Identification of candidate thermotolerance genes during early seedling stage in upland cotton (Gossypium hirsutum L.) revealed by comparative transcriptome analysis

High-temperature (HT) stress is a major environmental stress that limits cotton growth, metabolism, and yield worldwide. The identification and characterization of thermotolerance is restricted by the plant growth environment and growth stage. In this study, four genotypes of upland cotton (Gossypium hirsutum L.) with known field thermotolerance were evaluated under normal and HTs at the seedlings stage in a growth cabinet with 11 physiological, biochemical, and phenotypic assays. Consistent with previous field observations, the thermotolerance could be identified by genotype differences at the seedling stage under HT in a growth cabinet. Comparative transcriptome analysis was performed on seedlings of two contrasting cotton genotypes after 4 and 8 h of HT exposure. Gene ontology analysis combined with BLAST annotations revealed a large number of HT-induced differentially expressed genes (4,698) that either exhibited higher expression levels in the heat-tolerant genotype (Nan Dan Ba Di Da Hua) compared with the heat-sensitive genotype (Earlistaple 7), or were differentially expressed only in Nan Dan Ba Di Da Hua. These genes encoded mainly protein kinases, transcription factors, and heat-shock proteins, which were considered to play key roles in thermotolerance in upland cotton. Two heat-shock transcription factor genes (homologs of AtHsfA3, AtHsfC1) and AP2/EREBP family genes (homologs of AtERF20, AtERF026, AtERF053, and AtERF113) were identified as possible key regulators of thermotolerance in cotton. Some of the differentially expressed genes were validated by quantitative real-time PCR analysis. Our findings provide candidate genes that could be used to improve thermotolerance in cotton cultivars.