Plant Metabolomics: A Characterization of Plant Responses to Abiotic Stresses

Plants manage abiotic stress injuries due to adverse environmental conditions through a plethora of molecular, biochemical and physiological changes resulting in adjustment of metabolic pathways to reach a new homeostatic equilibrium. The degree of stress tolerance among plant species is supposed to depend on the quantitative and qualitative differences in the accumulation of specific compounds. Therefore, the identification of stress-induced metabolic changes is fundamental to improve crop productivity under unfavourable conditions. The new state of homeostasis reached under stress is characterized by the accumulation of specific compounds. Although some of them are by-products due to the cellular damage occurred under stress, most of them have a functional role both as signal transduction molecules and as compounds directly involved in stress tolerance mechanism. In this regard, the possibility to monitor a complete set of metabolites could largely improve the understanding of these adaptation mechanisms. This systematic study defined "metabolomics" is intended to provide an integrated view of the functional status of an organism, significantly contributing to study stress biology in plants. Depending on the question to be addressed, specific approaches or their combination can be used in metabolomic investigation: metabolic fingerprinting (a global screening of the different classes of metabolites), metabolic profiling (simultaneous quantification of all or a set of metabolites) and targeted analysis (focused measurement of specific or a set of metabolites). A variety of analytical techniques such as, GC-MS, LC-DAD-MS, FT-IR, NMR, are successfully employed for metabolic fingerprinting and profiling, whereas targeted analysis is performed using both the above mentioned techniques (integrated with the use of spiking experiments or the in vivo labelling) and the more traditional biochemical analysis. The huge volumes of data generated by these approaches require advanced bioinformatics tools, such as multivariate statistic analysis (supervised or unsupervised), to increase knowledge. Moreover, in the last years, metabolomics data handling has been improved because of the development of publicly available bioinformatics tools and databases. Here, we review a selection of recent publications in this field highlighting advantages and limits of the above mentioned approaches and techniques. We will focus on metabolite changes induced by single and multiple abiotic stresses. The meaning of specific and unspecific responses of plants to different abiotic stresses will be discussed, as well as differences observed in the response to multiple stress compared to a single one. Further information on the mechanism of plant stress response could be derived from the comparison of metabolite profiling among plants differing in their tolerance. Some authors analysed, through metabolic fingerprinting and profiling, the changes induced by stress in species and/or cultivars highlighting quantitative or qualitative differences correlating with the phenotype. Similarly, the metabolic content of wild type plants towards mutants or transgenics have been reported. These differences in compounds content can represent crucial predictors for stress tolerance both in varieties screening and plant breeding programs. Finally, we will discuss the potentiality of the global analysis of data obtained with different "omics" approaches, such as the integrated metabolome and transcriptome analyses on model plants, as a valuable strategy to attain an holistic view of mechanisms sustaining stress tolerance in plants.