Amino acids – A life between metabolism and signaling

Highlights

• Certain amino acids and derivatives have signaling functions in mammals and plants.

• Serine is essential for cell proliferation.

• GABA has a function in sexual reproduction and cell identity.

• Neolignans deriving from phenylalanine act as signals.

• Hydroxycinnamic acid amides play a profound role in the pathogen response of plants.

Abstract

Amino acids serve as constituents of proteins, precursors for anabolism, and, in some cases, as signaling molecules in mammalians and plants. This review is focused on new insights, or speculations, on signaling functions of serine, γ-aminobutyric acid (GABA) and phenylalanine-derived phenylpropanoids. Serine acts as signal in brain tissue and mammalian cancer cells. In plants, de novo serine biosynthesis is also highly active in fast growing tissues such as meristems, suggesting a similar role of serine as in mammalians. GABA functions as inhibitory neurotransmitter in the brain. In plants, GABA is also abundant and seems to be involved in sexual reproduction, cell elongation, patterning and cell identity. The aromatic amino acids phenylalanine, tyrosine, and tryptophan are precursors for the production of secondary plant products. Besides their pharmaceutical value, lignans, neolignans and hydroxycinnamic acid amides (HCAA) deriving from phenylpropanoid metabolism and, in the case of HCAA, also from arginine have been shown to fulfill signaling functions or are involved in the response to biotic and abiotic stress. Although some basics on phenylpropanoid-derived signaling have been described, little is known on recognition- or signal transduction mechanisms. In general, mutant- and transgenic approaches will be helpful to elucidate the mechanistic basis of metabolite signaling.

Introduction

All life depends on a constant flow of metabolites that provide building blocks as well as energy and reducing power for growth, development, and reproduction. Beside of their role in biochemistry, metabolic intermediates can also serve as signaling molecules contributing to the complex regulatory network that eventually adapts gene expression to altered requirements during the life cycle, or as a response to a changing environment. In this review we focus on the dual functions of certain amino acids and their derivatives as metabolic intermediates/end-products and signaling molecules. Such dual functions are well documented in the medical/mammalian field, and evidence for similar functions is also emerging for the plant system.

The amino acid serine has recently been suggested to act as a signal controlling the proliferation of mammalian cancer cells [1], [2]. As the demand for nutrients in fast growing cells is high, the nutritional state determines the rate of cell proliferation. In plants, de novo serine biosynthesis is highly active in fast growing tissues, such as meristems [3] suggesting a similar role of serine as signaling molecule in plants.

Likewise, in the mammalian brain glutamate-derived γ-amino butyric acid (GABA) is an inhibitory neurotransmitter that exerts its signaling effect after binding to specific receptors [4]. In plants, evidence for GABA-dependent signal transduction pathways exists and awaits a detailed characterization.

Besides their role as constituents of proteins, the aromatic amino acids phenylalanine, tyrosine and tryptophan are the precursors for a variety of secondary products [5], [6] among them compounds with signaling function. The phenylpropanoid pathway, starting from phenylalanine delivers, for instance, the neolignan dehydrodiconiferyl alcohol glucoside (DCG), which has been shown to exert cytokinine-like effects in plants [7], [8], [9]. Likewise, amines and polyamines deriving from the amino acid arginine together with the phenylpropanoid p-coumaric acid converge in the synthesis of hydroxycinnamic acid amides (HCAAs). HCAAs are involved in stress- and pathogen responses and might also act as signaling molecules during developmental processes [10].

Fig. 1 shows an overview on the compartmentation of anabolic and catabolic pathways in a mesophyll cell including branch points leading to those metabolic signals that are highlighted in this review. In contrast to catabolism, which is mainly localized in the cytosol or mitochondria, the majority of the anabolic reaction sequences are initiated in the plastid stroma. Chloroplasts are the site of CO₂-, ammonia- and sulphur assimilation and of a variety of pathways leading to the biosynthesis of building blocks like fatty acids [11], aromatic amino acids [5], [6], branched chain amino acids [12], isoprenoids via the mevalonate-independent way (methylerythritol 4-phosphate pathway; [13], serine [3], [14], and arginine [15]). The glycolytic intermediate phosphoenolpyruvate (PEP) obviously plays a central role both in anabolism and catabolism [16] and hence also in the production of amino acid derived signaling molecules.

In this review we elucidate the dual or multiple functions of serine, GABA, neolignans like DCG as well HCAAs with respect to metabolism and signaling. Mutant plants impaired in the biosynthesis of amino acids or downstream products might help to dissect the involvement of amino acid metabolism in cellular signaling processes.