Possible structure-activity profile of salicylate derivatives: their relationship on induction of systemic acquired resistance

In nature plants can be induced systemically to become more resistant to diseases through some biotic or abiotic inducers. One of the commonly used chemicals inducers is salicylic acid (SA), which appears to mimic the systemic effects of localized infection. The potential of some chemical inducers of systemic acquired resistance (SAR) to reduce Alternaria leaf spot disease on tomato was assessed in glasshouse trials. Possible effects of using SA and some of its 5position derivatives including, 5-chlorosalicylic acid, 5-methoxy salicylic acid, 5aminosalicylic acid, 5-fluorosalicylic acid, 5-methylsalicylic acid and 5-nitrosalicylic acid in host resistance induction have been investigated. Foliar applications of salicylate derivatives with concentrations of 400 µM have been tested for disease resistant induction. Among the selected mono-substituted salicylate derivatives that have been tested, applications of SA and 5methoxysalicylic acid have induced disease resistance significantly as well as increasing the SA accumulation in plant tissues, compared with not treated control plants. Treatments such as 5chlorosalicylic acid and 5-nitrosalicylic acid had a deleterious effect on SA accumulation in plant tissues as well as in disease resistance induction. The analysis results obtained by high performance thin layer chromatography (HPTLC) indicated a high correlation between the accumulation of endogenous level of free SA content in treated plants and the reduction of Alternariosis symptoms on tomato plant. The results indicated that, among the salicylate derivatives, the biochemical activators containing electron donating groups are more suitable for inducing disease resistance in tomato crop.