In plants, reactive oxygen species and, more particularly, hydrogen peroxide (H2O2) play a dual role as toxic by-products of normal cell metabolism and as regulatory molecules in stress perception and signal transduction. Peroxisomal catalases are an important sink for photorespiratory H2O2. Using ATH1 Affymetrix microarrays, expression profiles were compared between control and catalase-deficient Arabidopsis (Arabidopsis thaliana) plants. Reduced catalase levels already provoked differences in nuclear gene expression under ambient growth conditions, and these effects were amplified by high light exposure in a sun simulator for 3 and 8 h. This genome-wide expression analysis allowed us to reveal the expression characteristics of complete pathways and functional categories during H2O2 stress. In total, 349 transcripts were significantly up-regulated by high light in catalase-deficient plants and 88 were down-regulated. From this data set, H2O2 was inferred to play a key role in the transcriptional up-regulation of small heat shock proteins during high light stress. In addition, several transcription factors and candidate regulatory genes involved in H2O2 transcriptional gene networks were identified. Comparisons with other publicly available transcriptome data sets of abiotically stressed Arabidopsis revealed an important intersection with H2O2-deregulated genes, positioning elevated H2O2 levels as an important signal within abiotic stress-induced gene expression. Finally, analysis of transcriptional changes in a combination of a genetic (catalase deficiency) and an environmental (high light) perturbation identified a transcriptional cluster that was strongly and rapidly induced by high light in control plants, but impaired in catalase-deficient plants. This cluster comprises the complete known anthocyanin regulatory and biosynthetic pathway, together with genes encoding unknown proteins.

• Ϫ -dependent cell death, whereas impairment of ethylene perception by norbornadiene in rcd1 or ethylene insensitivity in the ethylene-insensitive mutant ein2 and in the rcd1 ein2 double mutant blocked O 2• Ϫ accumulation and lesion propagation. Exogenous methyl jasmonate inhibited propagation of cell death in rcd1 . Accordingly, the O 3 -exposed jasmonate-insensitive mutant jar1 displayed spreading cell death and a prolonged O 2• Ϫ accumulation pattern. These results suggest that ethylene acts as a promoting factor during the propagation phase of developing oxyradical-dependent lesions, whereas jasmonates have a role in lesion containment. Interaction and balance between these pathways may serve to fine-tune propagation and containment processes, resulting in alternate lesion size and formation kinetics.

We have isolated a codominant Arabidopsis mutant, radical-induced cell death1 (rcd1), in which ozone (O(3)) and extracellular superoxide (O(2)(*)-), but not hydrogen peroxide, induce cellular O(2)(*)- accumulation and transient spreading lesions. The cellular O(2)(*)- accumulation is ethylene dependent, occurs ahead of the expanding lesions before visible symptoms appear, and is required for lesion propagation. Exogenous ethylene increased O(2)(*)--dependent cell death, whereas impairment of ethylene perception by norbornadiene in rcd1 or ethylene insensitivity in the ethylene-insensitive mutant ein2 and in the rcd1 ein2 double mutant blocked O(2)(*)- accumulation and lesion propagation. Exogenous methyl jasmonate inhibited propagation of cell death in rcd1. Accordingly, the O(3)-exposed jasmonate-insensitive mutant jar1 displayed spreading cell death and a prolonged O(2)(*)- accumulation pattern. These results suggest that ethylene acts as a promoting factor during the propagation phase of developing oxyradical-dependent lesions, whereas jasmonates have a role in lesion containment. Interaction and balance between these pathways may serve to fine-tune propagation and containment processes, resulting in alternate lesion size and formation kinetics.

N -acyl-L-homoserine lactone (AHL) signal molecules are utilized by Gram-negative bacteria to monitor their population density (quorum sensing) and to regulate gene expression in a density-dependent manner. We show that Serratia liquefaciens MG1 and Pseudomonas putida IsoF colonize tomato roots, produce AHL in the rhizosphere and increase systemic resistance of tomato plants against the fungal leaf pathogen, Alternaria alternata . The AHLnegative mutant S. liquefaciens MG44 was less effective in reducing symptoms and A. alternata growth as compared to the wild type. Salicylic acid (SA) levels were increased in leaves when AHL-producing bacteria colonized the rhizosphere. No effects were observed when isogenic AHLnegative mutant derivatives were used in these experiments. Furthermore, macroarray and Northern blot analysis revealed that AHL molecules systemically induce SA-and ethylene-dependent defence genes (i.e. PR1a, 26 kDa acidic and 30 kDa basic chitinase). Together, these data support the view that AHL molecules play a role in the biocontrol activity of rhizobacteria through the induction of systemic resistance to pathogens.

SummaryA growing body of evidence suggests that nitric oxide (NO), an important signalling and defence molecule in mammals, plays a key role in activating disease resistance in plants, acting as signalling molecule and possibly as direct anti-microbial agent. Recently, a novel¯uorophore (diamino¯uorescein diacetate, DAF-2 DA) has been developed which allows bio-imaging of NO in vivo. Here we use the cellpermeable DAF-2 DA, in conjunction with confocal laser scanning microscopy, for real-time imaging of NO in living plant cells. Epidermal tobacco cells treated with cryptogein, a fungal elicitor from Phytophthora cryptogea, respond to the elicitor with a strong increase of intracellular NO. NO-induced uorescence was found in several cellular compartments, and could be inhibited by a NO scavenger and an inhibitor of nitric oxide synthase. The NO burst was triggered within minutes, reminiscent of the oxidative burst during hypersensitive response reactions. These results reveal additional similarities between plant and animal host responses to infection.

Transgenic tobacco deficient in the H 2 O 2 -removing enzyme catalase (Cat1AS) was used as an inducible and noninvasive system to study the role of H 2 O 2 as an activator of pathogenesis-related (PR) proteins in plants. Excess H 2 O 2 in Cat1AS plants was generated by simply increasing light intensities. Sustained exposure of Cat1AS plants to excess H 2 O 2 provoked tissue damage, stimulated salicylic acid and ethylene production, and induced the expression of acidic and basic PR proteins with a timing and magnitude similar to the hypersensitive response against pathogens. Salicylic acid production was biphasic, and the first peak of salicylic acid as well as the peak of ethylene occurred within the first hours of high light, which is long before the development of tissue necrosis. Under these conditions, accumulation of acidic PR proteins was also seen in upper leaves that were not exposed to high light, indicating systemic induction of expression. Short exposure of Cat1AS plants to excess H 2 O 2 did not cause damage, induced local expression of acidic and basic PR proteins, and enhanced pathogen tolerance. However, the timing and magnitude of PR protein induction was in this case more similar to that in upper uninfected leaves than to that in hypersensitiveresponse leaves of pathogen-infected plants. Together, these data demonstrate that sublethal levels of H 2 O 2 activate expression of acidic and basic PR proteins and lead to enhanced pathogen tolerance. However, rapid and strong activation of PR protein expression, as seen during the hypersensitive response, occurs only when excess H 2 O 2 is accompanied by leaf necrosis.

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SummaryIn plants, hydrogen peroxide (H 2 O 2 ) plays a major signaling role in triggering both a defense response and cell death. Increased cellular H 2 O 2 levels and subsequent redox imbalances are managed at the production and scavenging levels. Because catalases are the major H 2 O 2 scavengers that remove the bulk of cellular H 2 O 2 , altering their levels allows in planta modulation of H 2 O 2 concentrations. Reduced peroxisomal catalase activity increased sensitivity toward both ozone and photorespiratory H 2 O 2 -induced cell death in transgenic catalase-de®cient Arabidopsis thaliana. These plants were used as a model system to build a comprehensive inventory of transcriptomic variations, which were triggered by photorespiratory H 2 O 2 induced by high-light (HL) irradiance. In addition to an H 2 O 2 -dependent and -independent type of transcriptional response during light stress, microarray analysis on both control and transgenic catalase-de®cient plants, exposed to 0, 3, 8, and 23 h of HL, revealed several speci®c regulatory patterns of gene expression. Thus, photorespiratory H 2 O 2 has a direct impact on transcriptional programs in plants.