Analysis of metabolite pathway in leaf exposed to O3
The activity of cytosolic pyruvate kinase (PK), NAD-malic enzyme and fumarase in mitochondria were induced after exposure to ozone (Dizengremel et al ., 1994; Gerantet al ., 1996; Gaucher et al ., 2006; Dizengremel et al ., 2009). The changed metabolites in tricarboxylic acid (TCA) cycle under O3 treatment were isocitrate with increased 2.7 fold change in rice (Fig. 7, Table S3), 2-oxoglutarate with a reduction of aproximately 50% and citric of 50% in soybean (Fig. 8, Table S4). Additionally, O3 treatment leads to the high level of aspartate-derived asparagine or aspartate in wheat, rice and soybean. Subsequently, the level of arginine in soybean and agmatine in wheat are elevated in ornithine cycle. These results suggest that O3 affects TCA cycle in three crop plants and ornithine cycle in wheat andsoybean .
In mevalonate pathway, phosphoenolpyruvate is used for the synthesis of isoprenoids that play an antioxidant role and prevent photosynthetic electron transport damage (Loreto & Velikova, 2001; Magel et al ., 2006; Vickers et al ., 2009). The decreased level of sterols in wheat but the increased concentration in rice were observed under O3 stress (Fig. 6 and 7). Moreover, chloric O3 leads to the declined concentration of fatty acids, monoteroenes, sesquitepenes, saponins, diterpenes, terpene and polyterpenes in both wheat and rice. However, all these metabolites were not changed in soybean in response to O3 (Fig. 8).
Futhermore, in the glycerol phosphate pathway and pentose phosphate pathway, O3 motivated the synthesis of purines in the three species, and phospholipids only in wheat and rice. On the contrary, histidine was up-regulated in soybean, but not changed in wheat and rice.The shikimate pathway is also affected by O3 stress. In wheat, N-acetyltryptophan was highly accumulated with 10 fold, while levels of indole alkaloids, methyl salicylate, cinnamic acid, lignin, phenylpropanoids, flavonoids, and hydroxybenzoic acid were reduced in various extent (Fig. 6, Table S2). These metabolites are known to be involved in thickening of cell walls and also act as antimicrobial compounds (Yogendra et al ., 2015). In rice, metabolites of cinnamic acid, coumaric acid, flavonoids, salicylate, and indole alkaloids showed reduced abundance, while phenylalanine and kynurenic acid were increased (Fig. 7). In soybean, several products of 3-(4-hydroxyphenyl)pyruvate, 4-coumarate and flavonoids were reduced but methylsalicylate, lignin, phenylpropanoids and 4-hydroxybenzoic acid were elevated (Fig. 8).