Plant defense mechanisms against necrotrophic pathogens, such as Botrytis cinerea, are considered to be complex and to differ from those that are effective against biotrophs. In the abscisic acid-deficient sitiens tomato (Solanum lycopersicum) mutant, which is highly resistant to B. cinerea, accumulation of hydrogen peroxide (H 2 O 2 ) was earlier and stronger than in the susceptible wild type at the site of infection. In sitiens, H 2 O 2 accumulation was observed from 4 h postinoculation (hpi), specifically in the leaf epidermal cell walls, where it caused modification by protein cross-linking and incorporation of phenolic compounds. In wildtype tomato plants, H 2 O 2 started to accumulate 24 hpi in the mesophyll layer and was associated with spreading cell death. Transcript-profiling analysis using TOM1 microarrays revealed that defense-related transcript accumulation prior to infection was higher in sitiens than in wild type. Moreover, further elevation of sitiens defense gene expression was stronger than in wild type 8 hpi both in number of genes and in their expression levels and confirmed a role for cell wall modification in the resistant reaction. Although, in general, plant defense-related reactive oxygen species formation facilitates necrotrophic colonization, these data indicate that timely hyperinduction of H 2 O 2 -dependent defenses in the epidermal cell wall can effectively block early development of B. cinerea.

Abscisic acid (ABA) is one of the plant hormones involved in the interaction between plants and pathogens. In this work, we show that tomato (Lycopersicon esculentum Mill. cv Moneymaker) mutants with reduced ABA levels (sitiens plants) are much more resistant to the necrotrophic fungus Botrytis cinerea than wild-type (WT) plants. Exogenous application of ABA restored susceptibility to B. cinerea in sitiens plants and increased susceptibility in WT plants. These results indicate that ABA plays a major role in the susceptibility of tomato to B. cinerea. ABA appeared to interact with a functional plant defense response against B. cinerea. Experiments with transgenic NahG tomato plants and benzo(1,2,3)thiadiazole-7-carbothioic acid demonstrated the importance of salicylic acid in the tomato-B. cinerea interaction. In addition, upon infection with B. cinerea, sitiens plants showed a clear increase in phenylalanine ammonia lyase activity, which was not observed in infected WT plants, indicating that the ABA levels in healthy WT tomato plants partly repress phenylalanine ammonia lyase activity. In addition, sitiens plants became more sensitive to benzo(1,2,3)thiadiazole-7-carbothioic acid root treatment. The threshold values for PR1a gene expression declined with a factor 10 to 100 in sitiens compared with WT plants. Thus, ABA appears to negatively modulate the salicylic acid-dependent defense pathway in tomato, which may be one of the mechanisms by which ABA levels determine susceptibility to B. cinerea.Upon pathogen attack, infected plant cells generate signaling molecules to initiate defense mechanisms in surrounding cells to limit pathogen spread. The role of the plant hormones salicylic acid (SA), jasmonic acid (JA), and ethylene in this process is supported by well-documented observations and molecular characterization (Hammond-Kosack and Jones, 1996). This kind of information is not available for another plant hormone, abscisic acid (ABA), which participates in several processes. The role of ABA in developmental programs, such as seed dormancy, root geotropism, opening of stomata through stomatal guard cells, and dormancy of buds, has been most extensively documented (Walton, 1980). Furthermore, ABA is involved in the wound response (WR) activated upon insect feeding (Birkenmeier and Ryan, 1998).Regarding plant-pathogen interactions, information on ABA involvement is mainly based on indirect observations. Increased endogenous ABA levels were observed in response to infection with viruses, bacteria, and fungi (Whenham et al., 1986; Steadman and Sequeira, 1970; Kettner and Dö rffling, 1995). In addition, it is generally found that application of exogenous ABA increases the susceptibility of plants to fungal pathogens (Henfling et al., 1980; Ward et al., 1989; McDonald and Cahill, 1999). ABA also seems to interact with pathogen associated plant defense. In soybean (Glycine max), ABA suppressed Phe ammonia lyase (PAL) activity and transcription of PAL mRNA in hypocotyls inoculated with the incompatible pathogen P...

Summary666I.Introduction667II.Biosynthesis667III.Meta‐analysis669IV.The type of stress influences the total amount of GLVs released669V.Herbivores can modulate the wound‐induced release of GLVs669VI.Fungal infection greatly induces GLV production672VII.Monocots and eudicots respond differentially to different types of stress673VIII.The type of stress does not influence the proportion of GLVs per chemical class673IX.The type of stress does influence the isomeric ratio within each chemical class674X.GLVs: from signal perception to signal transduction676XI.GLVs influence the C/N metabolism677XII.Interaction with plant hormones678XIII.General conclusions and unanswered questions678Acknowledgements679References679 Summary Plants respond to stress by releasing biogenic volatile organic compounds (BVOCs). Green leaf volatiles (GLVs), which are abundantly produced across the plant kingdom, comprise an important group within the BVOCs. They can repel or attract herbivores and their natural enemies; and they can induce plant defences or prime plants for enhanced defence against herbivores and pathogens and can have direct toxic effects on bacteria and fungi. Unlike other volatiles, GLVs are released almost instantly upon mechanical damage and (a)biotic stress and could thus function as an immediate and informative signal for many organisms in the plant's environment. We used a meta‐analysis approach in which data from the literature on GLV production during biotic stress responses were compiled and interpreted. We identified that different types of attackers and feeding styles add a degree of complexity to the amount of emitted GLVs, compared with wounding alone. This meta‐analysis illustrates that there is less variation in the GLV profile than we presumed, that pathogens induce more GLVs than insects and wounding, and that there are clear differences in GLV emission between monocots and dicots. Besides the meta‐analysis, this review provides an update on recent insights into the perception and signalling of GLVs in plants.

The rhizobacterium Pseudomonas aeruginosa 7NSK2 produces secondary metabolites such as pyochelin (Pch), its precursor salicylic acid (SA), and the phenazine compound pyocyanin. Both 7NSK2 and mutant KMPCH (Pch-negative, SA-positive) induced resistance to Botrytis cinerea in wild-type but not in transgenic NahG tomato. SA-negative mutants of both strains lost the capacity to induce resistance. On tomato roots, KMPCH produced SA and induced phenylalanine ammonia lyase activity, while this was not the case for 7NSK2. In 7NSK2, SA is probably very efficiently converted to Pch. However, Pch alone appeared not to be sufficient to induce resistance. In mammalian cells, Fe-Pch and pyocyanin can act synergistically to generate highly reactive hydroxyl radicals that cause cell damage. Reactive oxygen species are known to play an important role in plant defense. To study the role of pyocyanin in induced resistance, a pyocyanin-negative mutant of 7NSK2, PHZ1, was generated. PHZ1 is mutated in the phzM gene encoding an O-methyltransferase. PHZ1 was unable to induce resistance to B. cinerea, whereas complementation for pyocyanin production or co-inoculation with mutant 7NSK2-562 (Pch-negative, SA-negative, pyocyaninpositive) restored induced resistance. These results suggest that pyocyanin and Pch, rather than SA, are the determinants for induced resistance in wild-type P. aeruginosa 7NSK2.Additional keywords: phenazine-1-carboxylate, siderophores.Induced resistance is a state of enhanced defensive capacity developed by a plant when appropriately stimulated (van Loon et al. 1998). Induced resistance is generally systemic and can be triggered by pathogens, certain chemicals, and nonpathogenic rhizosphere bacteria. The mechanisms involved in rhizobacteria-mediated induced systemic resistance (ISR) appear to vary among bacterial strains and pathosystems. Bacterial determinants of ISR which have been identified are lipopolysaccharides and siderophores (van Loon et al. 1998). Siderophores are high-affinity iron(III)-chelating compounds that are produced by most microorganisms under iron-limiting conditions. The catechol siderophore biosynthesis genes of Serratia marcescens are involved in ISR to Colletotrichum orbiculare on cucumber (Press et al. 2001). The purified pyoverdine-type siderophore of Pseudomonas putida WCS374 induced resistance to Fusarium wilt in radish (Leeman et al. 1996), while a pyoverdine-negative mutant of P. fluorescens CHA0 was less effective in inducing resistance to Tobacco necrosis virus on tobacco than the wild-type strain (Maurhofer et al. 1994). Another iron-chelating molecule that is well-studied with respect to induced plant defense is salicylic acid (SA). Although the siderophore capacity of SA is rather poor (Chipperfield and Ratledge 2000), it appears to be an important molecule in induced resistance by the rhizobacterium P. aeruginosa 7NSK2. This bacterium produces three siderophores under iron-limiting conditions (pyoverdine, pyochelin (Pch), and SA) and can induce resistance to plant diseases ca...

The mycotoxin deoxynivalenol (DON), produced by several Fusarium spp., acts as a virulence factor and is essential for symptom development after initial wheat infection. Accumulating evidence shows that the production of this secondary metabolite can be triggered by diverse environmental and cellular signals, implying that it might have additional roles during the life cycle of the fungus. Here, we review data that position DON in the saprophytic fitness of Fusarium, in defense and in the primary C and N metabolism of the plant and the fungus. We combine the available information in speculative models on the role of DON throughout the interaction with the host, providing working hypotheses that await experimental validation. We also highlight the possible impact of control measures in the field on DON production and summarize the influence of abiotic factors during processing and storage of food and feed matrices. Altogether, we can conclude that DON is a very important compound for Fusarium to cope with a changing environment and to assure its growth, survival, and production of toxic metabolites in diverse situations.