Metabolite profile and correlation in HP and LN
ANOVA analysis of the metabolites in the two maternal environments
indicated that contents of 36 of 58 known metabolites were significantly
changed between HP and LN maternal conditions. These metabolites
included mostly amino acids such as serine, pyroglutamate and GABA and
organic acids including TCA cycle intermediates such as galacterate,
malate, succinate and malonate (Table 1 ).
In general Log2 ratios of HP:LN in the RIL population showed that most
of the metabolites had a higher level in seeds from the HP environment
as compared to the LN environment (Figure 3, Table 1 ).
The metabolite profiles of seeds from the two maternal environments
showed that seeds grown in HP conditions contained a higher amount of
metabolites, such as many sugars, amino acids and organic acids
(Figure 3, 4 ). A high metabolite level was predominantly
observed for GABA, sugars (maltose, fructose and glucose), organic acids
including benzoate, salicylate, glycerate and also some TCA cycle
intermediates such as galacterate, malate, malonate and succinate. The
TCA cycle, including several catabolic reactions, plays a very critical
role in energy metabolism in plants. In addition, it contributes in many
other ways to the metabolome by being involved in a large number of
metabolic networks (Araujo et al. 2012). In plants, it has been
shown that amino acids such as leucine, iso-leucine and valine can be
degraded into new products which can be used as precursors for the TCA
cycle to provide additional energy for plants. The rate of the
degradation of these compounds can be increased due to sub-optimal
conditions such as abiotic stresses (Binder 2010).
Both HP and LN maternal environments could be sub-optimal environments
for plant growth and seed development. Our results revealed that most of
the amino acids were not significantly altered between the two maternal
environments; however some of them such as GABA, pyroglutamate, glycine,
leucine and aspartate showed significantly lower values in seeds grown
in LN. This result was consistent with findings for tomato and
Arabidopsis plants that reported a general decrease of amino acid levels
under low nitrate conditions (Tschoep et al. 2009;
Urbanczyk-Wochniak & Fernie 2004). GABA is one of the amino acids which
frequently shows higher levels under stress conditions (Michaeliet al. 2011; Renault et al. 2011; Shelp, Bown & McLean
1999). GABA is an amino compound which is produced via the so-called
GABA shunt pathway which has a primary role in keeping a balance in
central C/N metabolism (Bouche & Fromm 2004). It has been shown that
GABA levels increased rapidly under stressed growth conditions. Thus,
GABA is thought to be involved in the tolerance of plants to sub-optimal
environments (Fait et al. 2008; Kinnersley & Turano 2000;
Renault et al. 2011). In our study we have observed that the GABA
content was lower in seeds developed in LN conditions in comparison with
those of the HP maternal condition. Our findings confirmed previous
studies, where seeds also showed low amounts of GABA under LN maternal
conditions (He et al. 2016); (Geshnizjani et al. 2019).
It has previously been reported that nitrate starvation resulted in a
decrease in TCA cycle intermediates in tomato (Urbanczyk-Wochniak &
Fernie 2004) and Arabidopsis (Tschoep et al. 2009) leaves. We
also found that in comparison to the HP condition, seeds developed under
LN possessed a lower amount of TCA cycle intermediates such as malate,
succinate and malonate (Figure 4 ). This could be an indication
of higher consumption of TCA cycle intermediates to produce more energy
under LN to allow the plants to survive and continue growth.