Comparative Metabolic Analysis Reveals a Metabolic Switch in Mature, Hydrated, and Germinated Pollen in Arabidopsis thaliana

Abstract

Pollen germination is an essential process for pollen tube growth, pollination, and therefore seed production in flowering plants, and it requires energy either from remobilization of stored carbon sources, such as lipids and starches, or from secreted exudates from the stigma. Transcriptome analysis from in vitro pollen germination previously showed that 14 GO terms, including metabolism and energy, were overrepresented in Arabidopsis. However, little is understood about global changes in carbohydrate and energy-related metabolites during the transition from mature pollen grain to hydrated pollen, a prerequisite to pollen germination, in most plants, including Arabidopsis. In this study, we investigated differential metabolic pathway enrichment among mature, hydrated, and germinated pollen using an untargeted metabolomic approach. Integration of publicly available transcriptome data with metabolomic data generated as a part of this study revealed starch and sucrose metabolism increased significantly during pollen hydration and germination. We analyzed in detail alterations in central metabolism, focusing on soluble carbohydrates, non-esterified fatty acids, glycerophospholipids, and glycerolipids. We found that several metabolites, including palmitic acid, oleic acid, linolenic acid, quercetin, luteolin/kaempferol, and γ-aminobutyric acid (GABA), were elevated in hydrated pollen, suggesting a potential role in activating pollen tube emergence. The metabolite levels of mature, hydrated, and germinated pollen, presented in this work provide insights on the molecular basis of pollen germination.

Sucrose rather than GA transported by AtSWEET13 and AtSWEET14 supports pollen fitness at late anther development stages

“…4i ), substantially decreased in atsweet13;14 pollen compared to Col‐0 pollen. Lipid is another major form of carbon storage in mature pollen grains, especially for Arabidopsis (Shi & Yang, 2010 ; Wang et al ., 2022a ). Sugar is the primary source for lipid body biogenesis in pollen grains.…”

“…4j ). In addition, no significant differences were observed in each fatty acid species commonly found in Arabidopsis pollen (Wang et al ., 2022a ), including C16:0, C18:0, C18:1, C18:2 and C18:3 (Fig. S4 ).…”

Small extracellular vesicles released from germinated kiwi pollen (pollensomes) present characteristics similar to mammalian exosomes and carry a plant homolog of ALIX

“…Most of the enzymes present in GKP EVs were involved in catabolic processes, especially in the energetic metabolism, with glycolytic activity and nucleoside metabolic activity ( Supplementary Figures 4 , 5 ). Since pollen tube elongation must be sustained by an intense metabolic activity, the cytoplasmic concentrations of metabolism-related enzymes are particularly high in this phase ( Wang et al., 2022 ), hence they are more likely to be randomly internalized during EVs formation. Moreover, some of them, such as the pectinesterases or the pollen-specific leucine-rich repeat extensin-like protein ( Supplementary Table 1 ), can cooperate in the degradation of female tissues to open a passage for the pollen tube through the style ( Chae and Lord, 2011 ), and they might be released in a membranous compartment to extend their extracellular half-life.…”

Sucrose rather than GA transported by AtSWEET13 and AtSWEET14 supports pollen fitness at late anther development stages

“…4j ). In addition, no significant differences were observed in each fatty acids species commonly found in Arabidopsis pollen (Wang et al ., 2022a), including C16:0, C18:0, C18:1, C18:2, and C18:3 (Supporting Information Fig. S4 ).…”

“…4i ), substantially decreased in atsweet13;14 pollen compared to Col-0 pollen. Lipid is another major form of carbon storage in mature pollen grains, especially for Arabidopsis (Shi & Yang, 2010; Wang et al ., 2022a). Sugar is the primary source for lipid body biogenesis in pollen grains.…”