Metabolic changes associated with plasticity of studied
phenotypes to N availability
Primary metabolism is associated with growth and levels of several
primary metabolites are altered in response to changes in N availability
(Sulpice et al., 2013). However, it is not clear if the plasticity of
metabolites is associated with plasticity of the complex phenotypes
studied here. To address this question, we first measured the levels of
66 primary metabolites in a subset of 43 accessions grown in the three N
conditions (Table S6). To quantify plasticity, we calculated the CV of
every metabolite across the three mean values for each of the 43
accessions (Table S7). The majority of primary metabolites are essential
for plant growth and showed low plasticity across N conditions as
expected (Figure 5a). The organic acids citrate and fumarate, and the
amino acids ornithine, glutamine, lysine and arginine showed the highest
median CV across the accessions (Figures 5a; Table S7). Of these,
glutamine and glutamate are directly linked to N assimilation, whilst
citrate is a precursor for 2-oxoglutarate needed in this process.
Furthermore, malate and citrate valves are directly linked to carbon
fixation and the tricarboxylic acid (TCA) cycle, providing the
substrates for amino acids synthesis, and their conversion can favor the
accumulation of other metabolites, such as fumarate (Eprintsev, Fedorin,
Sazonova, & Igamberdiev, 2016).
To evaluate if metabolic plasticity relates to plasticity of the four
phenotypes, we performed correlation analysis between the CVs of
metabolites and CVs of phenotypes ERD, FRD, FT and YIE over the 43
accessions (Figure 5b). We found a significant positive correlation
between the plasticity of ERD and plasticities of benzoate,
dehydroascorbate (dimer), glutamine, and phosphorate levels, and a
significant negative correlation with plasticity of glucose levels.
Therefore, our findings suggest that stable levels of glucose over
different N conditions, in contrast to dehydroascorbate, are associated
with the higher plasticity of ERD. Further, plasticity of FRD showed
significant negative correlation with plasticity of fumarate and
glycerate levels, and positive correlation with plasticities in lactate
and fucose levels (Figure 5b). The plasticity of FT showed positive
correlation with homoserine levels, while that of YIE positively
correlated with plasticity of fructose, glucose, rhamnose and shikimate
levels (Figure 5b). The latter indicates that stable sugar contents
stabilizes yield in respond to N availability.
Next, we asked if the significant correlation between the plasticity of
a complex phenotype and the plasticity of metabolite levels was observed
between the mean metabolite levels and the mean levels of ERD, FRD, FT
and YIE in each N condition separately. Only two metabolites, benzoate
and lactate, showed significant correlation both between the mean and
plasticity values of the same trait; benzoate to ERD and lactate to FRD
respectively (Figure 5, Figure S4). This analysis supports the
hypothesis that the plasticities and the mean values of the studied
phenotypes are independently regulated.