About this Research Topic
Amino acids which are incorporated into proteins are called natural or proteinogenic amino acids (PAAs), and are understandably essential for all living organisms. But they harbor many other amino acids. These so-called non-proteinogenic amino acids (NPAAs) are by far not simply dispensable as their contrasting naming may apply. Their number is a multiple of the PAAs, their structure is far more diverse and they are fulfilling a plethora of functions from which many are not sufficiently understood or unknown at all.
In this Research Topic we aim to set the stage for understanding and advancing the knowledge of NPAAs in plants: there are a number of these amino acids which are precursors or metabolites of PAAs, being one major aspect of interest. At the same time, they are linking the synthesis of different ones like ornithine, citrulline, and L-arginosuccinate between arginine and aspartate biosynthesis, or homoserine, homocysteine and cystathionine between the biosynthesis of methionine and cysteine. There is a significant gap in our knowledge about their stability, tissue specificity and partitioning, sub-cellular and long-distance transport, and role in the feedback regulation of PAAs. Furthermore, they are still a matter of intensive research for a better understanding of primary metabolism and applied aspects of crop improvement, like their role in nitrogen assimilation, osmotic regulations against abiotic stresses and as defensive molecules against herbivores or pathogens.
Another aspect of interest is given by NPAA functions in plants, apart from metabolization, as it is the case, for instance, of γ-aminobutyric acid (GABA), 1-aminocyclopropane carboxylate (ACC), or pipecolic acid. These amino acids were found to have a direct influence on plant development and physiology. Partially, their functions have been unraveled long ago, like for ACC, but the regulation of this direct ethylene precursor is still not fully understood, though the conjugation of ACC to inactive forms is considered to be a way to control ethylene biosynthesis in plants. In contrast, it has been shown recently that pipecolic acid has a crucial impact on pathogen resistance and GABA on biotic and abiotic stress in plants. Nevertheless, the understanding of the functions of these NPAAs will contribute to a deeper understanding of plant physiology.
There are also other NPAAs in plants for which information is still scarce and which are also in the focus of this Research Topic. D-enantiomers of PAAs are such a class of compounds: They are found in reasonable amounts in soil, they are taken up and even synthesized by plants. In bacteria, these D-amino acids (D-AAs) are mainly an indispensable part of their cell walls, and in animals their functions range from poison precursors to signal transmitters. However, in plants, the physiological role of D-AAs is still unraveled and a matter of intensive debate.
This Research Topic aims to collect and present the current knowledge about transport, metabolism and functions of the large class of NPAAs in plants. Therefore, all tier one type articles (e.g. Original Research, Review, Methods and Perspective articles) are welcome.
Dr. Vijay Joshi affiliated to the Texas A&M AgriLife Research is collaborating with Dr. Uener Kolukisaoglu, Dr. Mark Stahl, Dr. Gyeong Mee Yoon and Dr. Georg Jander as an Editorial Assistant in this Research Topic.
Keywords: D-amino acids, ACC, GABA, Primary metabolism, Abiotic and biotic stress resistance
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