Spatial specificity of metabolism regulation of abscisic acid-imposed seed germination inhibition in Korean pine (Pinus koraiensis sieb et zucc)

Abstract

Introduction: Abscisic acid (ABA) can negatively regulate seed germination, but the mechanisms of ABA-mediated metabolism modulation are not well understood. Moreover, it remains unclear whether metabolic pathways vary with the different tissue parts of the embryo, such as the radicle, hypocotyl and cotyledon.

Methods: In this report, we performed the first comprehensive metabolome analysis of the radicle and hypocotyl + cotyledon in Pinus koraiensis seeds in response to ABA treatment during germination.

Results and discussion: Metabolome profiling showed that following ABA treatment, 67 significantly differentially accumulated metabolites in the embryo were closely associated with pyrimidine metabolism, phenylalanine metabolism, cysteine and methionine metabolism, galactose metabolism, terpenoid backbone biosynthesis, and glutathione metabolism. Meanwhile, 62 metabolites in the hypocotyl + cotyledon were primarily involved in glycerophospholipid metabolism and glycolysis/gluconeogenesis. We can conclude that ABA may inhibit Korean pine seed germination primarily by disrupting the biosynthesis of certain plant hormones mediated by cysteine and methionine metabolism and terpenoid backbone biosynthesis, as well as reducing the reactive oxygen species scavenging ability regulated by glutathione metabolism and shikimate pathway in radicle. ABA may strongly disrupt the structure and function of cellular membranes due to alterations in glycerophospholipid metabolism, and weaken glycolysis/gluconeogenesis in the hypocotyl + cotyledon, both of which are major contributors to ABA-mediated inhibition of seed germination. These results highlight that the spatial modulation of metabolic pathways in Pinus koraiensis seeds underlies the germination response to ABA.

Keywords: abscisic acid; cotyledon; hypocotyl; metabolism; radicle; seed germination.

Figure 1

Principal least squares-discriminant analysis of measured metabolites in the radicle of control seeds (R), the radicle of ABA-treated seeds (RA), the hypocotyl + cotyledon of control seeds (HC), and the hypocotyl + cotyledon of ABA-treated seeds (HCA).

Figure 2

A Metabolome view depicts the altered metabolic pathways in (A) the radicle and (B) the hypocotyl + cotyledon of control seeds compared to ABA-treated seeds. Each node stands for a metabolic pathway. The color of the node signifies the P-value associated with each metabolic pathway, and the size (or radius) of the node represents the impact value of each metabolic pathway. Dark red, large circles located in the top right corner of the “metabolome view” indicate the primary altered pathways, in contrast to the yellow, small circles positioned on the left side of the graph represent the metabolic pathways that are less affected by ABA.

Figure 3

Metabolites were enriched in the eight altered pathway in the radicle of control seeds (R) compared to the radicle of ABA-treated seeds (RA). The relative contents of differentially expressed metabolites were first log transformed (using a generalized logarithm transformation) and then auto scaled (mean-centered and divided by the standard deviation of each variable) for normalization purposes. The normalized values are presented on the Y-axis. Since only nicotinamide adenine dinucleotide is enriched in nicotinate and nicotinamide metabolism, and phenylpyruvate is enriched in phenylalanine metabolism, these two pathways are not included.

Figure 4

Metabolites were enriched in the two altered metabolic pathways in the hypocotyl + cotyledon of control seeds (HC) compared to the hypocotyl + cotyledon of ABA-treated seeds (HCA). The relative contents of differentially expressed metabolites were pareto scaled (mean-centered and divided by the square root of the standard deviation of each variable) for normalization purposes. The normalized values are presented on the Y-axis.

Figure 5

A Schematic diagram of metabolic regulation of ABA inhibition of Korean pine seed germination. The red upward arrows indicate upregulated metabolites, whereas the downward arrows represent the downregulated metabolites.