and cabbages and subcellular Cd distributions in cabbages and insects, and the proportions of them were not the highest among all chemical forms and subcellular distributions of Cd. Although exchangeable Cd was major Cd chemical form in cabbage planted soil, Cd bound to iron and manganese oxides and to organic matter were significantly correlated with growth indices and photosynthesis parameters of cabbages. Despite major part of Cd was precipitated in cell wall of roots, Cd in organelle fraction was closely associated with the fitness of cabbages. Metal-rich granules, not cytosolic fraction (the major subcellular Cd distribution), affected the food utilization of S. litura. Therefore, cabbage cultivars significantly affected Cd transfer and toxicity in B. campestris-S. litura system, and the use of Lvbao701 in Cd polluted soil could reduce potential risks for Cd entering food chains.

Defense of cabbages against herbivore cutworm Spodoptera litura under Cd stress and insect herbivory stress simultaneously.

Biotic (e.g., heavy metal) and abiotic stress (e.g., insect attack) can affect plant chemical defense, but little is known about the changes in plant defense when they occur concurrently. Herein, the impacts of heavy metal cadmium (Cd) stress and insect herbivory stress on the direct and indirect defense of two cultivar cabbages of Brassica campestris, the low-Cd cultivar Lvbao701 and the high-Cd cultivar Chicaixin No.4, against the herbivore cutworm Spodoptera litura were investigated. Although 10 mg kg-1 Cd stress alone inhibited leaf secondary metabolites (total phenolics, flavonoids), it reduced the feeding rate and odor selection of S. litura towards both cultivar cabbages, especially for Lvbao701, by increasing leaf Cd content and repellent volatile organic compounds (VOCs) (6-methyl-5-hepten-2-one, 7,9-di-tert-butyl-1-oxaspiro (4,5)deca-6,9-diene-2,8-dione), and reducing soluble sugar and attractive VOCs (3-methyl-3-pentanol, 2,5-hexanedione, tetradecanal). Under 2.5 mg kg-1 Cd and herbivory stress, although leaf total phenolics and flavonoids increased significantly, the feeding rate and odor selection of S. litura towards both cultivar cabbages increased, especially for Chicaixin No.4, indicating that the chemical defense of cabbages was depressed. Therefore, Cd stress alone improved the insect resistance of cabbages, whereas herbivory stress weakened the enhanced cabbages defence by Cd stress. The low-Cd cultivar Lvbao701 presented stronger insect resistance than Chicaixin No.4, suggesting that Lvbao701 application in Cd-polluted soil can not only decrease Cd transmission to higher levels in the food chain but also reduce pest occurrence.

Integration of transcriptome and proteome reveals molecular mechanisms underlying stress responses of the cutworm, Spodoptera litura, exposed to different levels of lead (Pb).

Heavy metals are major environmental pollutants that affect organisms across different trophic levels. Herbivorous insects play an important role in the bioaccumulation, and eventually, biomagnification of these metals. Although effects of heavy metal stress on insects have been well-studied, the molecular mechanisms underlying their effects remain poorly understood. Here, we used the RNA-Seq profiling and isobaric tags for relative and absolute quantitation (iTRAQ) approaches to unravel these mechanisms in the polyphagous pest Spodoptera litura exposed to lead (Pb) at two different concentrations (12.5 and 100 mg Pb/kg; PbL and PbH, respectively). Altogether, 1392 and 1630 differentially expressed genes (DEGs) and 58, 114 differentially expressed proteins (DEPs) were identified in larvae exposed to PbL and PbH, respectively. After exposed to PbL, the main up-regulated genes clusters and proteins in S. litura larvae were associated with their metabolic processes, including carbohydrate, protein, and lipid metabolism, but the levels of cytochrome P450 associated with the pathway of xenobiotic biodegradation and metabolism were found to be decreased. In contrast, the main up-regulated genes clusters and proteins in larvae exposed to PbH were enriched in the metabolism of xenobiotic by cytochrome P450, drug metabolism-cytochrome P450, and other drug metabolism enzymes, while the down-regulated genes and proteins were found to be closely related to the lipid (lipase) and protein (serine protease, trypsin) metabolism and growth processes (cuticular protein). These findings indicate that S. litura larvae exposed to PbL could enhance food digestion and absorption to prioritize for growth rather than detoxification, whereas S. litura larvae exposed to PbH reduced food digestion and absorption and channelized the limited energy for detoxification rather than growth. These contrasting results explain the dose-dependent effects of heavy metal stress on insect life-history traits, wherein low levels of heavy metal stress induce stimulation, while high levels of heavy metal stress cause inhibition at the transcriptome and proteome levels.