Lysine Catabolism Through the Saccharopine Pathway: Enzymes and Intermediates Involved in Plant Responses to Abiotic and Biotic Stress

Abstract

The saccharopine pathway (SACPATH) involves the conversion of lysine into α-aminoadipate by three enzymatic reactions catalyzed by the bifunctional enzyme lysine-ketoglutarate reductase/saccharopine dehydrogenase (LKR/SDH) and the enzyme α-aminoadipate semialdehyde dehydrogenase (AASADH). The LKR domain condenses lysine and α-ketoglutarate into saccharopine, and the SDH domain hydrolyzes saccharopine to form glutamate and α-aminoadipate semialdehyde, the latter of which is oxidized to α-aminoadipate by AASADH. Glutamate can give rise to proline by the action of the enzymes 1 -pyrroline-5-carboxylate synthetase (P5CS) and 1 -pyrroline-5-carboxylate reductase (P5CR), while 1 -piperideine-6-carboxylate the cyclic form of α-aminoadipate semialdehyde can be used by P5CR to produce pipecolate. The production of proline and pipecolate by the SACPATH can help plants face the damage caused by osmotic, drought, and salt stress. AASADH is a versatile enzyme that converts an array of aldehydes into carboxylates, and thus, its induction within the SACPATH would help alleviate the toxic effects of these compounds produced under stressful conditions. Pipecolate is the priming agent of N-hydroxypipecolate (NHP), the effector of systemic acquired resistance (SAR). In this review, lysine catabolism through the SACPATH is discussed in the context of abiotic stress and its potential role in the induction of the biotic stress response.

Connections Between Amino Acid Metabolisms in Plants: Lysine as an Example

“…Lysine metabolism is involved in the plant stress response in various forms. It is mainly catabolized through the saccharopine pathway, which has been shown to play a role in abiotic and biotic stress responses (Figures 1B, C) (Kiyota et al, 2015;Bernsdorff et al, 2016;Arruda and Barreto, 2020). Moreover, under salt and osmotic stress, LKR/SDH expression was found to increase, while downstream metabolite pipecolate was enhanced (Kiyota et al, 2015).…”

Rethinking of the Roles of Endophyte Symbiosis and Mycotoxin in Oxytropis Plants

“…In this context, it is also worth noticing that endophyte levels decrease in underground compared to aboveground tissues [46] (Figure 2). Therefore, it was speculated that locoweed endophytes may prefer to live in tissues with stronger photosynthetic activity and higher lysine levels [34], similar to the evidence demonstrated in grass and other plant species [47,48]. It is possible that the endophyte may preferentially infect certain tissue types or are distributed in a gradient as the plant grows from a seed to maturity plant.…”

“…These include such as the LKR/SDH and AASADH (aminoadipate semialdehyde dehydrogenase) and these many coupled to P5CR activities in order to keep the AASA concentrations to below toxic levels [75]. In the case of halophytes at least, this could be an adaptive pathway that sees increased levels of osmolytes proline and PA under salinity [47]. Being able to produce swainsonine from AASA, A. sect.…”

Bayesian Network Analysis of Lysine Biosynthesis Pathway in Rice

“…Over the last 50 years, lysine metabolism has been extensively studied. It has been shown that lysine is a self-regulating amino acid as the lysine biosynthesis pathway has two inhibition feedback loops [31][32][33]. These feedback loops are activated by the free lysine content, which negatively regulates the enzymes dihydrodipicolinate synthase (DHPS) and aspartate kinase (AK) [24,34].…”

“…The LKR and SDH enzymes are present in the saccharopine pathway and they initiate the lysine catabolism process through the TCA cycle (tricarboxylic acid cycle) [24]. The metabolic pathway of lysine biosynthesis and catabolism in presented in Figure 1 [31][32][33]. Thus, lysine can be enriched in cereal plants by enhancing its production in the biosynthesis pathway, preventing its catabolism, or combining these two approaches.…”