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The subject of this study was the participation of nitric oxide (NO) in plant responses to wounding, promoted by nicking of pelargonium (Pelargonium peltatum L.) leaves. Bio-imaging with the fluorochrome 4,5-diaminofluorescein diacetate (DAF-2DA) and electrochemical in situ measurement of NO showed early (within minutes) and transient (2 h) NO generation after wounding restricted to the site of injury. In order to clarify the functional role of NO in relation to modulation of the redox balance during wounding, a pharmacological approach was used. A positive correlation was found between NO generation and regulation of the redox state. NO caused a slight restriction of post-wounded O(2) (-) production, in contrast to the periodic and marked increase in H(2)O(2) level. The observed changes were accompanied by time-dependent inhibition of catalase (CAT) and ascorbate peroxidase (APX) activity. The effect was specific to NO, since the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5 tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) reversed the inhibition of CAT and APX, as well as temporarily enhancing H(2)O(2) synthesis. Finally, cooperation of NO/H(2)O(2) restricted the depletion of the low-molecular weight antioxidant pool (i.e. ascorbic acid and thiols) was positively correlated with sealing and reconstruction changes in injured pelargonium leaves (i.e. lignin formation and callose deposition). The above results clearly suggest that NO may promote restoration of wounded tissue through stabilisation of the cell redox state and stimulation of the wound scarring processes.

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In this study peroxynitrite (ONOO À ) is proposed as an important player in defence responses during the interaction of potato (Solanum tuberosum) and the oomycete pathogen Phytophthora infestans. The potato-avr P. infestans model system exhibited a transient programme of boosted ONOO À formation correlated in time with the burst of nitric oxide (NO) and superoxide during the first 6 h post-inoculation (hpi). The early ONOO À over-accumulation was not accompanied by TPx gene expression. In contrast, the compatible interaction revealed a 24 h delay of ONOO À formation; however, an enhanced level of NO and superoxide correlated with TPx up-regulation was recorded within the earlier stages of pathogen infection. Peroxynitrite over-accumulation in the susceptible potato coincided with an enhanced level of protein tyrosine nitration starting from 24 hpi. Surprisingly, the nitroproteome profile of the resistant potato did not show any visible difference after inoculation, apart from one band containing subtilisin-like protease-like proteins, which appeared 48 h after pathogen attack. An additional pharmacological approach showed that treatment of the susceptible genotype with ONOO À followed by inoculation with P. infestans contributed to slowing down of the colonization of host tissues by the pathogen via a faster and stronger up-regulation of the key defence markers, including the PR-1 gene. Taken together, the results obtained indicate that a precise control of emitted NO and superoxide in cooperation with thioredoxin-dependent redox sensors in sites of pathogen ingress could generate a sufficient threshold of ONOO À , triggering defence responses.