Plant monounsaturated fatty acids: Diversity, biosynthesis, functions and uses.

Monounsaturated fatty acids are straight-chain aliphatic monocarboxylic acids comprising a unique carbon­carbon double bond, also termed unsaturation. More than 50 distinct molecular structures have been described in the plant kingdom, and more remain to be discovered. The evolution of land plants has apparently resulted in the convergent evolution of non-homologous enzymes catalyzing the dehydrogenation of saturated acyl chain substrates in a chemo-, regio- and stereoselective manner. Contrasted enzymatic characteristics and different subcellular localizations of these desaturases account for the diversity of existing fatty acid structures. Interestingly, the location and geometrical configuration of the unsaturation confer specific characteristics to these molecules found in a variety of membrane, storage, and surface lipids. An ongoing research effort aimed at exploring the links existing between fatty acid structures and their biological functions has already unraveled the importance of several monounsaturated fatty acids in various physiological and developmental contexts. What is more, the monounsaturated acyl chains found in the oils of seeds and fruits are widely and increasingly used in the food and chemical industries due to the physicochemical properties inherent in their structures. Breeders and plant biotechnologists therefore develop new crops with high monounsaturated contents for various agro-industrial purposes.

Acyl-Acyl Carrier Protein Desaturases and Plant Biotic Interactions.

Interactions between land plants and other organisms such as pathogens, pollinators, or symbionts usually involve a variety of specialized effectors participating in complex cross-talks between organisms. Fatty acids and their lipid derivatives play important roles in these biological interactions. While the transcriptional regulation of genes encoding acyl-acyl carrier protein (ACP) desaturases appears to be largely responsive to biotic stress, the different monounsaturated fatty acids produced by these enzymes were shown to take active part in plant biotic interactions and were assigned with specific functions intrinsically linked to the position of the carbon-carbon double bond within their acyl chain. For example, oleic acid, an omega-9 monounsaturated fatty acid produced by Δ9-stearoyl-ACP desaturases, participates in signal transduction pathways affecting plant immunity against pathogen infection. Myristoleic acid, an omega-5 monounsaturated fatty acid produced by Δ9-myristoyl-ACP desaturases, serves as a precursor for the biosynthesis of omega-5 anacardic acids that are active biocides against pests. Finally, different types of monounsaturated fatty acids synthesized in the labellum of orchids are used for the production of a variety of alkenes participating in the chemistry of sexual deception, hence favoring plant pollination by hymenopterans.

Molecular Control of Oil Metabolism in the Endosperm of Seeds.

In angiosperm seeds, the endosperm develops to varying degrees and accumulates different types of storage compounds remobilized by the seedling during early post-germinative growth. Whereas the molecular mechanisms controlling the metabolism of starch and seed-storage proteins in the endosperm of cereal grains are relatively well characterized, the regulation of oil metabolism in the endosperm of developing and germinating oilseeds has received particular attention only more recently, thanks to the emergence and continuous improvement of analytical techniques allowing the evaluation, within a spatial context, of gene activity on one side, and lipid metabolism on the other side. These studies represent a fundamental step toward the elucidation of the molecular mechanisms governing oil metabolism in this particular tissue. In particular, they highlight the importance of endosperm-specific transcriptional controls for determining original oil compositions usually observed in this tissue. In the light of this research, the biological functions of oils stored in the endosperm of seeds then appear to be more diverse than simply constituting a source of carbon made available for the germinating seedling.

Abiotic stress signalling in extremophile land plants.

Plant life relies on complex arrays of environmental stress sensing and signalling mechanisms. Extremophile plants develop and grow in harsh environments with extremes of cold, heat, drought, desiccation, or salinity, which have resulted in original adaptations. In accordance with their polyphyletic origins, extremophile plants likely possess core mechanisms of plant abiotic stress signalling. However, novel properties or regulations may have emerged in the context of extremophile adaptations. Comparative omics of extremophile genetic models, such as Arabidopsis lyrata, Craterostigma plantagineum, Eutrema salsugineum, and Physcomitrella patens, reveal diverse strategies of sensing and signalling that lead to a general improvement in abiotic stress responses. Current research points to putative differences of sensing and emphasizes significant modifications of regulatory mechanisms, at the level of secondary messengers (Ca2+, phospholipids, reactive oxygen species), signal transduction (intracellular sensors, protein kinases, transcription factors, ubiquitin-mediated proteolysis) or signalling crosstalk. Involvement of hormone signalling, especially ABA signalling, cell homeostasis surveillance, and epigenetic mechanisms, also shows that large-scale gene regulation, whole-plant integration, and probably stress memory are important features of adaptation to extreme conditions. This evolutionary and functional plasticity of signalling systems in extremophile plants may have important implications for plant biotechnology, crop improvement, and ecological risk assessment under conditions of climate change.