Salicylic acid (SA) is widely distributed in many monocots and dicots and has many physiological effects. It can induce heat production in the thermogenic inflorescences of Arum lily([1]), block the biosynthesis of ethylene, and more attractively, it seems to be an important natural signal molecule in the induction of systemic acquired resistance (SAR) in tobacco, cucumber and other plants([2,3]). Studies in recent years showed that SA was also intimately related to the resistance of plants to aboitic stress, for example, SA increased chilling resistance of maize seedlings. Hence, SA has been accepted as a kind of new plant hormones. Up to date, the quantification of SA usually has been performed by HPLC[4,5], which often needs a large quantity of sample and a verbose pretreatment. Compared to HPLC, immunoassays, including radio-immunoassays (RIA) and enzyme-immunoassays (EIA), are easy to perform and have been widely used in the quantification of other plant hormones, such as IAA([6]), ABA([7,8]), GAs([9]), cytokinins et al([10]), and jasmonic acid (JA)([11]), and other low-molecular-weight, none-immunogenic compounds in plants([12]). Till now, only an indirect enzyme-linked immunosorbent assay (ELISA) for SA based on polyclonal antibodies (PAbs) has been developed by our group([13]), although Bennett et al([14]) had prepared SA PAbs using 4-aminosalicylic acid linked to KLH as immunogen in goat. However, the sensitivity of the ELISA we established formerly was relatively low, and also relatively larger quantity of sample is needed than other ELISAs for plant hormones. In this paper, an ELISA for SA based on monoclonal antibody raised against SA-NH-CH2-NH-KLH was introduced, and the fluctuation of SA content in cucumber leaves after inoculated with Pseudomonas syringae pv. syringae was determined.
