Castor Plant Adaptation to Salinity Stress during Early Seedling Stage by Physiological and Transcriptomic Methods

The early seedling stage is considered the most vulnerable period for plants, especially under salinity conditions. The castor plant (<i>Ricinus communis</i>) is a well-known oil and energy crop worldwide that can survive under stressful conditions. However, the specific mechanisms of this species during its early seedling stage under salt stress are still not clearly understood. Here, the physiological and transcriptome changes in the cotyledons and roots of the castor plant were evaluated. The results indicated that salt stress (150 mM NaCl, 6 d) increased malondialdehyde (MDA) and proline content, whereas it decreased dry weight (DW) and soluble sugar content. The Illumina Hiseq 2500 platform was used to analyze transcriptome profiles in the cotyledons and roots under salt stress conditions. The results showed that 1580 differentially expressed genes (DEGs) were found in the cotyledons (880 upregulated and 700 downregulated) and 1502 DEGs in the roots (732 upregulated and 770 downregulated). Furthermore, we found that salt stress significantly regulated 22 genes (e.g., <i>29520.t000005</i>, <i>29633.t000030</i>, and <i>29739.t000024</i>) involved in protein processing in the endoplasmic reticulum of the cotyledons. However, salt stress induced the expression of 25 genes (e.g., <i>30068.t000101</i>, <i>30076.t000022</i>, <i>29970.t000022</i>, and <i>29957.t000027</i>) involved in phenylpropanoid biosynthesis in the roots. In addition, a large number of genes participating in plant hormone signal transduction, starch and sucrose metabolisms, and arginine and proline metabolisms were induced in both cotyledons and roots. In conclusion, this study demonstrated that the different expression patterns in cotyledons and roots as well as their synergic relationship contributed to enhancing the salt tolerance of castor plants.