Soil Environ. 31(1): 30-36, 2012 www.se.org.pk Online ISSN: 2075-1141 Print ISSN: 2074-9546 *Email: kannikamsu13@yahoo.com © 2012, Soil Science Society of Pakistan (http://www.sss-pakistan.org) Selection of efficient salt-tolerant bacteria containing ACC deaminase for promotion of tomato growth under salinity stress Kannika Chookietwattana * and Kedsukon Maneewan Department of Biotechnology, Faculty of Technology, Mahasarakham University, Thailand Abstract For successful application of plant growth promoting bacteria (PGPB) in salt-affected soil, bioinoculant with salt- tolerant property is required in order to provide better survival and perform well in the field. The present study aimed to select the most efficient salt-tolerant bacterium containing 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase from eighty four bacterial strains and to investigate the effects of the selected bacterium on the germination and growth of tomato (Licopersicon esculentum Mill. cv. Seeda) under saline conditions. The Bacillus licheniformis B2r was selected for its ability to utilize ACC as a sole nitrogen source under salinity stress. It also showed a high ACC deaminase activity at 0.6 M NaCl salinity. Tomato plants inoculated with the selected bacterium under various saline conditions (0, 30, 60, 90 and 120 mM NaCl) revealed a significant increase in the germination percentage, germination index, root length, and seedling dry weight especially at salinity levels ranging from 30-90 mM NaCl. The work described in this report is an important step in developing an efficient salt-tolerant bioinoculant to facilitate plant growth in saline soil. Key words: ACC deaminase, salt stress-induced ethylene, salt-tolerant bacteria Introduction Salt-affected soil can result in either the reduction of crop productivity or abandonment of many agricultural areas (Maas, 1990). Under high soil salinity, plant biosynthesizes ethylene by using 1-aminocyclopropane-1- carboxylic acid (ACC) as a precursor, and hence called salt stress-induced ethylene. Numerous reports have documented that ethylene could inhibit the elongation of plant root and shoot (Jusaitis, 1986; Penrose and Glick, 2003), suppress leaf expansion (Peterson et al., 1991) and promote epinasty (Abeles et al., 1992). Thus, reduction of salt stress-induced ethylene production in plant is one of the most crucial strategies to increase agricultural production in saline soil. In the last decade, the concept of plant growth promoting bacteria (PGPB) containing ACC-deaminase for promotion of plant growth under environmental stress conditions has gained importance (Berg, 2009). These bacteria are capable of metabolizing ACC in the root of developing plants, thereby reducing the adverse effects of ethylene on plant growth (Ghosh et al., 2003; Glick, 2005; Hontzeas et al., 2006). However, the ability of inoculated bacteria to survive, outcompete with the native microflora, and colonize in the rhizosphere remains to be the challenge for successful application (Bashan, 1998) especially in saline soil since high salinity could influence the survival, growth and activity of microorganisms (Bremer and Krämer, 2000). The salt-tolerant ACC deaminase- containing bacteria could thus be advantageous over others to thrive in a new saline environment in the sufficient numbers to deliver beneficial effects on plants. Although the achievements of ACC deaminase-containing bacteria in promoting plant growth under various environmental stresses were reported immensely (Grichko and Glick, 2001; Dey et al., 2004; Mayak et al., 2004; Kausar and Shahzad, 2006; Shaharoona et al., 2006; Belimov et al., 2008; Zahir et al., 2008), little information is available on the effectiveness of salt-tolerant bacteria containing ACC deaminase under high salinity (Nadeem et al., 2006). Therefore, in this research study, attempts have been made to screen and select an efficient salt-tolerant ACC deaminase containing bacterium strain which could utilize ACC as a sole nitrogen source under salt- stressed conditions. Its effectiveness for growth promotion of tomato (Licopersicon esculentum Mill cv. Seeda) under in vitro and saline conditions has also been evaluated. Materials and Methods Microorganisms and culture conditions Eighty-four bacterial strains, which were isolated from saline soil in Thailand, and identified on the basis of phenotypic characterization, and 16S rDNA sequence analysis from the previous work of Chookietwattana (2003), were used in the experiment. For inoculum preparation, bacterial strains were individually cultured at