GABA, β-Alanine and Glycine in the Digestive Juice of Privet-Specialist Insects: Convergent Adaptive Traits Against Plant Iridoids

Gamma-aminobutyric acid (GABA) in flower nectar and its possible physiological and behavioral effects on insect pollinators

Floral nectar, a vital nutrition source for pollinators, contains diverse chemical compounds, including γ-Aminobutyric acid (GABA), a prevalent non-proteinogenic amino acid. While GABA's physiological role is known and well-studied, its ecological significance in plant-pollinator interactions remains unclear. Recent studies on GABA's effects on pollinators' preference, consumption, survival, physiology, and behavior show varying outcomes according to the species, indicating a complex relationship. GABA consumption impacts motor function and cognitive abilities, potentially influencing pollination efficiency. Future research addressing diverse concentrations, species, and behavioral aspects is crucial for comprehensively understanding GABA's ecological role in plant-pollinator interactions.

Cytochrome P450 Genes Expressed in Phasmatodea Midguts

Cytochrome P450s (CYPs) are xenobiotic detoxification genes found in most eukaryotes, and linked in insects to the tolerance of plant secondary chemicals and insecticide resistance. The number and diversity of CYP clans, families, and subfamilies that an organism produces could correlate with its dietary breadth or specialization. This study examined the CYP diversity expressed in the midguts of six species of folivorous stick insects (Phasmatodea), to identify their CYP complement and see if any CYPs correlate with diet toxicity or specialization, and see what factors influenced their evolution in this insect order. CYP genes were mined from six published Phasmatodea transcriptomes and analyzed phylogenetically. The Phasmatodea CYP complement resembles that of other insects, though with relatively low numbers, and with significant expansions in the CYP clades 6J1, 6A13/14, 4C1, and 15A1. The CYP6 group is known to be the dominant CYP family in insects, but most insects have no more than one CYP15 gene, so the function of the multiple CYP15A1 genes in Phasmatodea is unknown, with neofunctionalization following gene duplication hypothesized. No correlation was found between CYPs and diet specialization or toxicity, with some CYP clades expanding within the Phasmatodea and others likely inherited from a common ancestor.

Do amino and fatty acid profiles of pollen provisions correlate with bacterial microbiomes in the mason bee Osmia bicornis?

Bee performance and well-being strongly depend on access to sufficient and appropriate resources, in particular pollen and nectar of flowers, which constitute the major basis of bee nutrition. Pollen-derived microbes appear to play an important but still little explored role in the plant pollen-bee interaction dynamics, e.g. through affecting quantities and ratios of important nutrients. To better understand how microbes in pollen collected by bees may affect larval health through nutrition, we investigated correlations between the floral, bacterial and nutritional composition of larval provisions and the gut bacterial communities of the solitary megachilid bee Osmia bicornis. Our study reveals correlations between the nutritional quality of pollen provisions and the complete bacterial community as well as individual members of both pollen provisions and bee guts. In particular pollen fatty acid profiles appear to interact with specific members of the pollen bacterial community, indicating that pollen-derived bacteria may play an important role in fatty acid provisioning. As increasing evidence suggests a strong effect of dietary fatty acids on bee performance, future work should address how the observed interactions between specific fatty acids and the bacterial community in larval provisions relate to health in O. bicornis. This article is part of the theme issue 'Natural processes influencing pollinator health: from chemistry to landscapes'.

Responses of tea plants (Camellia sinensis) with different low-nitrogen tolerances during recovery from nitrogen deficiency

BACKGROUND

Tea plants have high nitrogen (N) consumptions, whereas molecular and physiological responses of tea plants to N recovery are still unclear.

RESULTS

By using non-invasive micro-test technology (NMT), ¹⁵ N tracer technique, ultra-performance liquid chromatography (UPLC), and transcriptome sequencing technology, we investigated the N recovery-induced changes in N absorptions, N tissue distributions, contents of free amino acids (FAAs), and global transcription of the low-N tolerant and intolerant tea genotypes [i.e. Wuniuzao (W) and Longjing43 (L)]. The results showed that the phenotype of Wuniuzao was better than that of Longjing43 under low-N condition. The N absorption and utilization of Wuniuzao were superior to Longjing43 under N recovery. The γ-aminobutyric acid (GABA) ratio (N recovery/N deficiency) in the root of Wuniuzao was significantly higher than that of Longjing43, while the glutamic acid ratio in the root of Wuniuzao was significantly lower than that of Longjing43. This findings suggested that Wuniuzao tended to enhance the GABA synthesis, while Longjing43 tended to inhibit the GABA synthesis under N recovery. The key genes in response to N recovery in Wuniuzao included N transport (AMT and NRT), N transformation (NR, NirA, and GAD), and amino acid transport (GAT) genes. In addition, some ribosome and flavonoid biosynthesis genes might help to maintain proteome homeostasis.

CONCLUSION

The N absorption and transport, and the conversion abilities of key amino acids (Glu and GABA) might improve the adaptability of tea plants to N recovery, which provided a basis for the breeding of N efficient tea varieties. © 2021 Society of Chemical Industry.

Deciphering the route to cyclic monoterpenes in Chrysomelina leaf beetles: source of new biocatalysts for industrial application?

The drastic growth of the population on our planet requires the efficient and sustainable use of our natural resources. Enzymes are indispensable tools for a wide range of industries producing food, pharmaceuticals, pesticides, or biofuels. Because insects constitute one of the most species-rich classes of organisms colonizing almost every ecological niche on earth, they have developed extraordinary metabolic abilities to survive in various and sometimes extreme habitats. Despite this metabolic diversity, insect enzymes have only recently generated interest in industrial applications because only a few metabolic pathways have been sufficiently characterized. Here, we address the biosynthetic route to iridoids (cyclic monoterpenes), a group of secondary metabolites used by some members of the leaf beetle subtribe Chrysomelina as defensive compounds against their enemies. The ability to produce iridoids de novo has also convergently evolved in plants. From plant sources, numerous pharmacologically relevant structures have already been described. In addition, in plants, iridoids serve as building blocks for monoterpenoid indole alkaloids with broad therapeutic applications. As the commercial synthesis of iridoid-based drugs often relies on a semisynthetic approach involving biocatalysts, the discovery of enzymes from the insect iridoid route can account for a valuable resource and economic alternative to the previously used enzymes from the metabolism of plants. Hence, this review illustrates the recent discoveries made on the steps of the iridoid pathway in Chrysomelina leaf beetles. The findings are also placed in the context of the studied counterparts in plants and are further discussed regarding their use in technological approaches.

Evolutionary dynamics of interactions between plants and their enemies: comparison of herbivorous insects and pathogens

Plants colonized land over 400 million years ago. Shortly thereafter, organisms began to consume terrestrial plant tissue as a nutritional resource. Most plant enemies are plant pathogens or herbivores, and they impose natural selection for plants to evolve defenses. These traits generate selection pressures on enemies. Coevolution between terrestrial plants and their enemies is an important element of the evolutionary history of both groups. However, coevolutionary studies of plant-pathogen interactions have tended to focus on different research topics than plant-herbivore interactions. Specifically, studies of plant-pathogen interactions often adopt a "gene-for-gene" conceptual framework. In contrast, studies of plants and herbivores often investigate escalation or elaboration of plant defense and herbivore adaptations to overcome it. The main exceptions to the general pattern are studies that focus on small, sessile herbivores that share many features with plant pathogens, studies that incorporate both herbivores and pathogens into a single investigation, and studies that test aspects of Thompson's geographic mosaic theory for coevolution. We discuss the implications of these findings for future research.

Transcriptomic responses of the olive fruit fly Bactrocera oleae and its symbiont Candidatus Erwinia dacicola to olive feeding

The olive fruit fly, Bactrocera oleae, is the most destructive pest of olive orchards worldwide. The monophagous larva has the unique capability of feeding on olive mesocarp, coping with high levels of phenolic compounds and utilizing non-hydrolyzed proteins present, particularly in the unripe, green olives. On the molecular level, the interaction between B. oleae and olives has not been investigated as yet. Nevertheless, it has been associated with the gut obligate symbiotic bacterium Candidatus Erwinia dacicola. Here, we used a B.oleae microarray to analyze the gene expression of larvae during their development in artificial diet, unripe (green) and ripe (black) olives. The expression profiles of Ca. E. dacicola were analyzed in parallel, using the Illumina platform. Several genes were found overexpressed in the olive fly larvae when feeding in green olives. Among these, a number of genes encoding detoxification and digestive enzymes, indicating a potential association with the ability of B. oleae to cope with green olives. In addition, a number of biological processes seem to be activated in Ca. E. dacicola during the development of larvae in olives, with the most notable being the activation of amino-acid metabolism.

Characterization of a Chitosanase from Jelly Fig (Ficus awkeotsang Makino) Latex and Its Application in the Production of Water-Soluble Low Molecular Weight Chitosans

A chitosanase was purified from jelly fig latex by ammonium sulfate fractionation (50–80% saturation) and three successive column chromatography steps. The purified enzyme was almost homogeneous, as determined by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) and gel activity staining. The molecular mass of the enzyme was 20.5 kDa. The isoelectric point (pI) was <3.5, as estimated by isoelectric focusing electrophoresis on PhastGel IEF 3-9. Using chitosan as the substrate, the optimal pH for the enzyme reaction was 4.5; the kinetic parameters K_m and V_max were 0.089 mg mL^-1 and 0.69 μmol min^-1 mg^-1, respectively. The enzyme showed activity toward chitosan polymers which exhibited various degrees of deacetylation (21–94%). The enzyme hydrolyzed 70–84% deacetylated chitosan polymers most effectively. Substrate specificity analysis indicated that the enzyme catalyzed the hydrolysis of chitin and chitosan polymers and their derivatives. The products of the hydrolysis of chitosan polymer derivatives, ethylene glycol (EG) chitosan, carboxymethyl (CM) chitosan and aminoethyl (AE) chitosan, were low molecular weight chitosans (LMWCs); these products were referred to as EG-LMWC, CM-LMWC and AE-LMWC, respectively. The average molecular weights of EG-LMWC, CM-LMWC and AE-LMWC were 11.2, 11.2 and 8.89 kDa, respectively. All of the LMWC products exhibited free radical scavenging activities toward ABTS^•+, superoxide and peroxyl radicals.

Impact of the dual defence system of Plantago lanceolata (Plantaginaceae) on performance, nutrient utilisation and feeding choice behaviour of Amata mogadorensis larvae (Lepidoptera, Erebidae)

Iridoid glycosides are plant defence compounds with potentially detrimental effects on non-adapted herbivores. Some plant species possess β-glucosidases that hydrolyse iridoid glycosides and thereby release protein-denaturing aglycones. To test the hypothesis that iridoid glycosides and plant β-glucosidases form a dual defence system, we used Plantago lanceolata and a polyphagous caterpillar species. To analyse the impact of leaf-age dependent differences in iridoid glycoside concentrations and β-glucosidase activities on insect performance, old or young leaves were freeze-dried and incorporated into artificial diets or were provided freshly to the larvae. We determined larval consumption rates and the amounts of assimilated nitrogen. Furthermore, we quantified β-glucosidase activities in artificial diets and fresh leaves and the amount of iridoid glycosides that larvae feeding on fresh leaves ingested and excreted. Compared to fresh leaves, caterpillars grew faster on artificial diets, on which larval weight gain correlated positively to the absorbed amount of nitrogen. When feeding fresh young leaves, larvae even lost weight and excreted only minute proportions of the ingested iridoid glycosides intact with the faeces, indicating that the hydrolysis of these compounds might have interfered with nitrogen assimilation and impaired larval growth. To disentangle physiological effects from deterrent effects of iridoid glycosides, we performed dual choice feeding assays. Young leaves, their methanolic extracts and pure catalpol reduced larval feeding in comparison to the respective controls, while aucubin had no effect on larval consumption. We conclude that the dual defence system of P. lanceolata consisting of iridoid glycosides and β-glucosidases interferes with the nutrient utilisation via the hydrolysis of iridoid glycosides and also mediates larval feeding behaviour in a concentration- and substance-specific manner.

Symbiotic bacteria enable olive fly larvae to overcome host defences

Ripe fruit offer readily available nutrients for many animals, including fruit fly larvae (Diptera: Tephritidae) and their associated rot-inducing bacteria. Yet, during most of their ontogeny, fruit remain chemically defended and effectively suppress herbivores and pathogens by high levels of secondary metabolites. Olive flies (Bactrocera oleae) are uniquely able to develop in unripe olives. Unlike other frugivorous tephritids, the larvae maintain bacteria confined within their midgut caeca. We examined the interaction between larvae, their associated bacteria, and fruit chemical defence, hypothesizing that bacterial contribution to larval development is contingent on the phenology of fruit defensive chemistry. We demonstrate that larvae require their natural complement of bacteria (Candidatus Erwinia dacicola: Enterobacteriaceae) in order to develop in unripe olives. Conversely, when feeding on ripe fruit, larval development proceeds independently of these bacteria. Our experiments suggest that bacteria counteract the inhibitory effect of oleuropein-the principal phenolic glycoside in unripe olives. In light of these results, we suggest that the unique symbiosis in olive flies, compared with other frugivorous tephritids, is understood by considering the relationship between the fly, bacteria and fruit chemistry. When applied in an evolutionary context, this approach may also point out the forces which shaped symbioses across the Tephritidae.

How insects overcome two-component plant chemical defence: plant β-glucosidases as the main target for herbivore adaptation

Insect herbivory is often restricted by glucosylated plant chemical defence compounds that are activated by plant β-glucosidases to release toxic aglucones upon plant tissue damage. Such two-component plant defences are widespread in the plant kingdom and examples of these classes of compounds are alkaloid, benzoxazinoid, cyanogenic and iridoid glucosides as well as glucosinolates and salicinoids. Conversely, many insects have evolved a diversity of counteradaptations to overcome this type of constitutive chemical defence. Here we discuss that such counter-adaptations occur at different time points, before and during feeding as well as during digestion, and at several levels such as the insects’ feeding behaviour, physiology and metabolism. Insect adaptations frequently circumvent or counteract the activity of the plant β-glucosidases, bioactivating enzymes that are a key element in the plant’s two-component chemical defence. These adaptations include host plant choice, non-disruptive feeding guilds and various physiological adaptations as well as metabolic enzymatic strategies of the insect’s digestive system. Furthermore, insect adaptations often act in combination, may exist in both generalists and specialists, and can act on different classes of defence compounds. We discuss how generalist and specialist insects appear to differ in their ability to use these different types of adaptations: in generalists, adaptations are often inducible, whereas in specialists they are often constitutive. Future studies are suggested to investigate in detail how insect adaptations act in combination to overcome plant chemical defences and to allow ecologically relevant conclusions.

Plant latex and other exudates as plant defense systems: roles of various defense chemicals and proteins contained therein

Plant latex and other exudates are saps that are exuded from the points of plant damage caused either mechanically or by insect herbivory. Although many (ca. 10%) of plant species exude latex or exudates, and although the defensive roles of plant latex against herbivorous insects have long been suggested by several studies, the detailed roles and functions of various latex ingredients, proteins and chemicals, in anti-herbivore plant defenses have not been well documented despite the wide occurrence of latex in the plant kingdom. Recently, however, substantial progress has been made. Several latex proteins, including cysteine proteases and chitin-related proteins, have been shown to play important defensive roles against insect herbivory. In the mulberry (Morus spp.)-silkworm (Bombyx mori) interaction, an old and well-known model system of plant-insect interaction, plant latex and its ingredients--sugar-mimic alkaloids and defense protein MLX56--are found to play key roles. Complicated molecular interactions between Apocynaceae species and its specialist herbivores, in which cardenolides and defense proteins in latex play key roles, are becoming more and more evident. Emerging observations suggested that plant latex, analogous to animal venom, is a treasury of useful defense proteins and chemicals that has evolved through interspecific interactions. On the other hand, specialist herbivores developed sophisticated adaptations, either molecular, physiological, or behavioral, against latex-borne defenses. The existence of various adaptations in specialist herbivores itself is evidence that latex and its ingredients function as defenses at least against generalists. Here, we review molecular and structural mechanisms, ecological roles, and evolutionary aspects of plant latex as a general defense against insect herbivory and we discuss, from recent studies, the unique characteristics of latex-borne defense systems as transport systems of defense substances are discussed based on recent studies.