Experimental Baj ıo, INIFAP-Celaya, Km 6.5 Carr. Celaya-San Miguel de Allende 38810, Guanajuato, M exico Summary 1. Plants that express resistance to herbivores emit volatile organic compounds (VOCs) that can trigger resistance responses in undamaged neighbours. Recent reports indicate that VOCs can also trigger the resistance to pathogens, an effect that might be due to different mechanisms: the priming of an induced expression of resistance genes in the receiver or direct inhibitory effects on microbial pathogens that cause a passive 'associational' resistance in the VOC-exposed plant. 2. We investigated whether VOCs emitted from a resistant common bean (Phaseolus vulgaris) cultivar enhance the resistance to the fungus Colletotrichum lindemuthianum in a susceptible cultivar and analysed whether specific VOCs are likely to directly affect the pathogen. 3. We found that susceptible plants exposed to the headspace of resistance-expressing plants over 6 h became phenotypically as resistant as the resistant cultivar. Several resistance marker genes (PATHOGENESIS-RELATED [PR] 1, 2 and 4) were primed in VOC-exposed susceptible plants. After challenging, these genes reached expression levels at least as high as in the resistant cultivar. Additionally, individual VOCs such as limonene, linalool, nonanal, methyl salicylate and methyl jasmonate at natural concentrations directly inhibited the germination of conidia as did also the headspace of a resistance-expressing plant. This inhibition of conidial germination was dosagedependent and irreversible. 4. Synthesis. We conclude that VOCs are involved in the resistance of bean to fungal pathogens. They can contribute to the direct resistance in the emitter itself, and resistance phenotypes of neighbouring receiver plants can result from induced as well as associational resistance. Plant VOCs play multiple roles in the resistance of plants to microbial pathogens.

BackgroundAnimal-derived elicitors can be used by plants to detect herbivory but they function only in specific insect–plant interactions. How can plants generally perceive damage caused by herbivores? Damaged-self recognition occurs when plants perceive molecular signals of damage: degraded plant molecules or molecules localized outside their original compartment.Methodology/Principal FindingsFlame wounding or applying leaf extract or solutions of sucrose or ATP to slightly wounded lima bean (Phaseolus lunatus) leaves induced the secretion of extrafloral nectar, an indirect defense mechanism. Chemically related molecules that would not be released in high concentrations from damaged plant cells (glucose, fructose, salt, and sorbitol) did not elicit a detectable response, excluding osmotic shock as an alternative explanation. Treatments inducing extrafloral nectar secretion also enhanced endogenous concentrations of the defense hormone jasmonic acid (JA). Endogenous JA was also induced by mechanically damaging leaves of lima bean, Arabidopsis, maize, strawberry, sesame and tomato. In lima bean, tomato and sesame, the application of leaf extract further increased endogenous JA content, indicating that damaged-self recognition is taxonomically widely distributed. Transcriptomic patterns obtained with untargeted 454 pyrosequencing of lima bean in response to flame wounding or the application of leaf extract or JA were highly similar to each other, but differed from the response to mere mechanical damage. We conclude that the amount or concentration of damaged-self signals can quantitatively determine the intensity of the wound response and that the full damaged-self response requires the disruption of many cells.Conclusions/SignificanceNumerous compounds function as JA-inducing elicitors in different plant species. Most of them are, contain, or release, plant-derived molecular motifs. Damaged-self recognition represents a taxonomically widespread mechanism that contributes to the perception of herbivore feeding by plants. This strategy is independent of insect-derived elicitors and, therefore, allows plants to maintain evolutionary control over their interaction with herbivores.

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To date, several classes of hormones have been described that influence plant development, including auxins, cytokinins, ethylene, and, more recently, brassinosteroids. However, it is known that many fungal and bacterial species produce substances that alter plant growth that, if naturally present in plants, might represent novel classes of plant growth regulators. Alkamides are metabolites widely distributed in plants with a broad range of biological activities. In this work, we investigated the effects of affinin, an alkamide naturally occurring in plants, and its derivates, N-isobutyl-2E-decenamide and N-isobutyl-decanamide, on plant growth and early root development in Arabidopsis. We found that treatments with affinin in the range of 10 Ϫ6 to 10 Ϫ4 m alter shoot and root biomass production. This effect correlated with alteration on primary root growth, lateral root formation, and root hair elongation. Low concentrations of affinin (7 ϫ 10 Ϫ6 -2.8 ϫ 10 Ϫ5 m) enhanced primary root growth and root hair elongation, whereas higher concentrations inhibited primary root growth that related with a reduction in cell proliferating activity and cell elongation. N-isobutyl-2E-decenamide and N-isobutyldecanamide were found to stimulate root hair elongation at concentrations between 10 Ϫ8 to 10 Ϫ7 m. Although the effects of alkamides were similar to those produced by auxins on root growth and cell parameters, the ability of the root system to respond to affinin was found to be independent of auxin signaling. Our results suggest that alkamides may represent a new group of plant growth promoting substances with significant impact on root development and opens the possibility of using these compounds for improved plant production.

This work demonstrates the fungistatic and bacteriostatic activities of affinin, the main alkamide of Heliopsis longipes (Gray) Blake (Asteraceae) roots and two alkamides obtained by catalytic reduction of affinin: N-isobutyl-2E-decenamide and N-isobutyl-decanamide. The bioactivity was tested against Rhizoctonia solani groups AG3 and AG5, Sclerotium rolfsii, Sclerotium cepivorum, Fusarium sp., Vertcillium sp., phytopathogenic fungi; Phytophthora infestans, a phytopathogenic Chromista; Saccharomyces cerevisiae, a nonphytopathogenic ascomycete; and Escherichia coli, Erwinia carotovora, and Bacillus subtilis, bacteria. Affinin, being the primary component of the lipidic fraction, is expected to be responsible for the fungitoxic activity observed in roots of this plant species. Four of the assayed fungi showed an important sensitivity to the presence of affinin: S. rolfsii, S. cepivorum, P. infestans, and R. solani AG-3 and AG-5, displaying a growth inhibition of 100%. S. cerevisiaeshowed a similar growth inhibition with affinin. None of the alkamides obtained by catalytic reduction of affinin showed a fungitoxic activity. Affinin had a definite negative effect on the growth of E. coli and B. subtilis, but E. carotovora carotovora was not sensitive to the highest dose of affinin assayed. N-Isobutyl-2E-decenamide displayed a higher bacteriostatic activity against E. coli and E. carotovora carotovora. In both cases, this alkamide was more potent than affinin. On the other hand, only N-isobutyl-decanamide displayed a significant activity on the growth of B. subtilis.

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