Amino acid biosynthesis in plants involves several key metabolic pathways, closely linked with various aspects of plant metabolism, including responses to environmental stress and regulation of energy metabolism.

➤ Proline Biosynthesis and Its Role in Stress Resistance

The biosynthesis of proline in plants is particularly important in response to stress. In many plant species, proline accumulates in response to various stresses, such as drought, salinity, or low temperatures. The key enzyme in this pathway is δ^1-pyrroline-5-carboxylate synthetase (P5CS), which catalyzes the rate-limiting step in proline synthesis. Interestingly, plants can regulate proline synthesis both at the transcriptional level and through post-translational mechanisms, adjusting proline production to the current redox state and nitrogen status of the cell.

➤ Lysine Metabolic Pathway and Its Significance

Lysine plays an important role in the energy metabolism of plants. Under energy-deficient conditions, lysine (and isoleucine) can be directly catabolized into the tricarboxylic acid (TCA) cycle. Additionally, lysine metabolism is involved in plant stress response, mainly through the saccharopine pathway, which plays a role in response to abiotic and biotic stress. In this context, an interesting phenomenon is the increase in LKR/SDH expression under salt and osmotic stress and the increase in the level of the metabolite pipecolate, which plays a role in plant defense.

There is also a link between lysine and other metabolites. In response to osmotic stress, the activity of AASADH (α-aminoadiposemialdehyde dehydrogenase) increases, suggesting its role in osmotic stress response. Lysine may also affect the metabolism of proline and tryptophan, participating in various signaling pathways.

➤ Significance of Isoleucine Metabolism

Isoleucine, a branched-chain amino acid (BCAA), plays an important role in plant stress resistance as an osmoregulation factor. Also, homoserine and threonine, derived from the aspartate pathway, may affect plant resistance to pathogens.

➤ Role of Aromatic Amino Acids

Aromatic amino acids in plants are synthesized from chorismate, the end product of the shikimate pathway, and are precursors to a wide range of secondary metabolites. These amino acids can play a role in plant resistance to biotic and abiotic stress.

The shikimate pathway is key to the synthesis of aromatic amino acids such as phenylalanine, tyrosine, and tryptophan. These amino acids are not only involved in protein synthesis but are also precursors to many secondary plant metabolites, including pigments, alkaloids, hormones, and cell wall components. This pathway can be regulated by various external factors, for example, exposure to ozone affects the regulation of the pre-chorismate part of this pathway in tobacco.

These examples highlight the complexity and multidimensionality of amino acid biosynthesis pathways in plants and their significant impact on plants’ ability to adapt and survive in different environmental conditions. Understanding these processes is crucial for agriculture and biotechnology, especially in the context of creating plants with increased resistance to environmental stresses.