Phytoalexin biosynthesis genes are regulated and involved in plant response to Ralstonia solanacearum infection

Genes encoding phytoalexin biosynthesis enzymes are transcriptionally regulated and are required for defense against fungi and oomycetes. Here, we studied the regulation of tobacco 5-epi-aristolochene synthase 4 (EAS4) promoters on bacterial infection and investigated the roles of tomato and Arabidopsis phytoalexin biosynthesis genes in defense against pathogenic bacteria. Our results showed that the Nicotiana glutinosa EAS4 (NgEAS4) promoter was significantly induced by treatments with several bacteria in treated and systemic leaves of transgenic plants. This promoter was also partially induced by treatments with type-III secretion-deficient mutants and total lysate of R. solanacearum (Rs), revealing that both bacterial pathogen-associated molecular patterns (PAMPs) and effectors are involved in R. solanacearum-induced local and systemic activation of the NgEAS4 promoter. Furthermore, the absence of a cis-element GT-1 box in the NgEAS4 promoter abolished systemic activation in non-treated leaves, whereas disruption of the GT-1 box of the N. tabacum EAS4 (NtEAS4) promoter led to constitutive expression. Moreover, silencing of tomato sesquiterpene synthase genes and disruption of the key gene PAD3 for Arabidopsis camalexin biosynthesis resulted in decreased tolerance to R. solanacearum. These results together, reveal the varied function of GT-1 boxes in regulating tobacco EAS4 promoters and the involvement of phytoalexin-biosythesis genes in plant defense against R. solanacearum.