Abiotic Stresses and Non-Protein Amino Acids in Plants

Plants have developed several strategies to overcome the broad scope of environmental challenges imposed by biotic and abiotic stresses, including chemical defenses known as secondary metabolites, natural products or, more recently, specialized metabolites, i.e. chemicals often not present in all plant species, which are mostly involved in ecochemical interactions. Nonprotein amino acids (NPAAs) comprise a large heterogeneous group of nitrogen-containing specialized metabolites with wide distribution in the Plant Kingdom, commonly found in several genera of Fabaceae. Various examples of toxic effects attributed to the NPAAs on animal, microbial and other plant cells are known and often related to their structural similarities to protein amino acids (PAAs). However, NPAAs have also been shown to play important roles in planta as protectant molecules against oxidative damage, besides increasing tolerance of different plant species to a variety of abiotic-induced stresses, such as drought, salinity, and temperature. In this review, we discuss well-established and novel functions recently unveiled for NPAAs, besides alternative modes of action proposed for these metabolites as key mediators and effectors in responses to abiotic stresses.