Abiotic Stress in Plants and Metabolic Responses - Abiotic Stress: Plant Responses and Applications in Agriculture - page 32

remain unresolved concerning hydric stress-plant metabolic response: How does drought
stress perturb metabolism in crop plants? How does hydric stress affect the metabolism of wild
plants? What modern strategies of “omics” could be exploited to support future programs of
crop breeding to lead to a more sustainable agriculture?
As previously described, one of the main mechanisms by which plants cope with water deficits
is osmotic adjustment. These adjustments maintain a positive cell turgor via the active
accumulation of compatible solutes. Traditionally, the analysis of metabolic responses to
drought stress was limited to analysis of one or two classes of compounds considered as “role
players” in the development of tolerance. Application of metabolomic approaches is providing
a less biased perspective of metabolic profiles of response and also is aiding in the discovery
of novel metabolic phenotypes. Unbiased GC-MS metabolomic profiling in Eucalyptus showed
that drought stress alters a larger number of leaf metabolites than the previously reported in
targeted analysis. Accumulation of shikimic acid and two cyclohexanepentol stereoisomers in
response to drought stress was described for the first time in Eucalyptus . Also, the magnitude
of metabolic adjustments in response to water stress correlates with the sensitivity/tolerant
phenotype observed; drought affected around 30-40% of measured metabolites in Eucalyptus
dumosa (a drought-sensitive specie) compared to 10-15% in Eucalyptus pauciflora (a drought-
tolerant specie) [86]. Similarly, critical differences in the metabolic responses were observed
when drought-tolerant (NA5009RG) and drought-sensitive (DM50048) soybean cultivars were
analyzed by 1 H NMR-based metabolomics. Interestingly, no enhanced accumulation of the
traditional osmoprotectants, such as proline, soluble sugars as sucrose or myo-inositol, organic
acids or other amino acids (except for aspartate), were detected in the leaves of either genotype
during water stress. In contrast, levels of 2-oxoglutaric acid, pinitol, and allantoin were affected
differentially in the genotypes when drought was imposed, suggesting possible roles as
osmoprotectants [87]. In contrast to soybean, levels of amino acids, including proline, trypto‐
phan, leucine, isoleucine, and valine, were increased under drought stress in three different
cultivars of wheat ( Triticum aestivum ) analyzed for 103 metabolites in a targeted GC-MS
approach [88]. Metabolic adjustments in response to adverse conditions are transient and
depend on the severity of the stress. In a 17-day time course experiment in maize ( Zea mays )
subjected to drought stress, GC-MS metabolic analysis revealed changes in concentrations of
28 metabolites. Accumulation of soluble carbohydrates, proline and eight other amino acids,
shikimate, serine, glycine, and aconitase, was accompanied by the decrement of leaf starch,
malate, fumarate, 2-oxoglutarate, and seven amino acids during the drought treatment course.
However, as the water potential became more negative, between the 8 th and 10 th days, the
changes in some metabolites were more dramatic, demonstrating their dependence on stress
severity [89].
Accumulation of compatible solutes is an evolutionary conserved trait in bacteria, plants,
animal cells, and marine algae. A recent GC-MS metabolomic analysis confirmed that the moss
Physcomitrella patens also triggers compatible solute accumulation in response to drought
stress. After two weeks of physiological drought stress, 26 metabolites were differentially
affected in gametophores, including altrose, maltitol, L-proline, maltose, isomaltose, and
butyric acid, comparable to metabolic adjustments previously reported in stressed Arabidop‐