In vivo cell wall loosening by hydroxyl radicals during cress seed germination and elongation growth

Abstract

Loosening of cell walls is an important developmental process in key stages of plant life cycles, including seed germination, elongation growth and fruit ripening. Here we report direct in vivo evidence for hydroxyl radical (•OH)-mediated cell wall loosening during plant seed germination and seedling growth. We used electron paramagnetic resonance (EPR)-spectroscopy to show that •OH is generated in the cell wall during radicle elongation and weakening of the endosperm of cress (Lepidium sativum L., Brassicaceae) seeds. Endosperm weakening precedes for radicle emergence, as demonstrated by direct biomechanical measurements. By 3H-fingerprinting we showed that wall polysaccharides are oxidised in vivo by the developmentally regulated action of apoplastic •OH in radicles and endosperm caps: the production and action of •OH increased during endosperm weakening and radicle elongation and were inhibited by the germination-inhibiting hormone abscisic acid. Both effects were reversed by gibberellin. Distinct and tissue-specific target sites of •OH attack on polysaccharides were evident. In vivo •OH attack on cell wall polysaccharides were not only evident in germinating seeds, but also in elongating maize (Zea mays L., Poaceae) seedling coleoptiles. We conclude that plant cell wall loosening by •OH is a controlled action of this type of reactive oxygen species (ROS).