New resistance to bacterial wilt in heat-stressed tomato is revealed by two-reference Genome Wide Association
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Bacterial wilt, caused by bacterial strains of the Ralstonia solanacearum species complex, is one of the most harmful diseases striking many crops including tomato. Its spread is dependent upon temperature and humidity, which are expected to fluctuate strongly due to climate change. Previous results have highlighted that temperature elevation led to an increase in disease severity in commercial cultivars, whose resistance is quantitative and mostly relies on the Quantitative Trait Loci ( QTL ) bwr-6 and bwr-12 . In this study, we focused on temperature-dependent quantitative disease resistance (QDR) to bacterial wilt with the aim to unravel new resistance mechanisms that remain efficient at higher temperatures. For this purpose, a new panel of 189 accessions composed of tomato wild relatives, was assembled and sequenced thus creating a unique genomic resource. Its response to the Ralstonia pseudosolanacearum strain GMI1000 from three- to ten-days post-inoculation at 28°C and 32°C was explored. To discover the genetic basis underlying the responses of the panel, Genome-Wide Association (GWA) studies were conducted using the disease symptom scores recorded daily and monitored throughout the kinetics of the infection. To improve QTL detection, we have proposed a new approach using two reference genomes from within the panel. By correcting part of a single reference genome, especially when the only reference genome is a cultivar, this approach may be considered an alternative to pangenomic studies. As panel sequencing was highly resolutive, QTL positions allowed the identification of 44 candidate genes, which seemed to follow a temporal dynamic of activation after pathogen inoculation. Interestingly, no candidate genes were found to be common between the two phenotyping temperatures, highlighting the importance of the experimental design in addressing this type of question. Most of our quantitative disease resistance candidate genes belong to gene families described as being involved in immunity. Moreover, a significant proportion appears to be expressed in roots where bacterial infection occurs. Among them, two candidates are closely linked to the genomic positions of the bwr-6 and bwr-12 QTLs , the main QTLs of bacterial wilt Quantitative Disease Resistance (QDR) studied whose mechanisms of action are still unknown.
Author summary
Bacterial wilt is a plant disease that affects more than 200 crop species (including tomato, potato and banana) leading to high yield losses. The most efficient way to deal with this disease is still the use of genetic resistance. However, previous studies have shown that numerous sources of resistance are negatively affected when plants face heat stress, which is alarming in a context of global warming. In this study, we have developed a new approach for discovering candidate genes in tomato capable of conferring thermostable resistance to bacterial wilt. Forty-four genes were sequentially detected over time, reflecting different temporal dynamics of induction after inoculation. Even if none were found to be common between the two temperatures, available information on their transcriptional regulation in roots and their involvement in immune processes confirms their relevance. Finally, we provide a short list of the candidate genes identified, some of which are currently undergoing functional validation and will be used in breeding programs to help overcome epidemics in future years.
