The Green Gut: Chlorophyll Degradation in the Gut of Spodoptera littoralis

Proteomic Variation in the Oral Secretion of Spodoptera exigua and Spodoptera littoralis Larvae in Response to Different food Sources

The Spodoptera genus is defined as the pest-rich genus because it contains some of the most destructive lepidopteran crop pests, characterized by a wide host range. During feeding, the caterpillars release small amounts of oral secretion (OS) onto the wounded leaves. This secretion contains herbivore-induced molecular patterns (HAMPs) that activate the plant defense response, as well as effectors that may inhibit or diminish the plant's anti-herbivory response. In this study, we explored the protein components of the OS of two Spodoptera species, Spodoptera exigua and Spodoptera littoralis. We identified 336 and 276 proteins, respectively, with a major role in digestion. Using a label-free quantitative proteomics approach, we investigated changes in protein abundance in the OS of both species after switching from a laboratory artificial diet to detached pepper and tomato leaves. Several proteins, such as various lipases, polycalin and a β-1,3-glucan binding protein, were more abundant in the OS of leaf-fed larvae in both species. Conversely, a tryptophan-aspartic acid (WD)-repeat containing protein significantly decreased upon feeding on plant leaves in both species. Phenotypic plasticity dependent on each Spodoptera-plant combination was observed for several peptidases, potentially related to the need to overcome the effects of proteinase inhibitors differentially produced by the two plant species, and for several REPAT proteins, possibly related to the specific modulation of each Spodoptera-plant interaction. Altogether, our results provide useful information for understanding the interaction of these two polyphagous Spodoptera species with the host plants, and help to identify evolutionary traits that may influence the outcome of herbivory in each of these two related species.

The fate of chlorophyll in phytophagous insects goes beyond nutrition

Chlorophyll (Chl) is a natural compound that is found in all autotrophic plants. Since phytophagous insects ingest the photosynthetically active material with the plant leaves, the question arises if and how herbivores deal with Chl and its degradation products. Here we review findings on Chl degradation in phytophagous insects and highlight the role of these ubiquitous plant metabolites for plant-feeding insects. Due to the anaerobic gut of many insects, the degradation is limited to the removal of the peripheral substituents, while the tetrapyrrole core remains intact. Proteins, such as red fluorescent protein, P252 (a novel 252-kDa protein), and chlorophyllide binding protein have been reported to occur in the insect gut and might be indirectly connected to Chl degradation. Besides of an nutritional value, e.g., by taking up Mg²⁺ ions or by sequestration of carbon from the phytol side chain, the Chl degradation products may serve the insect, after binding to certain proteins, as antimicrobial, antifungal, and antiviral factors. The protein complexes may also confer protection against reactive oxygen species. The antibiotic potential of proteins and degradation products does not only benefit phytophagous insects but also human being in medical application of cancer treatment for instance. This review highlights these aspects from a molecular, biochemical, and ecological point of view.

Pyrrolic and Dipyrrolic Chlorophyll Degradation Products in Plants and Herbivores

The degradation of chlorophyll, the omnipresent green pigment, has been investigated intensively over the last 30 years resulting in many elucidated tetrapyrrolic degradation products. With a comparison to the degradation of the structurally similar heme, we hereby propose a novel additional chlorophyll degradation mechanism to mono- and dipyrrolic products. This is the first proof of the occurrence of a family of mono- and dipyrrols in leaves that are previously only known as heme degradation products. This product family is also found in spit and feces of herbivores with specific metabolomic patterns reflecting the origin of the samples. Based on chromatographic and mass spectrometric evidence as well as on mechanistic considerations we also suggest several tentative new degradation products. One of them, dihydro BOX A, was fully confirmed as a novel natural product by synthesis and comparison of its spectroscopic data.