Loline alkaloid

A loline alkaloid is a member of the 1-aminopyrrolizidines (often referred to as lolines), which are bioactive natural products with several distinct biological and chemical features. The lolines are insecticidal and insect-deterrent compounds that are produced in grasses infected by endophytic fungal symbionts of the genus Epichloë (anamorph species: Neotyphodium). Lolines increase resistance of endophyte-infected grasses to insect herbivores, and may also protect the infected plants from environmental stresses such as drought and spatial competition. They are alkaloids, organic compounds containing basic nitrogen atoms. The basic chemical structure of the lolines comprises a saturated pyrrolizidine ring, a primary amine at the C-1 carbon, and an internal ether bridge—a hallmark feature of the lolines, which is uncommon in organic compounds—joining two distant ring (C-2 and C-7) carbons (see Fig. 1). Different substituents at the C-1 amine, such as methyl, formyl, and acetyl groups, yield loline species that have variable bioactivity against insects. Besides endophyte–grass symbionts, loline alkaloids have also been identified in some other plant species; namely, Adenocarpus species (family Fabaceae) and Argyreia mollis (family Convolvulaceae).

Discovery A member of the loline alkaloids was first isolated from the grass Lolium temulentum and its elemental composition determined in 1892. It was initially named temuline and later renamed norloline. Studies in the 1950s and 1960s by Russian researchers established the name loline and identified the characteristic 2,7 ether bridge in its molecular structure. Since then the analytical methods for purification and analysis of the lolines have been refined and several different loline species have been identified in many Lolium and related grasses infected by the Epichloë/Neotyphodium (epichloae) endophytes. Lolines are absent in grass plants that do not harbor the epichloae endophytes, and not all epichloae produce the lolines. Because of the very intimate association of plant and endophyte and difficulties to reproduce the symbiotic conditions in vitro, it was long unknown if the fungus was the producer of the lolines, or if they were synthesized by the plant in response to endophyte infection. In 2001, it was demonstrated that the endophyte Neotyphodium uncinatum produces lolines in some chemically defined growth media, which suggests that the endophyte is also the producer of the lolines in the grass plant. The lolines have also been reported from some plants in several plant families, suggesting a more widespread occurrence of these compounds in nature.

Mechanism of action Lolines are insecticidal and deterrent to a broad range of insects, including species in the Hemiptera, Coleoptera, Hymenoptera, Lepidoptera, and Blattodea, such as the bird cherry-oat aphid (genus Rhopalosiphum), large milkweed bug (Oncopeltus fasciatus), and American cockroach (Periplaneta americana). LC50 values of N-formylloline or N-acetylloline from grass seed extracts are 1-20 μg/ml for aphids and milkweed bugs and impair insect development and fecundity and cause avoidance of loline-containing grass tissues. However, results of feeding tests with grass extracts are occasionally difficult to interpret due to the presence of other endophyte alkaloids in these extracts, and the exact mechanisms of the insecticidal actions of the lolines are unknown. The lolines may be neurotoxic to the insects, and differences in the chemical groups at the C-1-amine result in different levels of insect toxicity; for example, N-formylloline (see Fig. 2), which occurs in higher concentrations in endophyte-infected grass plants, has greater insect toxicity than some other lolines, which occur at lower concentrations in the grass plant.

Loline bioactivities show some unexpected variability with variation in their concentration in grass tissues. For example, the tall fescue endophyte, N. coenophialum, has been associated with enhanced resistance to the migratory root-endoparasitic nematode, Pratylenchus scribneri. At low concentrations, N-formylloline serves as a chemoattractant for P. scribneri, but acts as a repellant at higher concentrations. However, ergot alkaloids also have repellent and immobilizing effects on P. scribneri, and an endophyte of perennial ryegrass lacking lolines, and genetically engineered to produce no ergot alkaloids, exhibits resistance to this nematode. Therefore, the relative importance of the loline and ergot alkaloids to nematode resistance remains unclear. Many epichloae endophytes—including N. coenophialum symbiotic with Lolium arundinaceum (syn. Festuca arundinacea, tall fescue)—also produce ergot alkaloids that are toxic to mammalian herbivores. The ergot alkaloids occur at relatively low concentrations in the plant and are often difficult to detect analytically. By contrast, the lolines frequently accumulate to very high levels in grass tissues, and were, therefore, initially associated also with toxicity to mammalian herbivores. Specifically, the lolines were thought to be responsible for toxic symptoms called fescue toxicosis displayed by livestock grazing on grasses infected by N. coenophialum. However, subsequently it was demonstrated that only the endophyte-produced ergot alkaloids are responsible for the symptoms of fescue toxicosis (or summer syndrome), and not the lolines which, even at high doses, have only very small physiological effects on mammalians feeders. Another group of alkaloids, the senecio-type alkaloids, are produced by various plants and like the lolines, the senecio alkaloids possess a pyrrolizidine ring structure. Unlike the lolines, however, the senecio alkaloids exhibit strong hepatotoxicity, owing to a double bond between C-1 and C-2 in their ring structure. This double bond is absent in the lolines, explaining the lack of hepatotoxicity of this group of compounds. The lolines have been suggested to inhibit seed germination or growth of other plants (allelopathy), and to increase resistance of infected grasses against drought, but such effects have not been substantiated under more natural conditions of cultivation or in habitats.

Production and distribution in the grass plant

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