NB-LRR genes: characteristics in three Solanum species and transcriptional response to Ralstonia solanacearum in tomato

NB-LRR genes in the three Solanum species showed specific constitution characteristics and evolved multiple clusters and duplicates. Some genes could respond to biotic stresses such as tomato bacterial wilt. Nucleotide-binding and leucine-rich repeat (NB-LRR, NLR) is a largest resistance gene family in plants, which plays a key role in response to biotic stresses. In this study, NB-LRR genes in cultivated tomato Solanum lycopersicum (Sl) and its wild relatives S. pennellii (Spe) and S. pimpinellifolium (Spi) were analyzed using bioinformatics approaches. In total, 238, 202 and 217 NB-LRR genes of 8 different types were found in Sl, Spe and Spi, respectively. The three species showed similar genomic characteristics. The NB-LRR genes were mainly distributed on chromosomes 4, 5 and 11 and located at the distal zones, forming multiple clusters and tandem duplicates. A large number of homologs appeared through gene expansion, with most Ka/Ks values being less than 1, indicating that purifying selection had occurred in evolution. These genes were mainly expressed in root and could respond to different biotic stresses. RT-qPCR analysis revealed that SlNLR genes could respond to tomato bacterial wilt, with SlNLR1 probably involved in the resistance response, whereas others being the opposite. The transcription factors (TFs) and interaction proteins that regulate target genes were mainly Dof, NAC and MYB families and kinases. The results provide a basis for the isolation and application of related genes in plant disease resistance breeding.