Salt Stress Leads to Morphological and Transcriptional Changes in Roots of Pumpkins (<i>Cucurbita</i> spp.)

Salinity stress poses a major challenge to agricultural productivity worldwide, including for pumpkin, a globally cultivated vegetable crop with great economic value. To deal with salt stress, plants exhibit an array of responses such as changes in their root system architecture. However, the root phenotype and gene expression of pumpkin in response to different concentrations of NaCl remains unclear. To this end, this study evaluated the effects of salinity stress on root architecture in <i>C</i>. <i>moschata</i> (Cmo-1, Cmo-2 and Cmo-3) and <i>C</i>. <i>maxima</i> (Cma-1, Cma-2 and Cma-3), as well as their hybrids of <i>C</i>. <i>moschata</i> and <i>C</i>. <i>maxima</i> (Ch-1, Ch-2 and Ch-3) at the germination and seedling stages. The results showed that the total root length and the number of root tips decreased by more than 10% and 5%, respectively, under 180 mM NaCl conditions compared to those under the 0 mM NaCl conditions. In contrast, the total root length and the number of root tips were increased or decreased under 60 mM NaCl conditions. Meanwhile, salt stress was considered severe when treated with more than 120 mM NaCl, which could be used to evaluate the salt tolerance of the germplasm resources of pumpkin. In addition, the transcriptional changes in the roots of both Cmo-3 and Cma-2 under salt stress were analyzed via RNA-sequencing. We found 4299 and 2141 differential expression genes (DEGs) in Cmo-3 and Cma-2, respectively. Plant hormone signal transduction, Phenylpropanoid biosynthesis and the MAPK signaling pathway were found to be the significant KEGG pathways. The expression of <i>ARF</i> (<i>auxin response factor</i>), <i>B-ARR</i> (<i>type-B response regulator</i>) and <i>PYR</i> (<i>pyrabactin resistance</i>)/<i>PYL</i> (<i>PYR-LIKE</i>) genes was downregulated by NaCl treatment. In contrast, the expression of <i>SnRK2</i> (<i>sucrose non-fermenting-1-related protein kinase 2</i>) and <i>AHP</i> (<i>histidine-containing phosphotransmitter</i>) genes was downregulated in Cmo-3 and upregulated in Cma-2. These findings will help us better understand the mechanisms of salt tolerance in pumpkins and potentially provide insight into enhancing salt tolerance in crop plants.