a colloquium entitled " Self - Defense by Plants : Induction and Signalling Pathways , " organized

We have used suspension-cultured parsley cells (Petroselinum crispum) and an oligopeptide elicitor derived from a surface glycoprotein of the phytopathogenic fungus Phytophthora megasperma f.sp. glycinea to study the signaling pathway from elicitor recognition to defense gene activation. Immediately after specific binding ofthe elicitor by a receptor in the plasma membrane, large and transient increases in several inorganic ion fluxes (Ca2+, H+, K+, Cl-) and H202 formation are the first detectable plant cell responses. These are rapidly followed by transient changes in the phosphorylation status of various proteins and by the activation of numerous defense-related genes, concomitant with the inactivation of several other, non-defense-related genes. A great diversity of cis-acting elements and trans-acting factors appears to be involved in elicitor-mediated gene regulation, similar to the apparently complex nature of the signal transduced intracellularly. With few exceptions, all individual defense responses analyzed in fungus-infected parsley leaves have been found to be closely mimicked in elicitor-treated, cultured parsley cells, thus validating the use of the elicitor/cell culture system as a valuable model system for these types of study. A crucial and rapidly expanding area of research concerns the chemical communication within and among organisms. This basic level of intercellular and interorganismic communication plays a critical role in determining the composition and dynamic behavior of ecosystems through such processes as the discrimination between self and nonself and the determination of symbiotic and pathogenic relationships. In functional terms, the chain of molecular events comprising these pathways can be divided into three parts: (i) generation and recognition of extracellular signals, (ii) intracellular signal conversion and/or transduction, and (iii) signal-specific responses of target cells. We are studying elements of all three functionally interconnected parts of such a signal-response chain, exploiting the fact that many details of the non-host-resistance response of parsley leaves (Petroselinum crispum) to infection with the soybean pathogen Phytophthora megasperma f.sp. glycinea (Pmg) can be mimicked by treatment of suspension-cultured parsley cells with an elicitor preparation from this fungus. We have focused our interest on the following elements: * the nature and mechanism of action of a Pmg-derived molecule with high elicitor activity on parsley cells; * rapid cell membraneand cell wall-associated changes, as well as intracellular changes, in metabolic activity; and The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact. * the elicitor-mediated activation and inactivation of numerous defense-related and non-defense-related genes. Prior to a discussion of some mechanistic details of the responses of cultured parsley cells to treatment with Pmg elicitor, the present state of knowledge of the whole-plant interaction of parsley leaves with the intact fungus will be briefly summarized. Responses of Parsley Leaves to Pmg Infection Young, primary parsley leaves were used for most of our studies (1-4), since a higher rate of infection with Pmg was obtained than with old leaves under the conditions used (1). Combining all of the results obtained so far with Pmg-infected parsley leaves, three major steps in the overall defense response can be distinguished (Fig. 1). Following leaf inoculation with fungal zoospores, cyst formation, germination, and formation of appressoria and infection vesicles (-4 hr postinoculation), the first microscopically visible sign of a plant defense response is hypersensitive (very rapid and highly localized) cell death. This hypersensitive response is associated with reinforcement of the affected cell wall-for example, by apposition of callose and incorporation of phenolics. The newly incorporated phenolics are readily detectable without histochemical staining by their autofluorescence under blue/UV light (1-4). Results obtained recently with a similar system, potato (Solanum tuberosum L.) leaves infected with Phytophthora infestans, indicate that the penetrating fungal infection vesicle is killed concomitantly with hypersensitive plant cell death (5). In both the parsley and potato systems, as well as in many others, hypersensitive cell death appears to be a particular early-plant-defense response initiated by those cells that are invaded by the fungus or in direct contact with fungal structures. Available evidence suggests a close correlation between the frequency of hypersensitive cell death (provided it occurs at all) and the degree of resistance (5). The second line of defense consists of numerous rapidly accumulated enzymes, structural proteins, and metabolites, at least some of which possess antifungal activity. Among these antifungal compounds are the so-called phytoalexins, plant speciesor family-specific classes of broad-range antibiotics. In parsley, the phytoalexins are a mixture of linear furanocoumarins (6-8), which are easily detectable by their blue autofluorescence under UV light. Many of these second-line defense reactions are activated transcriptionally, as demonAbbreviations: Pmg, Phytophthora megasperma; PAL, phenylalanine ammonia-lyase; 4CL, 4-coumarate:CoA ligase; C4H, cinnamate 4-hydroxylase; PR proteins, pathogenesis-related proteins; PRH, homeodomain-containing PR protein; BPF, box P-binding factor; BIF, box I-binding factor; CPRF, common plant regulatory factors.