Physiological and proteomics analyses reveal the resistance response mechanism to alkali stress in the early seedlings (cotyledons vs. roots) of castor plant (Ricinus communis L.)

Abstract

Soil alkalinity is a major environmental problem influencing plant growth and productivity in northeastern China. Castor plant (Ricinus communis) is one of the most important oilseed crops worldwide, and has good salt tolerance. The early seedling stage is extremely vulnerable to abiotic stresses, but little is known about the physiological and molecular mechanisms involved in the cotyledons and roots of castor seedlings under alkali stress. In this study, biomass, photosynthesis, root vitality, inorganic ions, organic solutes and antioxidant enzyme activities were measured. Two-dimensional gel electrophoresis-based proteomic approaches were also applied to identify differentially abundant proteins in alkali-treated seedlings. The results showed that alkali stress strongly inhibited seedling growth, especially of the roots. Na+ content increased with increasing alkalinity, and the roots had much higher Na+ and lower K+ than cotyledons. The proline and soluble sugars were both increased in cotyledons, but kept unchanged in the roots under alkali stress. Moreover, proteomic analysis revealed a total of 15 and 25 alkali-responsive proteins in the cotyledons and roots, respectively. For cotyledons, most of the identified proteins were involved in photosynthesis, stress and defense (＞10 %), while those for roots were mainly involved in carbohydrate and energy metabolism, genetic information, stress and defense (＞10 %). These results suggest that cotyledons and roots respond differently to alkali stress. Cotyledons retain efficiency of photosynthetic electron transport and accumulation of osmolytes, while the roots enhance transfer efficiency of the mitochondrial electron transport chain, adjust fatty acid biosynthesis, and promote phosphatidic acid biosynthesis under alkali stress. Additionally, the co-increased abundance of chaperonins indicated that both cotyledons and roots maintained folding of proteins into their proper 3D structures in response to alkali stress. These findings provide new insights into different physiological mechanisms in cotyledon and root responses to alkali stress during the early seedling stage in castor.