ABSTRACT
Ralstonia solanacearum is the causal agent of bacterial wilt (BW), one of the most important bacterial diseases worldwide. We used cDNA microarray to survey the gene expression profile in transgenic tomato (Solanum lycopersicum) overexpressing Arabidopsis (Arabidopsis thaliana) CBF1 (AtCBF1), which confers tolerance to BW. The disease-resistant phenotype is correlated with constitutive expression of the Related-to-ABI3/VP1 (RAV) transcription factor, ethylene-responsive factor (ERF) family genes, and several pathogenesis-related (PR) genes. Using a transient assay system, we show that tomato RAV2 (SlRAV2) can transactivate the reporter gene driven by the SlERF5 promoter. Virus-induced gene silencing of SlERF5 and SlRAV2 in AtCBF1 transgenic and BW-resistant cultivar Hawaii 7996 plants gave rise to plants with enhanced susceptibility to BW. Constitutive overexpression of SlRAV2 in transgenic tomato plants induced the expression of SlERF5 and PR5 genes and increased BW tolerance, while knockdown of expression of SlRAV2 inhibited SlERF5 and PR5 gene expression under pathogen infection and significantly decreased BW tolerance. In addition, transgenic tomato overexpressing SlERF5 also accumulated higher levels of PR5 transcripts and displayed better tolerance to pathogen than wild-type plants. From these results, we conclude that SlERFs may act as intermediate transcription factors between AtCBF1 and PR genes via SlRAV in tomato, which results in enhanced tolerance to BW.
Subject(s)
Arabidopsis Proteins/genetics , Disease Resistance , Gene Expression Regulation, Plant , Plant Diseases/immunology , Plant Proteins/genetics , Ralstonia solanacearum/physiology , Trans-Activators/genetics , Arabidopsis/genetics , Base Sequence , Chlorophyll/metabolism , DNA, Complementary/genetics , Gene Expression , Gene Expression Profiling , Gene Silencing , Genes, Reporter , Solanum lycopersicum/genetics , Solanum lycopersicum/immunology , Solanum lycopersicum/microbiology , Molecular Sequence Data , Oligonucleotide Array Sequence Analysis , Plant Diseases/microbiology , Plant Proteins/metabolism , Promoter Regions, Genetic/genetics , Sequence Analysis, DNA , Signal Transduction , Transcription Factors/genetics , Transcription Factors/metabolismABSTRACT
Bacterial wilt (BW), caused by Ralstonia solanacearum, is a devastating vascular disease of tomato worldwide. However, information on tomato's defense mechanism against infection by this soil-borne bacterium is limited. In this study, virus-induced gene silencing (VIGS) was employed to decipher signaling pathways involved in the resistance of tomato to this pathogen. Defined sequence fragments derived from a group of genes known or predicted to be involved in ethylene (ET) and salicylic acid (SA) signaling transduction pathways and mitogen-activated protein kinase (MAPK) cascades were subjected to VIGS in 'Hawaii 7996', a tomato cultivar with stable resistance to BW, and their effect on resistance was determined. The results indicated that silencing of ACO1/3, EIN2, ERF3, NPR1, TGA2.2, TGA1a, MKK2, MPK1/2 and MPK3 caused significant increase in bacterial proliferation in stembases and/or mid-stems. Partial wilting symptoms appeared on plants in which TGA2.2, TGA2.1a, MKK2 and MPK1/2 were silenced. These results suggested that ET-, SA- and MAPK-related defense signaling pathways are involved in the resistance of tomato to BW. This is the first report elucidating the multiple layers of defense governing the resistance of tomato to BW. The results are discussed to enlighten an important and complex interaction between tomato and a soil-borne vascular pathogen.
