Improvement of Drought Tolerance by Exogenous Spermidine in Germinating Wheat (Triticum aestivum L.) Plants Is Accompanied with Changes in Metabolite Composition

Abstract

Drought is one of the most important environmental factors reducing the yield and production of crops, including wheat. Polyamines are closely associated with plant stress tolerance. The present study investigated the mechanisms through seed germination with spermidine protecting wheat varieties from drought stress. In the first experiment, the effects of spermidine on the germination of wheat varieties, namely Rakhshan, Mihan, Sirvan and Pishgam, were investigated in three drought levels, namely 0, -2, and -4 MPa induced by polyethylene glycol 6000. Analysis of variance indicated that spermidine, drought stress and interaction between varieties and drought stress were significant for all traits, and with severity of stress, all traits significantly decreased. In the second experiment, detailed gene expression and non-targeted metabolomics analyses were carried out using the Rakhshan and Mihan varieties after germination, with or without spermidine treatment and/or drought stress. According to the biomass parameters, the Mihan variety showed relatively better growth compared to the other variety, but the Rakhshan one showed more pronounced responses at gene expression level to exogenous spermidine than the Mihan variety. Overall, these results showed that spermidine increased the drought tolerance of wheat at the germination stage, due to specific role of polyamine metabolism in the development of effective responses under drought stress.

Keywords: drought stress; exogenous spermidine; gene expression; metabolomics; seed germination; wheat.

Figure 1

Effect of spermidine (Spd) on radicle length of four wheat varieties under drought stress with three levels of PEG 6000 osmotic potential (−2, −4 Mpa and 0 using distilled water as control). Ck = distilled water. Different letters indicate statistically significant differences across all three levels of PEG 6000 at p < 0.05 level using LSD’s test.

Figure 2

Effect of spermidine (Spd) on radicle weight of four wheat varieties under drought stress with three levels of PEG 6000 osmotic potential (−2, −4 MPa and 0 using distilled water as control). Ck = distilled water. Different letters indicate statistically significant differences across all three levels of PEG 6000 at p < 0.05 level using LSD’s test.

Figure 3

Effect of spermidine (Spd) on coleoptile length of four wheat varieties under drought stress with three levels of PEG 6000 osmotic potential (−2, −4 MPa and 0 using distilled water as control). Ck = distilled water. Different letters indicate statistically significant differences across all three levels of PEG 6000 at p < 0.05 level using LSD’s test.

Figure 4

Effect of spermidine (Spd) on coleoptile weight of four wheat varieties under drought stress with three levels of PEG 6000 osmotic potential (−2, −4 MPa and 0 using distilled water as control). Ck = distilled water. Different letters indicate statistically significant differences across all three levels of PEG 6000 at p < 0.05 level using LSD’s test.

Figure 5

Effect of spermidine (Spd) on germination percentage of four wheat varieties under drought stress with three levels of PEG 6000 osmotic potential (−2, −4 MPa and 0 using distilled water as control). Ck = distilled water. Different letters indicate statistically significant differences across all three levels of PEG 6000 at p < 0.05 level using LSD’s test.

Figure 6

Effect of spermidine (Spd) on germination rate of four wheat varieties under drought stress with three levels of PEG 6000 osmotic potential (−2, −4 MPa and 0 using distilled water as control). Ck = distilled water. Different letters indicate statistically significant differences across all three levels of PEG 6000 at p < 0.05 level using LSD’s test.

Figure 7

Pearson correlation between germination characteristics in wheat varieties.

Figure 8

Principal component analysis for the effect of drought stress with three levels of PEG 6000 osmotic potential (−2, −4 MPa and 0 using distilled water as control) on germination and seedling growth of four wheat varieties (A), and for the effect of varieties on germination and seedling growth of four wheat varieties (B).

Figure 9

Effect of drought stress (0, −2 and −4 MPa) with Spd and Ck (control) in radicle of wheat genotypes (Mihan and Rakhshan) on relative expression of arginine decarboxylase (TaADC) (A), polyamine oxidase (TaPAO) (B), spermidine synthase (TaSPDS) (C), S-adenosylmethionine decarboxylase (TaSAMDC) (D) and ornithine decarboxylase (TaODC) (E) genes determined by qRT-PCR. All reactions for gene expression analyses were performed in triplicate using 3 biological and 3 technical repetitions. Bars show mean ± standard deviation (SD). Different letters indicate statistically significant differences at p < 0.05 level using LSD’s test.

Figure 10

Number of the differentially expressed metabolites in two varieties (Mihan and Rakshan) under PEG 6000 treatments. 0, 2, and 4 indicates control, −2 MPa, and −4 MPa PEG treatments, respectively. Up: statistically significant increased, down: statistically significant decreased levels.

Figure 11

The contents of metabolites found in an untargeted analysis in the coleoptiles and radicles of two wheat varieties Rakhshan and Mihan, with or without (C) treatment with exogenous Spd. 0: control plants treated with distilled water, 2 and 4 represents PEG treatments at −2 and −4 MPa, respectively. Darker red colour indicates higher abundance of the given metabolite.