Carotenoids synthesis affects the salt tolerance mechanism of Rhodopseudomonas palustris

Abstract

<i>Rhodopseudomonas palustris</i> CGA009 is a Gram-negative, purple non-sulfur, metabolically diverse bacterium with wide-ranging habitats. The extraordinary ability of <i>R. palustris</i> to decompose a variety of raw materials and convert them into high-value products makes it an attractive host for biotechnology and industrial applications. However, being a freshwater bacterium <i>R. palustris</i> has limited application in highly-saline environments. Therefore, it is of great significance to obtain the salt-tolerant strain of <i>R. palustris</i> and understand its tolerance mechanism. In this study, <i>R. palustris</i> CGA009 was successfully evolved into eight salt-tolerant strains using an adaptive laboratory evolution technique. RPAS-11 (<i>R. palustris</i> anti-salt strain 11) was selected as the best salt-tolerant strain and was used in further studies to explore the salt-tolerance mechanism. The expression of most genes associated with the carotenoid synthesis in RPAS-11 increased significantly under high concentration of salt stress, suggesting that carotenoid synthesis is one of the reasons for the salt tolerance of RPAS-11. Gene overexpression and knockout experiments were performed to get clear about the role of carotenoids in salt stress tolerance. RPAS-11-IDI, the mutant with overexpression of IDI (Isopentenyl diphosphate isomerase) exhibited enhanced salt tolerance, whereas the knockout mutant CGA009-∆<i>crtI</i> showed a decline in salt tolerance. In addition, the results indicated that rhodopin, a carotenoid compound, was the key pigment responsible for the salt tolerance in <i>R. palustris</i>. Furthermore, the production of lycopene, a widely-used carotenoid, was also increased. Taken together, our research helps to deepen the understanding of the salt tolerance mechanism of <i>R. palustris</i> and also widens the application of <i>R. palustris</i> in highly-saline environments.