Physiological Mechanisms Of Low Nitrogen Tolerance Alteration In Wheat During Hexaploidization

A large amount of archaeological and genetic evidence has shown that the hexaploid common wheat?Triticum aestivum L.,genome BBAADD?was formed about8,500 years ago by hybridization and doubling?hexaploidy?of domesticated tetraploid wheat?T.turgidum,genome AABB?and Tausch's goatgrass?Aegilops tauschii,genome DD?.Previous studies have shown that the stress resistance of the hexaploid common wheat is significantly higher than that of its tetraploid ancestors.However,the physiological basis and evolutionary course to acquire these enhanced adaptabilities by common wheat remain understood.In this study,we investigated the physiological mechanisms of alteration of low nitrogen tolerance during wheat hexaploidy.A synthetic allohexaploid wheat?neo-6×,BBAADD?,its tetraploid?4×,BBAA?and diploid?2×,DD?parents and natural common wheat?nat-6×,BBAADD?were cultured under low and normal nitrogen conditions.By measuring growth,photosynthesis,key enzymes of nitrogen metabolism,the electrophysiological parameters and gene expression of nitrogen absorption under the two nitrogen conditions,the alteration of low-nitrogen tolerance of the neo-6×and the physiological basis of the variation were discussed.The results showed that compared with its 4×and2×parents,the neo-6×maintained respectively high photosynthesis,nitrogen content and nitrogen assimilation efficiency under low nitrogen condition.At the morphological level,neo-6×has higher root/shoot ratio of biomass than its parents,which may be an adaptive growth strategy:the neo-6×used more roots to supply nitrogen to less shoots,thus promoting the accumulation of nitrogen in shoots.At the electrophysiological level,the H⁺excretion rate and the absorption rate of NO₃^-in neo-6×were significantly higher than in 4×parents under the condition of low nitrogen.The large outflow of H⁺can promote the rapid establishment of proton gradient in neo-6×to promote the absorption of NO₃^-,thereby increasing its low nitrogen tolerance.At the level of gene expression,the expression levels of neo-6×nitrogen uptake related genes?NPFs and NRTs?were also significantly higher than in 4×and 2×parents,suggesting that NPFs and NRTs play an important role in the evolution of nitrogen metabolism in wheat.In summary,the present study described the mechanisms of increasing low nitrogen tolerance in wheat during the process of hexaploid at physiological and molecular levels.The results will enrich the theory of wheat adaptive evolution and provide a theoretical reference for wheat resistance breeding.