A novel MYB transcription factor CaPHL8 provide clues about evolution of pepper immunity againstsoil borne pathogen

Abstract MYB TFs in plants are of crucial importance not only for growth and development but also for plant defense against pathogens. CaPHL8, an MYB TF, was identified as a positive regulator of pepper defense against Ralstonia solanacerum inoculation (RSI). Phylogenetic evaluation and functional characterization of CaPHL8 revealed its role in pepper defense evolution. Analysis of the amino acid sequence of PHL8 demonstrates its maximum similarity with the MYB family transcription factor in other plants. Up-regulation of CaPHL8 was observed in pepper plants facing Ralstonia attack.. Consistently the GUS activity of pCaPHL8 showed significantly high activity under RSI as compared to mock-treated plants. The loss of function studies of CaPHL8 conducted through VIGS (virus-induced gene silencing) confirmed the reduced pepper immunity to R. solanacearum and impaired plant growth accompanied by high pathogen growth. Compromised pepper immunity in silenced plants was coupled with a reduction in transcription of defense linked marker genes. On the other hand, transiently overexpressing CaPHL8 (35S::CaPHL8-HA) in pepper caused a hypersensitive response, elevated H2O2 production and high expression of immunity associated marker genes. Stable expression of CaPHL8-HA protein was confirmed by Western blot. Additionally, unlike many other TFs, CaPHL8 is not involved in high-temperature stress tolerance as evident by phenotype and non-significant transcription of high temperature-tolerance related marker genes in pepper. So, all these findings confirm that CaPHL8 is induced by RSI, not by high temperature and high humidity (HTHH). It provides adaptive plasticity to pepper by activating defense to RSI by direct or indirect regulation of different immunity -associated genes.