Ethylene, Nitric Oxide and Haemoglobins in Plant Tolerance to Flooding

As much as 12% of the world's soils may suffer excess water so that flooding is a major limiting factor on crop production in many areas. Plants attempt to deal with submergence by forming root aerenchyma to facilitate oxygen diffusion from the shoot to the root, initiating a hyponastic response where petiole elongation facilitates access to atmospheric oxygen or initiating a bio-energetically conserving quiescence phase. Ethylene has well established roles in the initiation of programmed cell death (PCD) to form air-spaces in aerenchyma and in the hyponastic responses in petioles. The flooding-tolerant species Rumex palustris and the model plant Arabidopsis thaliana have been extensively exploited to reveal some key molecular events. Our groups have recently demonstrated that nitric oxide (NO) triggers the biosynthesis of ethylene during stress and that NO plays key roles in PCD and the hyponastic response. NO is formed from the reduction of NO3/NO2 via several pathways, which are differentially utilized depending on the availability of O-2. In fact, NO production and responses to flooding can be directly dependent on the nitrogen status of soil, which reflects local agricultural practice. This chapter will detail our understanding of the roles of ethylene, NO and haemoglobin in flooding stres