3.3 Reduction of P-containing glycolytic intermediates in barley
and other plant species under Al stress
We first conducted metabolomics of the root tips of XZ29 and XZ9 under
6-72 h of Al treatment and identified a total of 52 primary metabolites
(Table S6). The temporal changes of these metabolites were measured and
analyzed with K-means clustering (Figure 4a). Most of the metabolites in
Clusters 1, 2 and 3 showed a gradual decrease during 24-72 h of Al
treatment. Interestingly, most of these metabolites belong to the
glycolysis pathway and its branches. The decrease in the five
P-containing glycolytic intermediates in Cluster 1 (glucose-6-phosphate,
Glu-6-P; fructose-6-phosphate, Fru-6-P; 3-phosphoglyceric acid, 3-PGA;
2-phosphoglyceric acid, 2-PGA; phosphoenolpyruvate, PEP) were larger and
occurred earlier in XZ9 than in XZ29 (Figure 4a, 4b). Many of the
metabolites downstream of glycolysis (inositol, galactinol, galactose,
serine, cysteine, leucine, isoleucine, threonine and valine) showed
similar patterns in XZ29 and XZ9, but the relative changes were greater
in XZ9 than for XZ29, especially at 72 h (Figures 4b, S7). Moreover,
highly significant correlations were detected between the glycolytic
intermediates and their downstream metabolites (Figure S8). In order to
understand the transcriptional regulation of the glycolysis pathway in
barley in response to Al stress, we examined the expression of key genes
in this pathway in XZ29 and XZ9 under 0.5h – 24h of Al treatments. The
up-regulations of Al-induced expression of most glycolysis pathway genes
were found earlier in XZ9 (6 h) than in XZ29 (12 h) (Figure 4b). The
opposing trends in gene expression and metabolite levels indicated that
the inhibition of glycolysis during Al stress may not occur at the
transcriptional level.
To further investigate whether the response of glycolytic intermediates
to Al stress is evolutionarily conserved in land plants, we performed
additional metabolome analysis in rhizoids of a liverwort
(Marchantia polymorpha ) and roots of a gymnosperm (Ginkgo
biloba ), a eudicot (Medicago truncatula ), and two monocots
(Zea mays and Triticum aestivum ). Al-induced reduction of
Glu-6-P and Fru-6-P were observed in roots of the examined plant
species, except for the rhizoids of liverwort (Figure 4c). Moreover,
Al-induced changes in sucrose, inositol, and threonine were detected in
all tested plant species (Figure 4c), indicating an early evolution of
these metabolites in Al response of land plants.