Minisymposium 21: Integrative Plant Biology

Abs # 35002: The NO world: a new frontier of plant biology

Presenter:	Yamasaki, Hideo , yamasaki@comb.u-ryukyu.ac.jp
Authors	Yamasaki, Hideo  (A)
Affiliations:	(A): Center of Molecular Biosciences (COMB), University of the Ryukyus

Most living organisms on earth are surrounded by air, a gaseous mixture consisting of 78% N₂, 21% O₂, and 0.03% CO₂. Plants play important roles in the circulation of these gasses on global scale through photosynthetic activities by green leaves and N₂ fixation activities by symbionts. In the past many decades, plant biology has highlighted on the beneficial side of these important plant metabolisms at both basic and applied levels. The discovery of reactive oxygen species (ROS) and reactive nitrogen species (RNS) endogenously produced in the cells has pointed out unexplored features of the metabolisms, namely, the presence of the mechanisms for coping with toxicity of oxygen and nitrogen in the air. Nitric oxide (NO), a RNS, is produced through industrial activities including combustion of fossil fuels. It has been shown, largely in medical science fields, that NO is also produced endogenously by the enzyme NO synthase (NOS) in vertebrate, invertebrates, insects and bacteria. In contrast, the mechanism for enzymatic NO production in plants and algae is still in debate. Until recently, NOS had been presumed as the only enzyme that could produce NO in plant cells. Despite many efforts to identify a gene and protein similar to mammalian-type NOS, however, there has been no substantial evidence to conclude the presence of such NOS in plants. Since 1999, we have proposed an alternative NO production mechanism for plants. Nitrate reductase (NR) is a well-known protein for plant biologists because it is a key enzyme of nitrate assimilation metabolism. The enzyme normally catalyzes the reduction of nitrate to form nitrite using NAD(P)H. We have shown that the enzyme is capable of further reducing the product nitrite to produce NO as the result. Importantly, NO can be subsequently converted to peroxynitrite, the most toxic RNS, under aerobic conditions. Although we cannot exclude a possibility for discovering a new type NOS that is unique to plants, it is now evident that plants do produce NO by a distinct mechanism from animals. These findings, the classical enzyme possessing unexplored new functions, offers us a new opportunity to re-consider and re-investigate plant biology in terms of "NO". Because there is increasing evidence to suggest that NO is involved in many fundamental plant activities, the molecule would be an important key to integrate multidisciplinary areas of plant biology from the molecular toward the global scale.