Pathways regulating threonine, methionine and isoleucine metabolism are very efficiently interconnected in plants. As both threonine and methionine serve as substrates for isoleucine synthesis, their synthesis and catabolism under different developmental and environmental conditions also influence isoleucine availability. Together, methionine gamma-lyase and threonine deaminase maintain the isoleucine equilibrium in plants under varied substrate availabilities. Isoleucine and the two other branched-chain amino acids (BCAAs) (leucine and valine) share four common enzymes in their biosynthesis pathways and thus are coordinately regulated. Induction of free amino acids as osmolytes in response to abiotic stress is thought to play a role in plant stress tolerance. In particular, the accumulation of BCAAs is induced many-fold during osmotic stress. However, unlike in the case of proline, not much research has been focused on understanding the function of the response involving BCAAs. This review describes pathways influencing branched-chain amino acid metabolism and what is known about the biological significance of their accumulation under abiotic stress. A bioinformatics approach to understanding the transcriptional regulation of the genes involved in amino acid metabolism under abiotic stress is also presented.

SummaryBeyond their essential function as the building blocks of proteins, amino acids contribute to many aspects of plant biochemistry and physiology. Despite this, there are relatively large gaps in our understanding of the biochemical pathways and regulation of amino acid synthesis in plants. A rapid (1.5 min versus 20-90 min for standard methods) HPLC-MS/MS assay for separating 19 amino acids was developed for quantifying levels of free amino acids in plant tissue. This assay was used to determine the free amino acid content in the seeds of 10,000 randomly mutagenized Arabidopsis lines, and 322 Arabidopsis lines with increased levels of one or more amino acids were identified. The heritability of the mutant phenotype was confirmed for 43 lines with increased seed levels of the aspartate-derived amino acids Ile, Lys, Thr, or Met. Genetic mapping and DNA sequencing identified a mutation in an Arabidopsis threonine aldolase (AT1G08630, EC 4.1.2.5) as the cause of increased seed Thr levels in one mutant. The assay that was developed for this project has broad applicability to Arabidopsis and other plant species.

Since research on plant interactions with herbivores and pathogens is often constrained by the analysis of already known compounds, there is a need to identify new defense-related plant metabolites. The uncommon nonprotein amino acid N d -acetylornithine was discovered in a targeted search for Arabidopsis thaliana metabolites that are strongly induced by the phytohormone methyl jasmonate (MeJA). Stable isotope labeling experiments show that, after MeJA elicitation, Arg, Pro, and Glu are converted to Orn, which is acetylated by NATA1 to produce N d -acetylornithine. MeJA-induced N d -acetylornithine accumulation occurs in all tested Arabidopsis accessions, other Arabidopsis species, Capsella rubella, and Boechera stricta, but not in less closely related Brassicaceae. Both insect feeding and Pseudomonas syringae infection increase NATA1 expression and N d -acetylornithine accumulation. NATA1 transient expression in Nicotiana tabacum and the addition of N d -acetylornithine to an artificial diet both decrease Myzus persicae (green peach aphid) reproduction, suggesting a direct toxic or deterrent effect. However, since broad metabolic changes that are induced by MeJA in wild-type Arabidopsis are attenuated in a nata1 mutant strain, there may also be indirect effects on herbivores and pathogens. In the case of P. syringae, growth on a nata1 mutant is reduced compared with wild-type Arabidopsis, but growth in vitro is unaffected by N d -acetylornithine addition.