Financial support: Ministry of Science, Research and Technology of Iran.

Keywords: Programmed Cell Death (PCD), rice, senescence.

Present address: #Research Centre for Applied Sciences, Jahade Daneshgahi, Shahid Beheshti University, Evin, Tehran, Iran. Tel: 0098-21-2402097 (ext. 213). Fax: 0098-21-9403884. §European Commission, DG12, Office SDME 9/28, 200 Ruela Lai, 1049, Brussels, Belgium.

Single Strand-Preferring Nucleases (SSPNs) have been implicated in the triggering and progress of cell death pathways, including senescence in higher plants, though the biological roles of such enzymes are still obscure. In the present study, heterotrophic cell suspension cultures of Oryza sativa L. (rice) cv Taipei 309 were used to investigate changes in Single Strand-Preferring Nuclease activity associated with cell death in vitro. An acid nuclease activity (pH 5.5) was found which was strongly stimulated in the presence of 10 mM Zn²⁺ and inhibited by 10 mM EDTA and EGTA. An increase in SSPN activity was concomitant with a loss of cell viability, total protein and the onset of stationary phase of growth in the cell cultures. Using DNA-SDS-PAGE two major SSPNs were detected with the molecular weights of 26 KDa and 53.5 KDa. There are possibilities that these two enzymes represent monomeric and dimeric forms of the same nuclease or they are two different enzymes. The 26 KDa nuclease was partially purified using heparin-Sepharose column chromatography. The results confirm the value of plant cell suspension cultures for the investigation of the molecular process underlying plant cell ageing, senescence processes and programmed cell death (PCD). Possible senescence-associated roles of SSPNs are discussed.