Rebirth and death: nitric oxide and reactive oxygen species in seeds.

Seed germination and early seedling growth are critical phases in the plant life cycle (Bewley and Black, 1994). Young seedlings need to grow rapidly in order to effectively compete with neighbours, yet young seedlings have limited nutrient reserves to draw on as they resume growth. Root systems are rudimentary, and this limits the ability of young seedlings to explore the environment for water and minerals. Developing the photosynthetic capacity required for autotrophic growth is another draw on stored reserves. Young seedlings are also vulnerable to adverse environmental conditions such as insufficient water and extremes of temperature. In order to increase the likelihood of early survival, most plant seeds are dormant at maturity, that is, they will not germinate under environmental conditions that would promote germination of nondormant seeds (Bewley, 1997; Bewley and Black, 1994; Finch-Savage and LeubnerMetzger, 2006). This is a highly adaptive property that allows plants to delay the growth of the next generation until the season or local environmental conditions are suitable. Dormancy loss may occur in dry storage after sufficient time has passed, a process referred to as after-ripening, or may require specific environmental signals. Examples of these are availability of soil nitrogen, exposure to light, a period of damp and cold, or exposure to compounds in smoke. These signals indicate times of the year or local environmental conditions that are likely to be favourable for seedling growth, and which, therefore, increase the likelihood of early seedling survival. The embryo and nutrient tissues within dormant seeds have the potential for great longevity. In many cases, seeds remain viable for years or decades. Following dormancy loss and imbibition of water, combinations of plant hormones and signalling molecules promote germination, and the renewed growth and development of the quiescent embryo. In some seeds, these same hormones and signals initiate the programmed death of nutritive tissues. This chapter will focus on the involvement of plant hormones, reactive oxygen species (ROS) and nitric oxide (NO) in seed germination and early seedling growth in two plant models, Arabidopsis thaliana (Arabidopsis) and Hordeum vulgare (barley). The data support the conclusion that NO functions to remove dormancy and promote