Physiological and Proteomic Responses of Diploid and Tetraploid Black Locust (Robinia pseudoacacia L.) Subjected to Salt Stress

Tetraploid black locust (Robinia pseudoacacia L.) is adaptable to salt stress. Here, we compared morphological, physiological, ultrastructural, and proteomic traits of leaves in tetraploid black locust and its diploid relatives under salt stress. The results showed that diploid (2x) plants suffered from greater negative effects than those of tetraploid (4x) plants. After salt treatment, plant growth was inhibited, photosynthesis was reduced, reactive oxygen species, malondialdehyde content, and relative electrolyte leakage increased, and defense-related enzyme activities decreased in 2x compared to those in 4x. In addition, salt stress resulted in distorted chloroplasts, swollen thylakoid membranes, accumulation of plastoglobules, and increased starch grains in 2x compared to those in 4x. However, 4x developed diverse responses under salt stress. A comparative proteomic analysis revealed that 41 and 37 proteins were differentially expressed in 2x and 4x, respectively. These proteins were mainly involved in photosynthesis, stress and defense, energy, metabolism, transcription/translation, and transportation. Distinct patterns of protein changes between 2x and 4x were analyzed. Collectively, our results suggest that the plants showed significantly different responses to salt stress based on ploidy level of the plant. The 4x possessed a better salt protection mechanism than that of 2x, suggesting salt tolerance in the polyploid plant.