Transcriptomic and metabolomic insights into drought response strategies of two Astragalus species

Drought is a major environmental constraint to plant productivity, and plants cope with water limitation using drought avoidance and/or tolerance strategies. Mesoxerophyte Astragalus mongholicus displayed better drought resistance than the mesophyte A. membranaceus. However, the underlying response mechanisms are still elusive. In this study, we characterized the morphological, physiological, molecular, and metabolic responses to drought stress to reveal the different response strategies in the two Astragalus species. The susceptible A. membranaceus, with a larger leaf area, showed significant stomatal closure at moderate drought stress, indicating drought avoidance strategy to minimize transpiration. A. mongholicus, with a smaller leaf area, was better in maintaining photosynthesis and activating antioxidant enzymes than A. membranaceus towards the drought tolerance strategy. Drought has greater disturbance on gene expression in A. membranaceus, compared to A. mongholicus, indicating that A. membranaceus was more sensitive in the face of drought. Meanwhile, highly conserved osmotic stress signal transduction pathways were discovered in two Astragalus species under drought treatment. The up-regulation of dehydration-responsive element-binding gene (DREB) expression in A. mongholicus, initiating a series of stress response pathways to help plants adapt and cope with water limitation, showed better “tolerance” than in A. membranaceus. An increase of compatible solutes in A. membranaceus indicates a positive response in regulating osmotic homeostasis in response to drought. The greater investment in antioxidant supplementation phenolic compounds of A. mongholicus suggests the “Tolerance” strategy. Furthermore, the greater antioxidant investment of A. mongholicus is evidenced by higher antioxidant efficacies in terms of antiradical activity. In summary, we conclude that the “Tolerance” strategy of A. mongholicus and the “Avoidance” strategy of A. membranaceus in the face of drought stress can be established.