No abstract

Inducible defensive responses in plants are known to be activated locally and systemically by signaling molecules that are produced at sites of pathogen or insect attacks, but only one chemical signal, ethylene, is known to travel through the atmosphere to activate plant defensive genes. Methyl jasmonate, a common plant secondary compound, when applied to surfaces of tomato plants, induces the synthesis of defensive proteinase inhibitor proteins in the treated plants and in nearby plants as well. The presence of methyl jasmonate in the atmosphere of chambers containing plants from three species of two families, Solanaceae and Fabaceae, results in the accumulation of proteinase inhibitors in leaves of all three species. When sagebrush, Artemisia tridentata, a plant shown to possess methyl jasmonate in leaf surface structures, is incubated in chambers with tomato plants, proteinase inhibitor accumulation is induced in the tomato leaves, demonstrating that interplant communication can occur from leaves ofone species of plant to leaves of another species to activate the expression of defensive genes.

Defensive genes in plants can be activated by several different types of nonpeptide signaling molecules. An endogenous polypeptide, consisting of 18 amino acids, was isolated from tomato leaves and was able at very low concentrations to induce the synthesis of two wound-inducible proteinase inhibitor proteins when supplied to young tomato plants. The sequence of the polypeptide was determined, and an identical polypeptide was synthesized that possessed full inducing activity. These data establish that a polypeptide factor can initiate signal transduction to regulate the synthesis of defensive proteins in plant tissues.

endogenous elicitor ͉ plant defense ͉ defensin ͉ hydrogen peroxide S imilarities have been noted among early signaling components of animal and plant innate immune systems, including leucine-rich repeat receptor-mediated recognition of pathogenassociated molecular patterns and͞or elicitors from pathogens and the resulting activation of defense gene transcription involved in early steps of immunity (1-14). Several peptides originating from pathogens can activate the plant innate immune response, including fungal elicitors Pep13, AVR9, and elicitins (1-3), and bacterial elicitors hrpZ, NPP1, flg22, and elf13 (4-7). We report here that a 23-aa peptide, isolated from extracts of Arabidopsis leaves and called AtPep1, exhibits characteristics of an endogenous elicitor of the innate immune response. Endogenous plant peptides that activate genes specifically for defense against pathogens have not been reported previously to our knowledge, although systemin peptides, which are found only in Solanaceae species, activate antiherbivore defense genes. AtPep1 was first identified in soluble extracts of Arabidopsis leaves by its ability, at subnanomolar concentrations, to cause an alkalinization of the medium of suspension cultured cells, a typical response of cell cultures to peptide elicitors (15-19). AtPep1 is derived from the C terminus of a 92-aa precursor protein AtproPep1. The peptide activates the transcription of defensin, a gene extensively studied for its role in innate immunity in Arabidopsis, the production of H 2 O 2 , and the expression of PROPEP1. Constitutive overexpression of PROPEP1 confers resistance against a root pathogen Pythium irregulare. PROPEP1 orthologs are found in numerous important agricultural crop species, including both dicots and monocots, and may provide novel genes for investigating crop productivity. Results and DiscussionAtPep1 was purified to homogeneity ( Fig. 1 A and B) and characterized as a peptide by its molecular mass (Fig. 1C) and amino acid sequence (Fig. 1D), which together indicated that the peptide was not posttranslationally modified. Chemically synthesized AtPep1 was found to be as active as native AtPep1, having a half-maximal activity of Ϸ0.25 nM in the alkalinization assay. Peptides present in an 1% trifluoroacetic acid͞water extract of Arabidopsis tissues were passed through a reverse-phase semipreparative C18 flash chromatography column and separated on a G-25 Sepharose column as described in Materials and Methods. The breakthrough peak was applied to a C18 HPLC column, and 10 l from 2-ml fractions from the column was assayed for alkalinization activity. (B) The peak identified in A as AtPep1 was further purified through two additional chromatography steps and finally purified by narrow-bore HPLC as described in Materials and Methods. Fractions were assayed as in A. The active peak is identified with arrows. (C) Analysis of the biologically active peak by MALDI-MS. (D) The amino acid sequence of the purified peptide, determined by Edman degradation. The daltons calculat...

Wounding of the leaves of potato or tomato plants by adult Colorado potato beetles, or their larvae, induces a rapid accumulation of a proteinase inhibitor throughout the plants' tissues that are exposed to air. This effect of insect damage can be simulated by mechanically wounding the leaves. The transport of a factor out of damaged leaves takes place rapidly after the wound is inflicted and the levels of proteinase inhibitor, in both damaged and adjacent leaves, rises strikingly within a few hours. The rapid accumulation of a powerful inhibitor of major intestinal proteinases of animals in response to wounding of the leaves is probably a defense mechanism.