Limited effect of reactive oxygen species on the composition of susceptible essential amino acids in the midguts of Lymantria dispar caterpillars

Herbivory meets fungivory: insect herbivores feed on plant pathogenic fungi for their own benefit

Plants are regularly colonised by fungi and bacteria, but plant-inhabiting microbes are rarely considered in studies on plant-herbivore interactions. Here we show that young gypsy moth (Lymantria dispar) caterpillars prefer to feed on black poplar (Populus nigra) foliage infected by the rust fungus Melampsora larici-populina instead of uninfected control foliage, and selectively consume fungal spores. This consumption, also observed in a related lepidopteran species, is stimulated by the sugar alcohol mannitol, found in much higher concentration in fungal tissue and infected leaves than uninfected plant foliage. Gypsy moth larvae developed more rapidly on rust-infected leaves, which cannot be attributed to mannitol but rather to greater levels of total nitrogen, essential amino acids and B vitamins in fungal tissue and fungus-infected leaves. Herbivore consumption of fungi and other microbes may be much more widespread than commonly believed with important consequences for the ecology and evolution of plant-herbivore interactions.

Acquiring nutrients from tree leaves: effects of leaf maturity and development type on a generalist caterpillar

The rapid growth and prolific reproduction of many insect herbivores depend on the efficiencies and rates with which they acquire nutrients from their host plants. However, little is known about how nutrient assimilation efficiencies are affected by leaf maturation or how they vary between plant species. Recent work showed that leaf maturation can greatly decrease the protein assimilation efficiency (PAE) of Lymantria dispar caterpillars on some tree species, but not on species in the willow family (Salicaceae). One trait of many species in the Salicaceae that potentially affects PAE is the continuous (or "indeterminate") development of leaves throughout the growing season. To improve our understanding of the temporal and developmental patterns of nutrient availability for tree-feeding insects, this study tested two hypotheses: nutrients (protein and carbohydrate) are more efficiently assimilated from immature than mature leaves, and, following leaf maturation, nutrients are more efficiently assimilated from indeterminate than determinate tree species. The nutritional physiology and growth of a generalist caterpillar (L. dispar) were measured on five determinate and five indeterminate tree species while their leaves were immature and again after they were mature. In support of the first hypothesis, caterpillars that fed on immature leaves had significantly higher PAE and carbohydrate assimilation efficiency (CAE), as well as higher protein assimilation rates and growth rates, than larvae that fed on mature leaves. Contrary to the second hypothesis, caterpillars that fed on mature indeterminate tree leaves did not have higher PAE than those that fed on mature determinate leaves, while CAE differed by only 3% between tree development types. Instead, "high-PAE" and "low-PAE" tree species were found across taxonomic and development categories. The results of this study emphasize the importance of physiological mechanisms, such as nutrient assimilation efficiency, to explain the large variation in host plant quality for insect herbivores.

Role of early signalling events in plant-insect interactions

The response of plants to the stress caused by herbivores involves several different defence mechanisms. These responses begin at the plant cell plasma membrane, where insect herbivores interact physically by causing mechanical damage and chemically by introducing elicitors or by triggering plant-derived signalling molecules. The earliest plant responses to herbivore contact are represented by ion flux unbalances generated in the plant cell plasma membrane at the damaged site. Differences in the charge distribution generate plasma transmembrane potential (V m) variation, the first event, which eventually leads to the initiation of signal transduction pathways and gene expression. Calcium signalling and the generation of reactive oxygen and nitrogen species are early events closely related to V m variations. This review provides an update on recent developments and advances in plant early signalling in response to herbivory, with particular emphasis on the electrophysiological variations of the plasma membrane potential, calcium signalling, cation channel activity, production of reactive oxygen and nitrogen species, and formation of a systemically moving signal from wounded tissues. The roles of calcium-dependent protein kinases and calcineurin signalling are also discussed.

Stability and oxidation products of hydrolysable tannins in basic conditions detected by HPLC/DAD-ESI/QTOF/MS

INTRODUCTION

Hydrolysable tannins occur in plants that are used for food or medicine by humans or herbivores. Basic conditions can alter the structures of tannins, that is, the oxidation of phenolic groups can lead to the formation of toxic quinones. Previously, these labile quinones and other oxidation products have been studied with colorimetric or electron paramagnetic resonance methods, which give limited information about products.

OBJECTIVE

To study the stability and oxidation products of hydrolysable tannins in basic conditions using HPLC with a diode-array detector (DAD) combined with electrospray ionisation (ESI) and quadrupole time-of-flight (QTOF) MS.

METHODS

Three galloyl glucoses, four galloyl derivatives with different polyols and three ellagitannins were purified from plants. The incubation reactions of tannins were monitored by HPLC/DAD at five pH values and in reduced oxygen conditions. Reaction products were identified based on UV spectra and mass spectral fragmentation obtained with the high-resolution HPLC/DAD-ESI/QTOF/MS. The use of a base-resistant HPLC column enabled injections without the sample pre-treatment and thus detection of short-lived products.

RESULTS

Hydrolysable tannins were unstable in basic conditions and half-lives were mostly less than 10 min at pH 10. Degradation rates were faster at pH 11 but slower at milder pH. The HPLC analyses revealed that various products were formed and identified to be the result of hydrolysis, deprotonation and oxidation. Interestingly, the main hydrolysis product was ellagic acid; it was also formed from galloyl glucoses that do not contain oxidatively coupled galloyl groups in their initial structures.

CONCLUSION

HPLD/DAD-ESI/QTOF/MS was an efficient method for the identification of polyphenol oxidation products and showed how different pH conditions determine the fate of hydrolysable tannins.

Importance of protein quality versus quantity in alternative host plants for a leaf-feeding insect

The nutritional value of alternative host plants for leaf-feeding insects such as caterpillars is commonly measured in terms of protein quantity. However, nutritional value might also depend on the quality of the foliar protein [i.e., the composition of essential amino acids (EAAs)]. A lack of comparative work on the EAA compositions of herbivores and their host plants has hampered the testing of this hypothesis. We tested the "protein quality hypothesis" using the tree-feeding caterpillars of Lymantria dispar (gypsy moth) and two taxonomically unrelated host plants, red oak (Quercus rubra) and sugar maple (Acer saccharum). Because L. dispar has higher fitness on oak than on maple, support for the hypothesis would be found if protein were of higher quality from oak than from maple. The whole-body EAA composition of L. dispar larvae was measured to estimate its optimum dietary protein composition, which was compared with the EAA compositions of oak and maple leaves. Contrary to the protein quality hypothesis, the EAA compositions of oak and maple were not significantly different in the spring. The growth-limiting EAAs in both tree species were histidine and methionine. Similar results were observed in the summer, with the exception that the histidine composition of oak was between 10 and 15 % greater than in maple leaves. The two main factors that affected the nutritional value of protein from the tree species were the quantities of EAAs, which were consistently higher in oak, and the efficiency of EAA utilization, which decreased from 80 % in May to <50 % in August. We conclude that the relative nutritional value of red oak and sugar maple for L. dispar is more strongly affected by protein quantity than quality. Surveys of many wild herbaceous species also suggest that leaf-feeding insects would be unlikely to specialize on plants based on protein quality.