AAB Bioflux, 2012, Volume 4, Issue 2. http://www.aab.bioflux.com.ro 36 AAB BIOFLUX Advances in Agriculture & Botanics- International Journal of the Bioflux Society Effect of exogenous proline on protein pattern changes in Citrus sinensis (L.) Osbeck under in vitro salt stress Muayed F. Abbas, Abbas M. Jasim and Huda A. Al-Taha College of Agriculture, Basrah University, Basrah, Iraq. Corresponding author: M. F. Abbas, muayedfadhil@yahoo.co.uk Abstract. Effect of exogenous proline treatment on growth and protein pattern changes of plantlets of Citrus sinensis (L.) Osbeck cv. Local orange under NaCl stress was investigated. Under salt stress conditions, the height and leaf number of the plantlets decreased significantly, whereas the addition of proline to the culture media significantly alleviated the inhibitory effect of salinity. SDS-PAGE analyses of extracted protein revealed that plantlets grown under NaCl stress (10, 40 and 50mM) showed the presence of a high molecular weight protein (82.7, 81.3 and 81.5 kDa). The addition of proline at 50 and 75mg l -1 to the culture media induced the synthesis of three new proteins (20.4-21.6, 40.8-41.4 and 69.7-70.2 kDa). It is concluded that exogenous proline treatment induced the synthesis of salt responsive proteins, in addition to its well known physiological roles. Key Words: Citrus sinensis (L.), Proline, SDS-PAGE, salt stress. Introduction. Salinity is generally detrimental to plant growth, adversely affect plant metabolism and causes important modification in growth, development and gene expression of plants. Such modifications may lead to the accumulation or depletion of certain metabolites, alteration in the behavior of many enzymes, overall changes in protein synthesis, and of particular interest is the synthesis of new proteins (Dubey 1999). Several researchers have shown the synthesis of new proteins in cultured plant cells when they are subjected to salinity (Erickson & Alfinto 1984; Singh et al 1985; Bressan et al 1988; Ben-Hayyim et al 1989; Amini et al 2007). These specifically synthesized proteins under salt stress appear to have a role in providing tolerance or adaptation to the plants, although the mechanism of how such new proteins could provide adaptation to salinity is far from clear. Compatible solutes, such as proline, are known to accumulate under salt stress in many crops (Munns & Tester 2008). The exogenous application of proline has been suggested to be an effective approach in improving salt tolerance. Although, in the past most attention has been concerned with the role of proline as compatible solute and osmoprotectant. Its further role in salinity appears to involve the induction of salt responsive genes, with the resultant formation of new proteins which may improve the adaptation to salinity stress (Khedr et al 2003). Citrus is an important fruit crop worldwide. It is very sensitive to salinity, its critical level of salinity is 17mM NaCl, with production affected adversely with increasing salinity, up to 80Mm NaCl, where production is decreased by 50% (Storey & Walker 1999). No information is available in the literature regarding the effect of exogenous proline application on protein pattern changes in citrus under in vitro salt stress. Accordingly, the present work was undertaken to study the effect of exogenous proline on protein pattern changes in plantlets of Citrus sinensis (L.) Osbeck cv. Local orange under NaCl stress. Such information is utmost important to understand the molecular basis of salinity tolerance in crop plants.