β-aminobutyric acid induces the accumulation of pathogenesis-related proteins in tomato (Lycopersicon esculentum L.) plants and resistance to late blight infection caused by Phytophthora infestans

Tomato (Lycopersicon esculentum L.) plants were sprayed with aqueous solutions of isomers of aminobutyric acid and were either analyzed for the accumulation of pathogenesis-related (PR) proteins or challenged with the late blight fungal agent Phytophthora infestans. The β isomer of aminobutyric acid induced the accumulation of high levels of three proteins: P14a, β-1,3 glucanase, and chitinase. These proteins either did not accumulate or accumulated to a much lower level in α- or γ-aminobutyric acid-treated plants. Plants pretreated with α-, β-, and γ-aminobutyric acid were protected up to 11 d to an extent of 35, 92, and 6%, respectively, against a challenge infection with P. infestans. Protection by β-aminobutyric acid was afforded against the blight even when the chemical was applied 1 d postinoculation. Examination of ethylene evolution showed that α-aminobutyric acid induced the production of 3-fold higher levels of ethylene compared with β-aminobutyric acid, whereas γ-aminobutyric acid induced no ethylene production. In addition, silver thiosulfate, a potent inhibitor of ethylene action, did not abolish the resistance induced by β-aminobutyric acid. The results are consistent with the possibility that β-aminobutyric acid protects tomato foliage against the late blight disease by a mechanism that is not mediated by ethylene and that PR proteins can be involved in induced resistance.