Reactive oxygen species (ROS), such as superoxide (O2 •‾), hydrogen peroxide (H2O2), singlet oxygen (1O2) and the hydroxyl radical (•OH) can damage essential biomolecules including nucleic acids, proteins and lipids, causing damage to various cellular components. However, ROS also participate in signalling networks that are essential in plant stress responses, and also in the regulation of growth and development. Given the apparent importance of ROS, it is surprising that very little of their beneficial aspects have been researched in seeds and seedlings. Using desiccation sensitive ("recalcitrant") seeds of sweet chestnut (Castanea sativa Mill.) and desiccation tolerant ("orthodox") seeds of garden pea (Pisum sativum L.), the production of ROS was investigated during germination and seedling development, and in response to abiotic stress. Putative extracellular ROS-producing enzymes in both species were characterised to elucidate mechanisms of ROS production. Desiccating C. sativa seeds led to viability loss while the intracellular antioxidant glutathione became increasingly oxidised. Wounding and desiccation induced extracellular ROS production in C. sativa embryonic axes and P. sativum seedling axes. A pivotal role for extracellular peroxidases in producing O2 •‾ as a stress response became evident in both species as well as for the development of P. sativum seedlings. Wounding also induced amine oxidases in P. sativum embryonic axes to produce a burst of H2O2 that was essential for O2 •‾ production. Lipoxygenases were identified as putative O2 •‾- producing enzymes that may contribute to stress signalling in response to wounding. Treating desiccation-stressed material with H2O2 improved seed germination, seedling vigour and the establishment of secondary root growth. In conclusion, cell wall peroxidases, amine oxidases and lipoxygenases may work in synergy to produce O2 •‾required for stress signalling. Such extracellular ROS produced by seeds appear to be important signalling components involved in wound and desiccation response, regeneration and growth
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