Plant-microbe interactions chemicals

This volume treats pheromones (Chapters 4.01—4.06), defensive substances and toxins (Chapters 4.08—T. 10), antifeedants (Chapters 4.11-4.12), compounds employed in plant-plant and plant-microbe interactions (Chapter 4.13), plant-insect interactions (Chapter 4.14) and microbe-microbe interactions (Chapter 4.07). Hormones of plants (Chapter 4.02) and insects (Chapter 4.03) are also treated in this volume. A unique attempt in the present volume is to regard flavor and fragrance (Chapter 4.15) and taste (Chapter 4.16) as phenomena of human-environmental interactions or human chemical ecology. 

We can distinguish between secondary metabolites that are already present prior to an attack or wounding, so-called constitutive compounds, and others that are induced by these processes and made de novo. Inducing agents, which have been termed elicitors by phytopathologists, can be cell wall fragments of microbes, the plant itself, or many other chemical constituents (4,17,22-24). The induced compounds are called phytoalexins, which is merely a functional term, since these compounds often do not differ in structure from constitutive natural products. In another way this term is misleading, since it implies that the induced compound is only active in plant-microbe interactions, whereas in reality it often has multiple functions that include antimicrobial and antiherbivoral properties (see below). 

Most plants have trichomes on their aerial surfaces. The trichomes may be simple hairs or more specialized glandular trichomes, whose main function may be the production and accumulation of chemicals such as essential oils. The vast majority of these consists of monoterpenoids, sesquiterpenoids and diterpenoids with a high vapour pressure. They may be absorbed on the cuticular wax layer. The trichome secretions are closely related to plant-insect or plant-microbe interactions. Terpenoids can attract, rep>el or initiate defence reactions in insects. Apart from their ecological roles, plant terpenoids are widely used in the pharmaceutical and fragrance industries. The properties of essential oils are correlated with their qualitative and quantitative compositions. 

There is some confusion in the literature as to when it is appropriate to apply the term allelochemical to phenolic acids. Since phenolic acids and their derivatives are found essentially in all terrestrial soils, it should be understood that the presence of phenolic acids in soil does not automatically imply that these phenolic acids are functionally allelochemicals. In theory, phenolic acids in soils, depending on their chemical state, concentrations, and the organisms involved, can have no effect, a stimulatory effect, or an inhibitory effect on any given plant or microbial process. For phenolic acids in the soil to be classified as allelochemicals requires that a) the phenolic acids are in an active form (e.g., free and protonated), b) they are involved in chemically mediated plant, microbe, or plant/microbial interactions and c) the concentrations of the active forms in the soil solution are sufficient to modify plant or microbial behavior, either in a positive or negative manner.8,49 However, changes in microbial behaviour associated with the utilization of phenolic acids as a carbon or energy source would not qualify as an allelopathic response. 

Blum, U., 1995. The value of model plant-microbe-soil systems for understanding processes associated with allelopathic interactions One example. In Inderjit, Dakshini, K. M. M., Einhellig, F. A. (Eds.), Allelopathy organisms, processes, and applications. ACS Symposium Series No. 582. American Chemical Society, Washington D.C, 127-131... 

In this review we point out the value of bioassays in the study of allelopathic interactions but do not attempt to critically evaluate or prioritize bioassay techniques that have been published. Since it is also not possible to propose a general bioassay for researchers, we present selected examples of bioassays that have been used to understand these plant-plant and plant-microbe chemical interactions. Furthermore, we attempt to provide an overview of some bioassays that are useful or that may be adaptable to allelochemicals. The general focus will be on laboratory bioassays since they are paramount to determine quantitative effects of allelochemicals and to ascertain actual mechanisms of allelopathy in nature. 

Molisch,77 in 1937, coined the term allelopathy as chemical interactions among plants including microorganisms. In 1984, Rice89 defined allelopathy as the effect(s) of one plant (including microorganisms) on another plant via release of chemicals into the environment. Here we also take this broader view of allelopathy, and discuss plant vs. plant, microbe vs. plant, and microbe vs. microbe allelopathic interactions in regard to bioassay systems. 

Allelopathic interactions are complex. Tm aware of no case where one chemical has been unequivocally proven to explain the entire situation. Almost all allelopathic interactions involve not only products of higher plants but also those of microbes, either as enhancers or detoxifiers. All cases require chemical characterization work followed by intensive studies by plant physiologists. Seldom can all the work be accomplished within one group. Simply, this means we must work together. 

It is now known that the initial interaction between plants and bacteria of the Rhizobiaceae is a chemical detection by the microbe of a susceptible host, i.e., the host produces compounds which act as signals for the microbial pathogen or symbiont. The microbe responds to these signals by expression of genes necessary in subsequent stages of the interaction. For a few of the Rhizobiaceae some signal compounds involved have been identified (1-7). 

The bioavailability of a metal(loid) is controlled by its physicochemical speciation [e.g., Cr(OH)3 vs. HCrO4 ] in the soil environment and tlie biological (microbes, plant roots), physical (point of zero charge, soil moisture content), and chemical factors (pH, ionic strength, redox potential) interacting with metal(loid)s in soils (McBride, 1994 Sparks, 2003 Krishnamurti and Naidu, Chapter 11, this... 

---