Genome-Wide Identification of Lettuce GRAS Gene Family Reveals That LsGRAS13 Is Negative Regulator of Thermally Induced Bolting in Lactuca sativa

Lettuce is susceptible to high-temperature stress during cultivation, which leads to bolting and affecting yield. However plant-specific transcription factors known as GRAS proteins play an important role in regulating plant growth development and abiotic stress responses. In this study, the whole LsGRAS genome of lettuce was identified. The results showed that 59 LsGRAS genes were distributed unevenly across nine chromosomes, 100% of which were located in the nucleus. Phylogenetically classified into nine conserved subfamilies. Chromosome localization and gene structure analysis suggested that duplication events and a large number of intronless genes may be the reason for the massive expansion of the LsGRAS gene family. To investigate the expression profiles of these genes in lettuce, we analyzed the transcription levels of all 59 LsGRAS genes in RNA-Seq data under high-temperature treatment with exogenous melatonin. We found that the expression level of LsGRAS13 was higher on 6, 9, 15, 18, and 27 days under high-temperature treatment with melatonin compared to the same treatment days without melatonin. Functional assays revealed that silencing LsGRAS13 resulted in accelerated bolting of lettuce, whereas the flower bud differentiation rate was faster in LsGRAS13-silenced plants than in control plants.  In this study, the LsGRAS gene was comprehensively annotated and analyzed. Meanwhile, the expression pattern of the LsGRAS gene under high-temperature treatment was deeply explored, which is of great significance for the response mechanism of plants to high-temperature stress and the improvement of high-temperature stress resistance of lettuce, and provides valuable information and candidate genes.