Microbiol. Res. (1998) 153, 297-307 © Gustav Fischer Verlag Review Plant-cyanobacterial symbiotic somaclones as a potential bionitrogen-fertilizer for paddy agriculture: biotechnological approaches R. P. Sinha 1,2, A. Vaishampayan I, D.-P. Hader? I Photobiological Nitrogen Fixation Research Laboratory, Department of Genetics and Plant Breeding, Institute of Agricultural Sciences, Banaras Hindu University, Varanasi-221005,India z Institutflir Botanikund PharmazeutischeBiologie, Friedrich-Alexander-Universitat, Staudtstr.5, D-91058, Erlangen,Germany Accepted: November 22, 1998 Abstract The role of Ny-fixing cyanobacteria in improving the fertility of rice paddy fields is well known. Among all symbiotic as- sociations, involving cyanobacteria, the Azalia-Anabaena complex is very efficient in water-logged rice paddy fields, contributing significant levels of nitrogen and organic matter thereby improving the physico-chemical properties of soils. This review focuses on some of the recent advances made in the areas of developing genetically improved somaclones of Azalia-Anabaena associative complex, and artificial plant- cyanobacterial symbiotic associations. Key words: Azalia - cyanobacteria - somaclonal mutation - symbiosis - higher plants 1 Introduction The majority of cyanobacterial species display a unique combination of 'higher plant-type' oxygenic photo- synthesis and 'bacteria-type' oxyphobic nitrogen fixa- tion (Stewart 1980). The significant role of these N 2 - fixing microorganisms in improving the soil fertility of paddy fields, at the sole expense of photosynthetic energy produced on their own, is well documented (Singh 1961; Roger and Kulasooriya 1980; Venkatara- man 1981; Watanabe 1984; Sinha and Hader 1996a). Corresponding author: D.-P. Hader Tel.: +49 9131858216, Telefax: +49 9131858215 e-mail: dphaeder@biologie.uni-erlangen.de However, the viability and nitrogen-fixing potential of these nitrogen fixers is affected by abiotic stress factors (Hader et al. 1995; Sinha and Hader 1996 a, 1996 b, and 1997; Hader and Worrest 1997). Also modern agri- cultural fields are generally treated with heavy doses of synthetic agro-chemicals such as herbicides and nitro- genous fertilizers to maintain the high-yield crop varieties. Of these, the herbicides are generally designed to be photosynthetic electron transport inhibitors or uncouplers (Dodge 1975), thereby killing the target weeds as well as the cyanobacteria because of their identical photosynthesis (Singh and Vaishampayan 1978; Vaishampayan 1984 a, 1984b, 1985 a, and 1985 b). Synthetic nitrogen fertilizers repress N 2 fixation above a threshold level as these organisms switch off their N 2 - fixing machinery as soon as the concentration of NH 4 + in the external environment exceeds its biological con- sumption (Singh et al. 1978 a, and 1978 b; Vaisham- payan 1981, 1982 a, 1982 b, 1983 a, 1983 b, and 1995) thereby converting the N 2 fixer into an NH 4 + consumer. The net result is the fixation of no more than 27-30 kg N per ha per annum by the diverse cyanobacterial spe- cies (Singh and Singh 1989). The cyanobacteria, however, maintain their fast growth rate, simple genome organization, response to DNA transformation and an efficient homologous re- combination system, apart from their 'plant-type' 02- evolving photosynthesis, chlorophyll biosynthesis and amino acid synthesis (Vaishampayan and Sahay 1984; Vaishampayan 1988; Haselkorn 1992 and 1995). Be- Microbiol, Res. 153 (1998) 4 297