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 Δ¹-pyrroline-5-carboxylate synthetase (P5CS) and Δ¹-pyrroline-5-carboxylate reductase (P5CR), while Δ¹-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.

Keywords: Saccharopine pathway; abiotic stress; biotic stress; drought stress; lysine catabolism.

FIGURE 1

The aspartate pathway for lysine biosynthesis and saccharopine pathway for lysine catabolism. (A) Schematic representation of the aspartate pathway. Only the key biosynthetic steps are represented. Aspartate is converted into β-aspartyl phosphate that is used in a serial enzymatic step by steps to produce lysine, threonine, methionine and Isoleucine. The aspartate pathway is localized in the plastids and thus lysine needs to be transported to the cytosol and other organelles. (B) The core reactions of the SACPATH involve the conversion of L-lysine into α-aminoadipate. The first two steps of the pathway are catalyzed by the bifunctional enzyme LKR/SDH; the LKR domain condenses L-lysine and α-ketoglutarate into saccharopine using NADPH as a cofactor and the SDH domain hydrolyzes saccharopine into α-aminoadipate semialdehyde and glutamate using NAD(P)⁺ as cofactors. The α-aminoadipate semialdehyde is then oxidized into α-aminoadipate by the enzyme AASADH using NAD(P)⁺ as cofactors.

FIGURE 2

The SACPATH metabolic flux that gives rise to proline and pipecolate. As depicted in Figure 1, the hydrolysis of saccharopine gives rise to glutamate and α-aminoadipate semialdehyde. Glutamate can be converted into proline by the action of P5CS and P5CR. P5CS converts glutamate into Δ¹-pyrroline 5-carboxylate that is than transformed into proline by P5CR. The α-aminoadipate semialdehyde spontaneously cyclizes to Δ¹-piperidine 6-carboxylate, which can serve as a substrate for P5CR to produce pipecolate.

FIGURE 3

Expression profiling of the core genes of the SACPATH and NHP pathway for lysine catabolism in Arabidopsis subjected to biotic and abiotic stresses. Expression profiling datasets of the genes encoding LKR/SDH and AASADH of the SACPATH and ALD1, SARD4, and FMO1 of the NHP pathway, were retrieved from public databases and analyzed using the Genevestigator platform. Only the data of wild type (Col-0 or Ws-0) subjected to control and stress treatments were used in this analysis. The following datasets were used: GSE18978, leaves inoculated or not inoculated with Pseudomonas syringae ES4326; E-MEXP-546, leaves inoculated or not inoculated with the P. syringae pv. tomato avirulent strain DC3000 avrRpm1; GSE19255; leaves inoculated or not inoculated with P, syringae pv. tomato; GSE13739, leaves inoculated or not inoculated with the G. orontii MGH isolate; GSE14961, seedlings grown for 1 day in 2 mM salicylic acid; GSM131295, 16-day-old seedlings grown in 300 mM mannitol (osmotic stress); GSM131319, 16-day-old seedlings grown in 150 mM NaCl (salt stress); GSE10670, plants subjected to drought stress; and RIKENGODA21B, seedlings treated with 10 μM ABA. The values are expressed as log2 of the fold change between treated and untreated plants.

FIGURE 4

Proposed model for the role of the SACPATH in abiotic and biotic stress responses in plants. Upon abiotic stress (AbS, 1) such as that imposed by osmotic, salt, and drought, the cellular concentration of free lysine increase. As the lysine concentration increases, the transcripts encoding bifunctional LKR/SDH, monofunctional SDH, and AASADH accumulate, increasing the lysine-to-α-aminoadipate metabolic flux. In general, the expression of P5CS and P5CR is also upregulated under abiotic stress, and by the action of these two enzymes, glutamate is converted into proline (AbS, 2), and Δ¹-piperidine 6-carboxylate is converted into pipecolate (AbS, 3). Proline and pipecolate are the two most effective osmolytes and help sustain cellular homeostasis under abiotic stress. The upregulation of AASADH helps metabolize aldehydes generated by stress conditions, including, for example, betaine aldehyde, whose conversion generates betaine, another important osmolyte (AbS, 4). Upon pathogen attack (BS, 5) lysine is also catabolized to pipecolate through the NHP pathway by the enzymes ALD1, which deaminates lysine to Dhp, and SARD4, which reduces Dhp to pipecolate. Dhp is then N-hydroxylated by FMO1 to generate NHP, the effector of SAR (BS, 6). In the proposed model, the pipecolate generated by the SACPATH could also serve as a substrate for FMO1 to produce NHP, therefore contributing to SAR.