An overview of the metabolites detected upon O3treatment
To gain first insights into how O3 affects the metabolism of crop plants, we performed a comparative metabolic analysis of leaves in wheat, rice and soybean between charcoal filtered air and elevated O3 treatment. As shown in Table 1, a total of six hundred and forty-four compounds were detected and identified in leaves of the three species. The detected compounds were predominantly grouped into fifty-eight classes, such as, carboxylic acids and derivatives, fatty acyls, prenol lipids, flavonoids, glycerophospholipids, benzene and substituted derivatives.
The score plots based on the model of principal component analysis (PCA) and partial least-squares discriminate analysis (PLS-DA) showed that six repeats in treatment and control group were clustered together and data from two groups could be clearly distinguished in space, respectively (Fig. 3). This suggests smaller system errors during the whole test, data with good repeat ability and differences in metabolites between the two groups for each crop, which enable them for further analysis.
One hundred and ninety-three differential metabolites were found in three species under elevated O3. Comparative analysis revealed that twenty differential metabolites that overlapped between wheat and rice was higher than the other two comparison sets (four in wheat and soybean, five in rice and soybean). Aspartate was the only one differential metabolite, which was common to the three species and up-accumulated in these species (Fig. 4A). It belongs to the class of carboxylic acids and derivatives, which represent the core metabolome responsive to O3 stress. The number of soybean-specific differential metabolites was less than that of rice-specific and wheat-specific metabolites.
In wheat, one hundred and four kinds of metabolites were putatively identified, belonging to twenty six kinds of different chemical groups. The relative larger proportions of chemical groups were flavonoids (22.3%), fatty acyls (7.7%), prenol lipids (5.8%), carboxylic acids and derivatives (5.8%), respectively (Fig. 4B). The important and high-fold change metabolites were observed, such as carboxylic acids and derivatives, morphinans, purine nucleotides, glycerolipids and glycerophospholipids (Table S2).
Likewise, eighty-nine metabolites were putatively identified in rice, belonging to twenty kinds of different chemical groups. The relative larger proportions of chemical groups were carboxylic acids and derivatives (30.3%), prenol lipids (10%), flavonoids (8.9%), and fatty acyls (5.6%) (Fig. 4C). Among these metabolites, high fold changes were detected in carboxylic acids and derivatives, glycerophospholipids, quinolines and derivatives (Table S3).
In soybean, twenty-nine differential metabolites were assigned with putative names of identity, belonging to eleven different chemical groups. The relative larger proportions of chemical groups were carboxylic acids and derivatives (37.9%), benzene and substituted derivatives (13.7%), flavonoids (10.3%), prenol lipids (10.3%) and Phenols (6.9%) (Fig. 4D). High fold change metabolites were fatty acyls, phenols, carboxylic acids and derivatives (Table S4).