Soil chemistry determines whether defensive plant secondary metabolites promote or suppress herbivore growth

Plant secondary (or specialized) metabolites mediate important interactions in both the rhizosphere and the phyllosphere. If and how such compartmentalized functions interact to determine plant–environment interactions is not well understood. Here, we investigated how the dual role of maize benzoxazinoids as leaf defenses and root siderophores shapes the interaction between maize and a major global insect pest, the fall armyworm. We find that benzoxazinoids suppress fall armyworm growth when plants are grown in soils with very low available iron but enhance growth in soils with higher available iron. Manipulation experiments confirm that benzoxazinoids suppress herbivore growth under iron-deficient conditions and in the presence of chelated iron but enhance herbivore growth in the presence of free iron in the growth medium. This reversal of the protective effect of benzoxazinoids is not associated with major changes in plant primary metabolism. Plant defense activation is modulated by the interplay between soil iron and benzoxazinoids but does not explain fall armyworm performance. Instead, increased iron supply to the fall armyworm by benzoxazinoids in the presence of free iron enhances larval performance. This work identifies soil chemistry as a decisive factor for the impact of plant secondary metabolites on herbivore growth. It also demonstrates how the multifunctionality of plant secondary metabolites drives interactions between abiotic and biotic factors, with potential consequences for plant resistance in variable environments.

Organismal traits are commonly coopted for multiple functions (1–4). In complex and fluctuating environments, multifunctionality may lead to fitness trade-offs with important consequences for ecological and evolutionary dynamics (5–7).Plant secondary (or specialized) metabolites are important mediators of species interactions in natural and agricultural systems (8). Many plant secondary metabolites have been documented to protect plants against insect herbivores by acting as toxins, digestibility reducers, and/or repellents (9). Plant secondary metabolites also serve other functions: they can, for instance, act as signaling molecules (10), photoprotectants (11), antibiotics (12), soil nutrient mobilizers (13), and precursors of primary metabolites (14). Recent genetic work has highlighted that the same plant secondary metabolites may engage in multiple functions (4, 15, 16), leading to potentially important interactions between different environmental factors (6, 17). How this multifunctionality influences plant ecology and plant–insect interactions in complex environments is not well understood.The soil environment can have a major impact on plant defense expression and plant–herbivore interactions. Soil nutrients and micronutrients can reprogram plant defenses through cross talk between defense and nutrient signaling (18, 19) or by influencing soil microbes, which subsequently modulate plant defense responses (20, 21). Elements such as silicon (Si) can also act as defenses directly by forming crystals on the leaf surface (22). In addition, soil nutrients can also influence plant–herbivore interactions by changing the nutritional value of the plant to herbivores (23). Thus, plant secondary metabolites with dual functions in the rhizosphere and phyllosphere may mediate interactions between soil chemistry and herbivores (24).Benzoxazinoids are shikimic acid–derived secondary metabolites that are produced in high abundance by grasses such as wheat and maize. They evolved multiple times within the plant kingdom and are also found in various dicot families (25). Initially, benzoxazinoids were described as defense compounds that suppress and repel insect herbivores (26). Later genetic work revealed that benzoxazinoids also act as within-plant signaling compounds by initiating callose deposition upon pathogen and aphid attack (27, 28). Benzoxazinoids are also released into the rhizosphere in substantial quantities (29), where they can chelate iron (30), thus making it bioavailable (31). By consequence, benzoxazinoids can influence plant iron homeostasis. Recently, a link was documented between the iron chelating capacity and the interaction between maize plants and the western corn rootworm. This highly adapted insect is attracted by iron benzoxazinoid complexes and can use them for its own iron supply (31). Thus, it is conceivable that the multiple functions of benzoxazinoids may lead to trade-offs between their function as defenses and their functions as providers of essential micronutrients.Here, we explore how the multifunctionality of benzoxazinoids shapes interactions between soil conditions and a leaf herbivore. By comparing soils that differ in their trace element composition, we uncover that the protective effect of maize benzoxazinoids against the fall armyworm can be reversed to a susceptibility effect in certain soils. Using micronutrient analyses and manipulative laboratory experiments, we document that this phenomenon can be explained by the interaction of benzoxazinoids with free iron in the soil. We further document that iron and benzoxazinoids interact to control leaf defenses but that the benzoxazinoid-dependent susceptibility is best explained by increased iron supply to the fall armyworm. Taken together, these results provide a mechanistic link between soil properties and leaf–herbivore interactions and illustrate how plant secondary metabolite multifunctionality shapes plant–herbivore interactions.     

Structure Elucidation and Toxicity Analysis of the Byproducts Formed after Biodegradation of Aflatoxins B1 and B2 Using Extracts of Mentha arvensis

The aqueous extracts of leaves and shoots of Mentha arvensis were checked for their potential to biodegrade aflatoxin B1 and B2 (AFB1; 100 µg/L and AFB2; 50 µg/L) through in vitro assays. Overall, the results showed that leaf extract degrades aflatoxins more efficiently than the shoot extract. First, the pH, temperature and incubation time were optimized for maximum degradation by observing this activity at different temperatures between 25 and 60 °C, pH between 2 and 10 and incubation time from 3 to 72 h. In general, an increase in all these parameters significantly increased the percentage of biodegradation. In vitro trials on mature maize stock were performed under optimized conditions, i.e., pH 8, temperature 30 °C and an incubation period of 72 h. The leaf extract resulted in 75% and 80% biodegradation of AFB1 and AFB2, respectively. Whereas the shoot extract degraded both toxins up to 40–48%. The structural elucidation of degraded toxin products by LCMS/MS analysis showed seven degraded products of AFB1 and three of AFB2. MS/MS spectra showed that most of the products were formed by the loss of the methoxy group from the side chain of the benzene ring, the removal of the double bond in the terminal furan ring and the modification of the lactone group, indicating less toxicity compared to the parent compounds. The degraded products showed low toxicity against brine shrimps, confirming that M. arvensis leaf extract has significant potential to biodegrade aflatoxins.     

Maize food allergy: a double‐blind placebo‐controlled study

Background Maize allergy is not very common especially in Europe. The number of studies that address IgE mediated maize allergy is all too few. Objective Evaluate subjects with a history of maize allergy by double‐blind, placebo‐controlled food challenge; identify the spectrum of symptoms manifested during challenge; determine the lowest provocation dose (PD) during challenge; determine the performance characteristics of maize skin prick test and specific IgE. Methods Twenty‐seven patients with a history of maize allergy were enrolled to be evaluated by skin test, specific IgE and double‐blind placebo‐controlled maize challenge. Results Forty‐eight percent of the patients were challenge positive. PD range was 0.1–25 g. Fifty‐four percent of the maize allergic subjects had a PD that was 2.5 g; two subjects reacted to 100 mg of maize. Comparison of maize specific IgE levels and skin test results to the challenge results revealed the following (specific IgE level/skin testing): sensitivity 1.00/0.846, specificity 0.077/0.384, positive predictive value 0.520/0.579, and negative predictive value 1.00/0.714. Conclusion Maize is a cause of IgE‐mediated allergic reactions to foods in adults and children. Nearly half of the subjects recruited were confirmed by challenge to be allergic to maize. Twenty‐three percent of the positive challenge patients manifested symptoms that involved two organ systems, thus fulfilling the criteria for maize induced anaphylaxis. Maize is allergenic and can pose a risk for symptomatic food allergy at a dose of 100 mg.     

玉米须抗菌活性的初步研究

目的：探讨玉米须的体外抗菌活性。方法：采用琼脂扩散法和试管连续稀释法测定玉米须对常见致病菌和条件致病菌的体外抗菌活性及最低抑菌浓度（MIC）。结果：（1）玉米须对金黄色葡萄球菌和乙型溶血性链球菌皆有明显的抗菌活性，抑菌圈直径分别为21mm和16mm。对金黄色葡萄球菌的最低抑菌浓度为0．5g／ml。（2）玉米须对大肠埃希菌、福氏志贺氏菌、伤寒沙门氏菌、铜绿假单胞菌无抗菌活性。结论：玉米须对金黄色葡萄球菌和乙型溶血性链球菌有较好的体外抑菌活性。     

玉米紫色植株花色苷色素抗脂质过氧化作用

目的 探讨玉米紫色植株色素抗脂质过氧化的作用.方法 体外实验测定玉米紫色植株花色苷色素在Fe2 引发的卵磷脂脂质体体系中抗氧化活性.体内实验,取50只小鼠随机分成5组,分别给予溶媒和不同剂量的色素,然后采用溴代苯致实验性肝损伤,测定肝匀浆的丙二醛含量.结果 在由Fe2 引发的卵磷脂脂质体体系中玉米紫色植株色素对脂质过氧化有明显的抑制作用,抑制率随样品的浓度增高而增大,并且明显优于抗坏血酸.在溴代苯致小鼠实验性肝损伤模型实验中,中、高剂量组的丙二醛含量均低于损伤模型组.低剂量组和损伤模型组比较丙二醛含量差异无统计学意义.结论 玉米紫色植株色素具有较强的抗脂质过氧化作用.     

玉米SOD的分离纯化与部分性质研究

目的建立1种高效而简单的玉米SOD分离纯化方法。方法采用初步提取、40%~80%硫酸铵分级沉淀、Cu2+螯合亲和色谱和DEAE-纤维素离子交换色谱处理的方法从玉米中提取SOD。结果获得了比活为1.18×104U/mg的SOD-A组分,活力回收率为11.7%;纯化后的SOD-A组分经PAGE和SDS-PAGE均为一条带;等电聚焦结果表明该组分的等电点为7.2;经鉴定,该组分属于Cu/Zn-SOD。稳定性研究表明,该组分对热、酸碱及常见变性剂(如尿素)的稳定性较好。结论利用Cu2+螯合亲和色谱和DEAE-纤维素离子交换色谱相结合的方法可以纯化得到高比活、稳定性好的玉米SOD-A组分。     

Opaque-2 maize in the prevention and treatment of pellagra

In order to determine the usefulness of opaque-2 maize in the prevention of pellagra, people from the third and fifth brigades of a commune in a maize-eating district of Northwest China received opaque-2 maize in their diet and served as the experimental subjects. People from the neighboring sixth brigade ate conventional maize and served as the control subjects. After three months there were no cases of pellagra in the experimental groups, but there were three cases in the control group; the incidence was 1.8%.Prior to the study, urinary excretion values of N-methylniacinamide (N-MN) of the subjects were measured. There were no significant differences among the three brigades (P>0.05). By the conclusion of the study the differences in urinary excretion of N-MN between the experimental and control groups were highly significant: 0.21±0.02 and 0.21±0.02 mg/2 h for the experimental groups and 0.12±0.01 mg/2 h for the control group.Opaque-2 maize also was used in a study of the treatment of pellagra. Persons with pellagra were divided into three groups: Group 1 received opaque-2 maize, Group 2 received conventional maize plus niacinamide (10 mg per day), and Group 3 received conventional maize. Before treatment, the average values of the urinary excretion of N-MN of each group were not significantly different. After one month of the treatment, a loading dose of 50 mg niacinamide was administered orally. Four-hour urines were collected and analyzed for N-MN. The differences between Group 1 and Group 3 or Group 2 and Group 3 were significant. Although the results showed that opaque-2 maize could be used in the prevention and treatment of pellagra, meat, milk, and milk products should be recommended to people in all of the areas where pellagra is endemic. The planting of opaque-2 maize, however, should provide an effective means to supply utilizable niacin and is a simple, economical way to prevent pellagra for the present.     

Identification and Quantification of a Toxigenic and Non-Toxigenic Aspergillus flavus Strain in Contaminated Maize Using Quantitative Real-Time PCR

Aflatoxins, which are produced by Aspergillus flavus, are toxic to humans, livestock, and pets. The value of maize (Zea mays) grain is markedly reduced when contaminated with aflatoxin. Plant resistance and biological control using non-toxin producing strains are considered effective strategies for reducing aflatoxin accumulation in maize grain. Distinguishing between the toxin and non-toxin producing strains is important in determining the effectiveness of bio-control strategies and understanding inter-strain interactions. Using polymorphisms found in the fungal rRNA intergenic spacer region (IGS) between a toxigenic strain of A. flavus (NRRL 3357) and the non-toxigenic strain used in the biological control agent Afla-Guard^® (NRRL 21882), we developed a set of primers that allows for the identification and quantification of the two strains using quantitative PCR. This primer set has been used to screen maize grain that was inoculated with the two strains individually and co-inoculated with both strains, and it has been shown to be effective in both the identification and quantification of both strains. Screening of co-inoculated ears from multiple resistant and susceptible genotypic crosses revealed no significant differences in fungal biomass accumulation of either strain in the field tests from 2010 and 2011 when compared across the means of all genotypes. Only one genotype/year combination showed significant differences in strain accumulation. Aflatoxin accumulation analysis showed that, as expected, genotypes inoculated with the toxigenic strain accumulated more aflatoxin than when co-inoculated with both strains or inoculated with only the non-toxigenic strain. Furthermore, accumulation of toxigenic fungal mass was significantly correlated with aflatoxin accumulation while non-toxigenic fungal accumulation was not. This primer set will allow researchers to better determine how the two fungal strains compete on the maize ear and investigate the interaction between different maize lines and these A. flavus strains.     

Vegetable relishes,high in β-carotene,increase the iron,zinc and β-carotene nutritive values from cereal porridges

Iron, zinc and vitamin A deficiencies are serious public health problems in sub-Saharan Africa, which can be alleviated by dietary diversification. The effects of adding cowpea leaf (CL) and orange-fleshed sweet potato (OFSP) relishes to sorghum and maize porridges on iron, zinc and β-carotene contents and bioaccessibilities were determined. Despite the high iron content of the CL relish (14.59?mg/100?g), the vegetable relishes had little effect on the iron bioaccessibility from the cereal porridges. Importantly, the addition of the CL relish increased the percentage and amount of bioaccessible zinc 2- and 3-fold, respectively. Addition of CL and OFSP relishes resulted in β-carotene contents of 10–13?mg/100?g. The β-carotene from the OFSP relish meals was double as bioaccessible than that from the CL relish meals. Addition of the vegetable relishes has real potential to improve especially the vitamin A and zinc nutritive value of cereal diets.