Allelopathy

Chemistry and Mode of Action of Allelochemicals

Francisco A. Macias, Juan C.G. Galindo, Jose M. G. Molinillo, Francisco A. Macias, Juan C.G. Galindo, Jose M. G. Molinillo

• Publisher: CRC Press
• Year: 2003
• Print ISBN:
  9780849319648
• eBook ISBN:
  9781135493370
• Topic: Ciencias biológicas
• Subtopic: Ecología

Most, if not all, books on allelopathy cover the ecological, agronomic, and descriptive physiological aspects. And although the amount of papers published on the chemical aspects and mode of action of these compounds continues to rise, there has been, until now, no book available that reflects the latest literature. Written by experts, Allelopathy:

1 Ecophysiology and Potential Modes of Action for Selected Lichen Secondary Metabolites

J. G. Romagni, R. C. Rosell, N. P. D. Nanayakkara, and F. E. Dayan

CONTENT

Abstract

Introduction

Results and Discussion

Usnic Acid

Anthraquinones

Whitefly Bioassays

Methodology

References

ABSTRACT

Lichens, a symbiosis between a fungal and algal partner, produce secondary compounds that are unique to the symbiosis. Due to the high energy investment in these compounds, which can comprise up to 25% of the dry thallus weight, they must have an important role in lichen ecology. Our group is beginning to elucidate specific allelopathic roles and modes of action for these compounds. One lichen compound, (-)-usnic acid, was found to inhibit 4-hydroxyphenylpyruvate dioxygenase, a key enzyme in carotenoid biosynthesis. A series of lichen emodin analogues have been found to cause bleaching in grasses. Continued research suggested a decrease in photosystem II (PSII) activity, but the putative mode of action for these compounds remains to be determined. Another group of anthraquinone analogues has been found to inhibit germination and primary root formation. The preliminary data suggest that each lichen secondary compound has several ecological roles. Many inhibit pathways crucial for seedling development. This may decrease interspecific competition, especially in the canopy. Finally, we determined several compounds to be effective against phloem-feeding insects, particularly whiteflies (Bemisia tabaci). Both (-)-usnic acid and vulpinic acid caused highly significant mortality in whiteflies. Other functions of the same compounds, such as (-)-usnic, may include antiherbivory mechanisms.

INTRODUCTION

Lichens are a classic example of symbiosis. This partnership may contain up to three kingdoms, including a fungal (mycobiont) and algal and/or cyanobacterial (photobiont) partners. These organisms produce a variety of secondary compounds, most of which arise from the secondary metabolism of the fungal component and are deposited on the surface of the hyphae rather than compartmentalized in the cells. Many of these compounds are unique to lichens, with a small minority (ca. 60) occurring in other fungi or higher plants.¹² Due to a long history of chemotaxonomic study, the secondary chemistry of lichen compounds is better documented than in any other phylogenetic group; however, the bioactivity associated with these compounds has been generally ignored.

Of the more than 20,000 known species of lichens, only a few have been analyzed and identified as containing biologically active secondary compounds. Most of the unique secondary metabolites that are present in lichens are derived from the polyketide pathway, with a few originating from the shikimic acid and mevalonic acid pathways (Table 1.1). Previous studies have suggested that the para-depsides are precursors to meta-depsides, depsones, diphenyl ethers, depsidones and dibenzofurans.^9,12

Lichen secondary products may comprise up to 20% of thallus dry weight,¹⁵ although 5-10% is more common. Due to the high cost of carbon allocation, it is probable that these compounds have important ecological roles, either as protection against biotic factors such as herbivory³⁷ and competition or abiotic factors such as UV light.¹⁵ Of those species tested, over 50% of them synthesize substances with some degree of antimicrobial activity. This may play some role in general lichen ecology and/or ecosystem dynamics. The antimicrobial activity, however, appears to be unrelated to other ecological roles, such as herbivory.²⁸ Several anthraquinones with high antimicrobial activity have been isolated and characterized from some species in the lichen genus Xanthoria.³⁰

Table 1.1
Major classes of secondary lichen metabolites

Antiherbivory roles of metabolites have been well documented.^9,25,26 Proksch³¹ reported that lichens produced secondary metabolites that acted as feeding deterrents which protected them from animal consumption. Several insects appeared to selectively avoid the medullary region, which contained most of the lichen metabolites, grazing primarily on the algal layer.²⁵

Several lichen metabolites are known to inhibit the growth and development of fungal species. For example, crude aqueous extracts of lichens inhibit wood-decaying fungi, and other lichen products inhibit certain pathogenic fungi. Crude lichen extracts inhibit spore germination and may also cause decreased mycorrhyzal growth.^18,27,28

The potential role of lichen metabolites in allelopathic interactions has recently been reviewed.^9,27 The phytotoxic effect of certain lichen metabolites may play a role in the establishment of lichen populations. The depsides, barbatic acid and lecanorin, and the tridepside, gyrophoric acid, have been shown to inhibit photosynthetic electron transport in isolated chloroplasts.^13,34 Another aspect of the allelopathic potential of lichens is related to the ability of (-)-usnic, one of the two enantiomers known to exist in nature, to inhibit carotenoid biosynthesis through the enzyme 4-hydroxyphenyl pyruvate dioxygenases.³³ The in vitro activity of usnic acid is superior to that of other synthetic inhibitors of this herbicide target site.

Despite these experimental results, the ecological impact of these lichen secondary metabolites is not well understood. Primary lichen successional species do not have fewer secondary compounds than do subsequent successional species. There are also those species that thrive although they do not have high levels of secondary metabolites. Some theories attempting to explain why certain species produce more secondary products than others include the possibility that those producing high levels of compounds are able to grow in more severe environments, such as those with limited nutrient supplies or those with high nitrogen and phosphorus content.¹⁵

The objectives of this paper are broad. Our first objective is to describe the primary mechanism of action of usnic acid on plants as ascertained by our laboratory.³³ A second objective is to describe the phytotoxic activity of selected lichen anthraquinone analogues. In addition to the phytotoxic activity, we describe the effects of these secondary metabolites on phloem-feeding insects. Finally, we provide a hypothesis to explain the functional roles of these metabolites in the ecosystem.

RESULTS AND DISCUSSION

USNIC ACID

(-)-Usnic acid [2,6-diacetyl-7,9-dihydroxy-8,9b-dimethyl-1,3(2H,9βH)- dibenzofurandione] is one of two naturally occurring biologically active enantiomers (Fig. 1.1) that are found in most yellow-green lichens. This compound is biosynthesized via the polyketide pathway and is categorized as either a dibenzofuran or triketone. The enantiomers, which differ in the orientation of the methyl group at 9b on the otherwise rigid molecule, have been identified as showing different biological activities and mechanisms of action. Usnic acid has been documented to have antihistamine, spasmolytic, antiviral, and antibacterial activities.¹² Proska et al.³² reported that (-)-usnic acid inhibited urease and arginase activity. There are several reports²⁴ that the (+)-enantiomer is a more effective antimicrobial agent, although no specific mode of action was determined.

Figure 1.1
Structure illustrating triketone moiety of A. (-)-usnic, B. (+)-usnic, and C. sulcotrione.

Limited studies have documented phytotoxic effects of usnic acid including inhibition of transpiration and oxygen evolving processes in maize and sunflower seedlings.²³ Studies of mouse mitochondria have suggested that (+)-usnic acid uncouples oxidative phosphorylation at levels of 1 μM.¹ However, a definitive explanation of the phytotoxicity of usnic acid had, to our knowledge, never been reported. Thus, we have attempted to determine the phytotoxic mode of action for (-)-usnic acid.³³

(-)-Usnic caused a dose-dependent bleaching of the cotyledonary tissues (Fig. 1.2) that ultimately led to the death of the seedlings, whereas (+)-usnic did not cause any significant changes in chlorophyll content. Loss of chlorophylls in response to phytotoxins can be associated with light-dependent destabilization of cellular and subcellular membranes, but usnic acid apparently acts differently since both enantiomers caused membrane leakage in the absence of light (F...

Table of contents

• COVER PAGE
• TITLE PAGE
• COPYRIGHT PAGE
• PREFACE
• CONTRIBUTORS
• INTRODUCTION-REALITY AND FUTURE OF ALLELOPATHY
• 1: ECOPHYSIOLOGY AND POTENTIAL MODES OF ACTION FOR SELECTED LICHEN SECONDARY METABOLITES
• 2: BIOACTIVE COMPOUNDS FROM POTAMOGETONACEAE ON AQUATIC ORGANISMS
• 3: FATE OF PHENOLIC ALLELOCHEMICALS IN SOILS - THE ROLE OF SOIL AND RHIZOSPHERE MICROORGANISMS
• 4: BENZOXAZOLIN-2(3H)-ONES-GENERATION, EFFECTS AND DETOXIFICATION IN THE COMPETITION AMONG PLANTS
• 5: HELIANNANES-A STRUCTURE-ACTIVITY RELATIONSHIP (SAR) STUDY
• 6: CHEMISTRY OF HOST-PARASITE INTERACTIONS
• 7: APPLICATION OF ANALYTICAL TECHNIQUES TO THE DETERMINATION OF ALLELOPATHIC AGENTS IN WHEAT ROOT EXUDATES -PRACTICAL CASE STUDY
• 8: THE IMPORTANCE OF ALKALOIDAL FUNCTIONS
• 9: ALLELOCHEMICAL PROPERTIES OF QUINOLIZIDINE ALKALOIDS
• 10: MODE OF ACTION OF PHYTOTOXIC TERPENOIDS
• 11: MODE OF ALLELOCHEMICAL ACTION OF PHENOLIC COMPOUNDS
• 12: MODE OF ACTION OF THE HYDROXAMIC ACID BOA AND OTHER RELATED COMPOUNDS
• 13: MODE OF ACTION OF PHYTOTOXIC FUNGAL METABOLITES
• 14: PROTEOMIC TECHNIQUES FOR THE STUDY OF ALLELOPATHIC STRESS PRODUCED BY SOME MEXICAN PLANTS ON PROTEIN PATTERNS OF BEAN AND TOMATO ROOTS
• 15: APPLICATION OF MICROSCOPIC TECHNIQUES TO THE STUDY OF SEEDS AND MICROALGAE UNDER OLIVE OIL WASTEWATER STRESS
• 16: BIOASSAYS-USEFUL TOOLS FOR THE STUDY OF ALLELOPATHY