The effect of soil salinity and pepino mosaic virus infection on the expression of tomato defence-related genes

Abstract

To reflect field conditions where cultivated plants are frequently exposed to multiple stresses simultaneously, the effect of salinity on the expression of defence-related genes in tomato plants was examined in the absence and presence of pepino mosaic virus (PepMV) infection. Tomato seedlings were grown under three soil salinity regimes from the fourth day after transplantation: (a) no-added salt (control), (b) low salinity (20 mM NaCl) and (c) high salinity (100 mM NaCl). The plants were non-, mock- or PepMV-inoculated 14 days after transplantation and systemic leaves were collected 4 days postinoculation for analysis by reverse transcription-quantitative PCR (RT-qPCR). In noninoculated tomato plants exposed to low and high salinity, expression of the salicylic acid (SA)-, jasmonic acid-, ethylene-related genes was typically down-regulated compared to control plants, with only a slight (nonsignificant) increase in ABA2 expression. In PepMV-infected tomato plants, a strong induction of SA-related genes was observed, which was compromised under low and high salt stress. The induction of PR1 gene expression by PepMV infection was the highest (an 82-fold increase) in control plants, but was significantly less (a 20-fold increase) under conditions of low and high soil salinity. No direct correlation between SA gene expression levels and PepMV RNA accumulation became apparent. In RT-qPCR, the levels of PepMV genomic RNA and tomato LoxD and ACS1 RNA transcripts under mild salt stress were significantly reduced (the former 6-fold), but under high salt stress the levels of all were similar to those in the nonstressed controls.