Determination of proline concentration, an abiotic stress marker, in root exudates of excised root cultures of fruit tree rootstocks under salt stress

Root exudates have important, yet incompletely known, roles in plant physiology, mainly in rhizosphere interactions with other organisms. Proline might play a critical role in protecting plants under stress, particularly under saline conditions. However, as far as we know, no attempts have been made to determine the proline content of root exudates. Here, we present preliminary results of the quantification of proline in root exudates of excised root cultures of Prunus rootstocks. Roots from micro-propagated Prunus rootstocks ( ̳Masto de Montaðana,‘ GF677 and ̳Adesoto 101‘) were cultured in vitro under increasing NaCl concentrations (0, 20, 60, 180 mM) to detect their early tolerance to salt stress. After three weeks of culture, the proline content of both the root tissues and the MS-based liquid medium in which the roots were cultured was determined. The proline concentration in root tissues and root exudates from all rootstocks increased as salt concentration in the medium increased, following a trend similar to that of whole plant tissues. This indicated that the role of proline exudates in plant responses to salt stress could be studied using non-destructive methods. In addition, proline exudates could be useful for the early detection of salt stress tolerance, provided that a relationship between proline and salt stress tolerance could be found.