Salicylic acid glucoside acts as a slow inducer of oxidative burst in tobacco suspension culture

Abstract

Salicylic acid β-glucoside (SAG) is a storage form of a defense signal against pathogens, releasing free salicylic acid (SA), to meet the requirements in plants. Since excess SA induces locally restricted cell death following oxidative burst and Ca2+ influx in plants, the effects of SAG on cell viability, Ca2+ influx, and generation of superoxide (O2•-) were examined in suspension-cultured tobacco BY-2 cells expressing aequorin. Among SA-related chemicals tested, only SAG induced the slow and long-lasting O2•- generation, although SAG was less active in acute O2•- generation, Ca 2+ influx and induction of cell death. The prolonging action of SAG is likely due to gradual release of SA and the data suggested that a peroxidase-dependent reaction is involved. Notably, pretreatment with low-dose SA (50 μM) enhanced the response to SAG by 2.5-fold. There are four possible secondary messengers in early SA signaling detectable in the BY-2 culture, namely O2•-, H2O2, Ca 2+ and protein kinase (PK). If these messengers are involved in the low-dose SA-dependent priming for SAG response, they should be inducible by low-dose SA. Among the four SA-inducible signaling events, PK activation was excluded from the low-dose SA action since a much higher SA dose (> 0.4 mM) was required for PK activation.