Alterations in wheat pollen lipidome during high day and night temperature stress

Abstract

Understanding the adaptive changes in wheat pollen lipidome under high temperature (HT) stress is critical to improving seed set and developing HT tolerant wheat varieties. We measured 89 pollen lipid species under optimum and high day and/or night temperatures using electrospray ionization-tandem mass spectrometry in wheat plants. The pollen lipidome had a distinct composition compared with that of leaves. Unlike in leaves, 34:3 and 36:6 species dominated the composition of extraplastidic phospholipids in pollen under optimum and HT conditions. The most HT-responsive lipids were extraplastidic phospholipids, phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylinositol, phosphatidic acid, and phosphatidylserine. The unsaturation levels of the extraplastidic phospholipids decreased through the decreases in the levels of 18:3 and increases in the levels of 16:0, 18:0, 18:1, and 18:2 acyl chains. PC and PE were negatively correlated. Higher PC:PE at HT indicated possible PE-to-PC conversion, lower PE formation, or increased PE degradation, relative to PC. Correlation analysis revealed lipids experiencing coordinated metabolism under HT and confirmed the HT responsiveness of extraplastidic phospholipids. Comparison of the present results on wheat pollen with results of our previous research on wheat leaves suggests that similar lipid changes contribute to HT adaptation in both leaves and pollen, though the lipidomes have inherently distinct compositions.

Keywords: direct infusion automated electrospray ionization tandem mass spectrometry; extraplastidic phospholipids; lipid co-occurrence; lipid remodelling; lipid unsaturation; phosphatidylcholine; phosphatidylethanolamine; pollen lipids.

Figure 1.

Composition of extraplastidic phospholipids of pollen grains (a, c, e, g, and i) and leaves (b, d, f, h, and j) of wheat genotype Karl 92. Leaf lipid compositions are reproduced from Narayanan et al. (2016a) for novel comparison with pollen lipid compositions. Values shown are mean ± SE; n = 10 [two experiments and five replications (plants)]. OT, optimum temperature; HN, high night temperature; HD, high day temperature; HDN, high day and night temperature; PC, phosphatidylcholine; PE, phosphatidylethanolamine; PI, phosphatidylinositol; PA, phosphatidic acid; PS, phosphatidylserine.

Figure 2.

Composition of various lipid species as a percentage of total PC and PE of pollen grains and leaves of wheat genotype Karl 92. Leaf lipid compositions are reproduced from Narayanan et al. (2016a) for novel comparison with pollen lipid compositions. OT, optimum temperature; HN, high night temperature; HD, high day temperature; HDN, high day and night temperature; PC, phosphatidylcholine; PE, phosphatidylethanolamine.

Figure 3.

Effects of temperature on extraplastidic phospholipid molecular species of wheat genotype Karl 92. Values shown are mean ± SE; n = 10 [two experiments and five replications (plants)]. Means with different letters are significantly different according to the least significant difference (LSD) test at P<0.05. Breaks on the y-axis indicate a change in scale. OT, optimum temperature; HN, high night temperature; HD, high day temperature; HDN, high day and night temperature; PC, phosphatidylcholine; PE, phosphatidylethanolamine; PI, phosphatidylinositol; PA, phosphatidic acid; PS, phosphatidylserine.

Figure 4.

Effects of temperature on unsaturation index of extraplastidic phospholipid classes of wheat genotype Karl 92. The unsaturation index of each lipid molecular species was calculated as the product of the amount of that lipid molecular species and the average number of double bonds per acyl chain, where the average number of double bonds per acyl chain was calculated by dividing the number of double bonds in the lipid molecular species by the number of acyl chains. Finally, the unsaturation index of a lipid head group class was calculated as the sum of the unsaturation indices of individual lipid molecular species in that class. Values shown are mean ± SE; n = 10 [two experiments and five replications (plants)]. Means with different letters are significantly different according to the least significant difference (LSD) test at P<0.05. OT, optimum temperature; HN, high night temperature; HD, high day temperature; HDN, high day and night temperature; PC, phosphatidylcholine; PE, phosphatidylethanolamine; PI, phosphatidylinositol; PA, phosphatidic acid; PS, phosphatidylserine.

Figure 5.

Heat map showing the correlation among phosphatidylcholine (PC) and phosphatidylethanolamine (PE) species of wheat genotype Karl 92 (a) based on Spearman’s correlation coefficient, ρ. Blue and red colors on the heat map indicate negative and positive correlations, respectively. Effects of temperature on PC:PE ratio (unitless) of Karl 92 (b). OT, optimum temperature; HN, high night temperature; HD, high day temperature; HDN, high day and night temperature.

Figure 6.

Lipid dendrogram of wheat genotype Karl 92. Eighty nine lipid analytes were clustered using a single-linkage hierarchical algorithm based on Spearman’s correlation coefficient, ρ. Co-occurring lipid groups (Groups 1-10) with ρ ≥ 0.85 are indicated by red and blue bars on the dendrogram. The arrows on the dendrogram indicate the directionality of differences in levels of each lipid (based on % of total signal) under high day and night temperature stress conditions compared to optimum temperature conditions; lipids that decreased in amount are indicated by green-colored downward arrows, and lipids that increased in amount are indicated by pink-colored upward arrows.