Phytohormone signaling pathway for eliciting leaf phenolic synthesis in honeysuckle (Lonicera japonica Thunb.) under coastal saline environment

Abstract

To date, it remains unclear how saline stress induces leaf phenolic synthesis in honeysuckle (Lonicera japonica Thunb.), a traditional Chinese medicinal crop. This study was to investigate hormone signaling for salt-induced leaf phenolic synthesis in honeysuckle from molecular, physiological and ecological levels by pot experiments and field trials. As a major ingredient of soil salinity in coastal land, NaCl was used in salt treatment for simulating coastal saline environment. NaCl stress significantly increased leaf abscisic acid (ABA) and salicylic acid (SA) concentrations, potentiated leaf phenolic synthesis by elevating phenylalanine ammonia-lyase (PAL) activity and PALs transcription, and enhanced leaf phenolic accumulation. Under NaCl stress, tungstate and paclobutrazol pretreatments inhibited leaf ABA and SA accumulation, respectively, and lowered PAL activity, PALs transcription, and phenolic accumulation. Thus, ABA and SA participated in transducing salt-induced leaf phenolic synthesis. Paclobutrazol pretreatment hardly influenced leaf ABA concentration in salt-treated plants, but tungstate pretreatment abated salt-induced leaf SA accumulation, indicating that ABA was an upstream signal of SA. Under NaCl stress, SA application restored leaf SA level with no effect on ABA level but incompletely restored PAL activity and phenolic accumulation in tungstate-pretreated plants, confirming the downstream role of SA and implying the existence of other downstream signals. Leaf ABA and SA concentrations were higher in plants in coastal saline plots than non-saline plots, and the roles of ABA and SA in mediating salt-induced leaf phenolic synthesis passed ecological test, according to a significant positive correlation of leaf ABA and SA levels with phenolic concentration. In conclusion, ABA acted as an upstream signal of SA to elicit leaf phenolic synthesis in honeysuckle under coastal saline environment.