A Genome-wide association study identifies candidate genes for heat tolerance in adult cucumber plants

Heat stress causes overgrowth, leaf dryness and fruit malformation, which negatively impacts cucumber quality and yield. Yet, in spite of the devastating consequences of this abiotic stress, few genes for heat tolerance in cucumber have been identified. Here, the heat injury indices of 88 cucumber accessions representing diverse ecotypes were collected in two open-field environments, with naturally occurring high temperatures over two years. Seventeen of the 88 accessions were identified as highly heat-tolerant. Using a genome-wide association study, five loci (gHII3.1, gHII3.2, gHII3.3, gHII4.1 and gHII6.1) on three chromosomes associated with heat tolerance were detected. Pairwise linkage disequilibrium correlation, sequence polymorphisms, and qRT-PCR analyses at these loci, identified five candidate genes predicted to be casual for heat stress response in cucumber. CsaV3_3G04883, CsaV3_4G029050 and CsaV3_6G005370 each had nonsynonymous SNPs, and were significantly up-regulated by heat stress in the heat-tolerant genotypes. CsaV3_3G031890 was also induced by heat stress, but in the heat-sensitive genotypes, and sequence polymorphism was only found in the promoter region. Identifying these candidate genes lays a foundation for understanding cucumber thermotolerance mechanisms. Our study is one of the few to examine heat stress in adult cucumber plants and it therefore fills a critical gap in knowledge. It is also an important first-step towards accelerating the breeding of robust heat-tolerant varieties.