A newly formed hexaploid wheat exhibits immediate higher tolerance to nitrogen-deficiency than its parental lines

Abstract

Background: It is known that hexaploid common wheat (Triticum aestivum L.) has stronger adaptability to many stressful environments than its tetraploid wheat progenitor. However, the physiological basis and evolutionary course to acquire these enhanced adaptabilities by common wheat remain understudied. Here, we aimed to investigate whether and by what means tolerance to low-nitrogen manifested by common wheat may emerge immediately following allohexaploidization.

Results: We compared traits related to nitrogen (N) metabolism in a synthetic allohexaploid wheat (neo-6×, BBAADD) mimicking natural common wheat, together with its tetraploid (BBAA, 4×) and diploid (DD, 2×) parents. We found that, under low nitrogen condition, neo-6× maintained largely normal photosynthesis, higher shoot N accumulation, and better N assimilation than its 4× and 2× parents. We showed that multiple mechanisms underlie the enhanced tolerance to N-deficiency in neo-6×. At morphological level, neo-6× has higher root/shoot ratio of biomass than its parents, which might be an adaptive growth strategy as more roots feed less shoots with N, thereby enabling higher N accumulation in the shoots. At electrophysiological level, H⁺ efflux in neo-6× is higher than in its 4× parent. A stronger H⁺ efflux may enable a higher N uptake capacity of neo-6×. At gene expression level, neo-6× displayed markedly higher expression levels of critical genes involved in N uptake than both of its 4× and 2× parents.

Conclusions: This study documents that allohexaploid wheat can attain immediate higher tolerance to N-deficiency compared with both of its 4× and 2× parents, and which was accomplished via multiple mechanisms.

Keywords: Adaptation; Allopolyploidy; Gene expression; Nitrate transporter; Nitrogen uptake; Wheat.

Fig. 1

Effects of low N condition on growth, photosynthesis and nitrogen content in a newly formed hexaploid (neo-6×), its diploid (2×) and tetraploid (4×) parents, and natural allohexaploid (nat-6×). 17-day-old seedlings were subjected to low N condition (0.1 mM) for 31 days. The values are means of four biological replicates. a Growth status of four wheat lines under 0.1 mM and 5 mM N conditions. b Photosynthesis and nitrogen content: g_s − stomatal conductance; P_N − net photosynthetic rate; Chl – chlorophyll; Car–carotenoid; E − transpiration rate. Asterisks indicated significant difference (t test, P < 0.05) between control and low N-stressed plants for a given genotype. The means of any two of all four lines at the same N condition were compared using t test (P < 0.05), and means followed by different letters at the same N condition are significant

Fig. 2

Effects of low N condition on activities of enzymes involved in nitrogen assimilation in a newly formed hexaploid (neo-6×), its diploid (2×) and tetraploid (4×) parents, and natural allohexaploid (nat-6×). The enzymes in the fresh mature leaves at the same leaf position for each wheat line were assayed. Ten mature leaves from five individual plants for each wheat line was pooled as a biological replicate. The values are means of four biological replicates. Asterisks indicated significant difference (t test, P < 0.05) between control and low N-stressed plants for a given genotype. The means of any two of all four lines at the same N condition were compared using t test (P < 0.05), and means followed by different letters at the same N condition are significant. The seedlings were subjected to low N condition (0.1 mM) for 7 days. NR, nitrate reductase; GS, glutamine synthetase; GDH, glutamate dehydrogenase; and GO, Glycolate oxidase

Fig. 3

Effects of low N condition on the contents of amino acids in shoots of a newly formed hexaploid (neo-6×), its diploid (2×) and tetraploid (4×) parents, and natural allohexaploid (nat-6×). The values are means of four biological replicates. Asterisks indicated significant difference (t test, P < 0.05) between control and low N-stressed plants for a given genotype. The means of any two of all four lines at the same N condition were compared using t test (P < 0.05), and means followed by different letters at the same N condition are significant. The seedlings were subjected to low N condition (0.1 mM) for 31 days

Fig. 4

Effects of low N condition on the expression of root nitrate transporter genes (a-d), H⁺ efflux (e), NO₃⁻ influx (f), dry weight (DW) (g-h) and root/shoot ratio (i) in a newly formed hexaploid (neo-6×), its diploid (2×) and tetraploid (4×) parents, and natural allohexaploid (nat-6×). (a-d) The fold increase of the gene expression was calculated according to (treatment-control) /control, and the percentage of gene expression decrease was calculated according to (control-treatment)*100% /control. (e-i) Asterisks indicated significant difference (t test, P < 0.05) between control and low N-stressed plants for a given genotype. The means of any two of all four lines at the same N condition were compared using t test (P < 0.05), and means followed by different letters at the same N condition are significant. The values are means of 3–7 biological replicates. When the seedlings were subjected to low N condition (0.1 mM) for 7 days, gene expression, NO₃⁻ influx and H⁺ efflux were measured. The dry weights of the roots and shoots were measured and root DW /shoot DW ratio (root /shoot ratio) was calculated at 31 days of the stress. Under 5 mM N condition, we did not detect any NO₃⁻ flux signal due to high background signal of NO₃⁻, thus, NO₃⁻ influx data of only 0.1 mM N treatment was displayed in the Figure f

Fig. 5

Nitrogen-deficiency tolerance mechanism of a newly formed hexaploidy (neo-6×). a Comparative characteristics of H⁺, H₂O and NO₃⁻ uptakes of a newly formed hexaploid (genome BBAADD) and its tetraploid (genome BBAA) parent under low N condition. b Differences between a newly formed hexaploid (neo-6×) and its diploid (2×) and tetraploid (4×) parents in photosynthesis and nitrogen metabolism under low N condition