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).