Metabolic responses to P-stress
Ion, lipid and primary metabolite levels were determined to uncover
metabolic responses of switchgrass to P limitation. Levels (mg/kg
tissue) of all anions and cations tested changed significantly in shoots
and/or roots in response to P-stress. Pi levels decreased substantially
in both shoots (up to 3-fold) and roots (>10-fold) under
P-stress, while nitrate and sulfate levels tended to increase in these
organs and conditions (Figure 3a ). In contrast, chloride and
malate levels tended to decrease under P-stress, possibly in response to
the increasing levels of nitrate in order to maintain charge balance. In
this context, potassium (K) and sodium (Na) levels decreased
significantly in shoots and/or roots under P-stress.
P limitation affected lipid composition of switchgrass shoots and roots
(Figure 3b ). Generally, phospholipid levels, including
phosphatidylcholine (PC), phosphatidylethanolamine (PE) and
phosphatidylserine (PS) decreased with increasing P-stress, in both
shoots and roots, with the largest decreases observed under severe
P-stress. In contrast, non-P lipids, including digalactosyl
diacylglycerol (DGDG), monogalactosyl diacylglycerol (MGDG) and
sulphoquinovosyl diacylglycerol (SQDG) increased significantly under
moderate and severe P stress compared to P-replete roots. Interestingly,
while these glyco- and sulfolipids increased under mild and moderate
P-stress in shoots, their levels in severely P-stressed shoots were more
similar to those in P-sufficient plants. Levels of triacylglycerol (TAG)
remained stable under mild or moderate P-stress in shoots and declined
by ~30% under severe P-stress, while levels of TAG
increased in roots with increasing P-stress.
Primary metabolites were profiled by GC/MS to gain insight into the
metabolic consequences of P-stress. A total of 95 compounds were
identified, 55 of which were selected after t-test filter
(P <0.05) for further principal component analysis (PCA;Figure 3c ). Principal component 1 (PC1) and PC2 accounted for
52 and 38 % of the total variation, respectively. PCA of the metabolite
data revealed distinct metabolic signatures for shoots and roots, and at
different levels of P-stress (Figure 3c ). Profiled metabolites
can be classified into five categories, including phosphate, sugars and
sugar alcohols, organic acids, amino acids, and miscellaneous
metabolites. In general, most sugars and sugar alcohols decreased with
increasing P stress, with a few exceptions including sucrose,
gluconolactone, erythritol and erythrose. Organic acid levels increased
under mild and moderate P-stress in both shoots and roots, although this
trend reversed under severe P-stress, especially in shoots. Likewise,
levels of most amino acids increased with mild and moderate P-stress,
and declined especially in shoots under severe P-stress (Figure
4 ).