Abstract
Bacterial spot, incited by several Xanthomonas sp., is a serious disease in tomato (Solanum lycopersicum L.). Although genetics of resistance has been widely investigated, the interactions between the pathogen and tomato plants remain unclear. In this study, tanscriptomes of X. perforans race T3 infected tomato lines were compared to those of controls. An average of 7 million reads were generated with approximately 21,526 genes mapped in each sample post-inoculation at 6 h (6 HPI) and 6 days (6 DPI) using RNA-sequencing technology. Overall, the numbers of differentially expressed genes (DEGs) were higher in the resistant tomato line PI 114490 than in the susceptible line OH 88119, and the numbers of DEGs were higher at 6 DPI than at 6 HPI. Fewer genes (78 in PI 114490 and 15 in OH 88119) were up-regulated and most DEGs were down-regulated, suggesting that the inducible defense response might not be fully activated at 6 HPI. Accumulation expression levels of 326 co-up regulated genes in both tomato lines at 6 DPI might be involved in basal defense, while the specific and strongly induced genes at 6 DPI might be correlated with the resistance in PI 114490. Most DEGs were involved in plant hormone signal transduction, plant-pathogen interaction and phenylalanine metabolism, and the genes significantly up-regulated in PI 114490 at 6 DPI were associated with defense response pathways. DEGs containing NBS-LRR domain or defense-related WRKY transcription factors were also identified. The results will provide a valuable resource for understanding the interactions between X. perforans and tomato plants.
Highlights
Bacterial spot caused by at least four distinct species of Xanthomonas (X. euvesicatoria, X. vesicatoria, X. perforans, and X. gardneri), severely affects marketability of both fresh-market and processing tomato (Jones et al, 2000; Stall et al, 2009)
In addition to identification and genetic mapping of genes or quantitative trait loci (QTL), efforts have been making on discovery of genes participating in a complex molecular network of regulation during the time-course of hypersensitive response (HR) to race T3 in the unimproved tomato line Hawaii 7981 using microarray analysis approach (Gibly et al, 2004; Balaji et al, 2007), and identification of genes differentially expressed in the resistant line PI 114490 and a susceptible line OH 88119 during the timecourse of the race T3 infection using cDNA-AFLP techniques (Du et al, 2014)
Since the plant-pathogen interaction pathways (KEGG: 04626) summarized the genes involved in defense network of PTI and ETI, so we focused on the transcript dynamics of this Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways and enumerated the differentially expressed genes (DEGs) imparting the plant immune response (Table 5)
Summary
Bacterial spot caused by at least four distinct species of Xanthomonas (X. euvesicatoria, X. vesicatoria, X. perforans, and X. gardneri), severely affects marketability of both fresh-market and processing tomato (Jones et al, 2000; Stall et al, 2009). Comparison of genes differentially expressed in PI 114490 and OH 88119 provides us some information to understand the mechanism of resistance to bacterial spot race T3 in tomato during the process of symptom development at the 3, 4, and 5 days post spray-inoculation stages (Du et al, 2014). Both microarray and cDNA-AFLP can only identify part of the genes involved in resistance or defense processes due to their low throughput limitation. To fully unravel the mechanisms of field resistance to race T3 of bacterial spot, it is necessary to identify more genes differentially expressed during different infection times in tomato
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