Comparative adaptive strategies of old and young leaves to alkali-stress in hexaploid wheat

Abstract

Alkali stress is an important limiting factor for agricultural production. The aim of this study was to investigate whether old and young leaves have different biochemical responses to alkali stress in hexaploid wheat, and to provide molecular dissection of these differential responses on the basis of proteomic profiling. Under alkali stress, hexaploid wheat accumulated higher concentrations of Na+ in older leaves to protect young leaves. Young leaves accumulated amino acids, carbohydrates, and dehydrin proteins to relieve Na+ toxicity. Under alkali stress, 14-3-3 protein abundance was enhanced in old leaves to accelerate Na+ compartmentation in the vacuole. Abundances of 27 ribosomal proteins were significantly decreased in old leaves but not in young leaves, and many proteinases and ribonucleases also were significantly upregulated by alkali stress in old leaves. Interestingly, one E3 ubiquitin-protein ligase, one ubiquitin protein, and one proteasome protein were simultaneously upregulated in old leaves but not in young leaves. We propose that during the response to alkali stress, old leaves may programmatically degrade ribosomes through the ubiquitin-proteasome pathway to generate amino acids, which will increase tissue tolerance. Alkali stress induced remarkable enhancement of almost all free amino acids and ribose contents in old leaves, thus supporting this hypothesis. In addition, we observed that, for some salinity-tolerant protein-triads, the A, B, and D homoeologous proteins showed different response to alkali stress. This suggests that alkali stress may influence the translation of homoeologous genes, resulting in imbalanced translation of A, B, and D homoeologous genes in some gene triads.