Abscisic Acid Determines Basal Susceptibility of Tomato to Botrytis cinerea and Suppresses Salicylic Acid-Dependent Signaling Mechanisms

Abstract

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...

Powdery Mildew Resistance Conferred by Loss of the ENHANCED DISEASE RESISTANCE1 Protein Kinase Is Suppressed by a Missense Mutation in KEEP ON GOING, a Regulator of Abscisic Acid Signaling

“…As described above though, strain DC3000 up-regulates endogenous ABA levels in Arabidopsis (de Torres-Zabala et al, 2007); thus, exogenous ABA may have little effect on this strain. ABA application also enhances the susceptibility of Arabidopsis to Fusarium oxysporum (Anderson et al, 2004) and the susceptibility of tomato (Solanum lycopersicum) to Botrytis cineria (Audenaert et al, 2002). Consistent with this, ABA-deficient Arabidopsis mutants are more resistant to F. oxysporum (Anderson et al, 2004) and B. cinerea (Adie et al, 2007), and the ABA-deficient sitiens mutant of tomato is more resistant to B. cineria (Audenaert et al, 2002) and to P. syringae (Thaler and Bostock, 2004).…”

NPR1: the spider in the web of induced resistance signaling pathways

“…These include systemic acquired resistance (SAR), which is triggered by necrotizing pathogens [1]; induced systemic resistance (ISR), which is activated upon colonization of roots by selected strains of non-pathogenic rhizobacteria [2]; and woundinduced defense, which is typically elicited upon tissue damage such as that caused by feeding insects ( [3]; Figure 1). Induced defense responses are regulated by a network of interconnecting signal transduction pathways in which the hormonal signals salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) play a major role [4][5][6], and other hormones such as brassinosteroids and abscisic acid can also be involved [7][8][9].…”

The multifaceted role of ABA in disease resistance

“…Early post-invasive penetration resistance against oomycetes and fungi Although it has been known for two decades that ABA has a negative role in disease resistance against oomycetes [18], first indications for a regulatory role of ABA in fungal disease resistance came from the discovery that the ABAdeficient tomato mutant sitiens is resistant to the necrotrophic fungus Botrytis cinerea [19]. Recently, it was demonstrated that this resistance is based on increased accumulation of ROS during the early stages of tissue penetration [20].…”