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.

Differential Activation of Partially Redundant Δ9 Stearoyl-ACP Desaturase Genes Is Critical for Omega-9 Monounsaturated Fatty Acid Biosynthesis During Seed Development in Arabidopsis.

The spatiotemporal pattern of deposition, final amount, and relative abundance of oleic acid (cis-ω-9 C18:1) and its derivatives in the different lipid fractions of the seed of Arabidopsis (Arabidopsis thaliana) indicates that omega-9 monoenes are synthesized at high rates in this organ. Accordingly, we observed that four Δ9 stearoyl-ACP desaturase (SAD)-coding genes (FATTY ACID BIOSYNTHESIS2 [FAB2], ACYL-ACYL CARRIER PROTEIN5 [AAD5], AAD1, and AAD6) are transcriptionally induced in seeds. We established that the three most highly expressed ones are directly activated by the WRINKLED1 transcription factor. We characterized a collection of 30 simple, double, triple, and quadruple mutants affected in SAD-coding genes and thereby revealed the functions of these desaturases throughout seed development. Production of oleic acid by FAB2 and AAD5 appears to be critical at the onset of embryo morphogenesis. Double homozygous plants from crossing fab2 and aad5 could never be obtained, and further investigations revealed that the double mutation results in the arrest of embryo development before the globular stage. During later stages of seed development, these two SADs, together with AAD1, participate in the elaboration of the embryonic cuticle, a barrier essential for embryo-endosperm separation during the phase of invasive embryo growth through the endosperm. This study also demonstrates that the four desaturases redundantly contribute to storage lipid production during the maturation phase.

AtMYB92 enhances fatty acid synthesis and suberin deposition in leaves of Nicotiana benthamiana.

Acyl lipids are important constituents of the plant cell. Depending on the cell type, requirements in acyl lipids vary greatly, implying a tight regulation of fatty acid and lipid metabolism. The discovery of the WRINKLED1 (WRI1) transcription factors, members of the AP2-EREBP (APETALA2-ethylene-responsive element binding protein) family, has emphasized the importance of transcriptional regulation for adapting the rate of acyl chain production to cell requirements. Here, we describe the identification of another activator of the fatty acid biosynthetic pathway, the Arabidopsis MYB92 transcription factor. This MYB and all the members of the subgroups S10 and S24 of MYB transcription factors can directly activate the promoter of BCCP2 that encodes a component of the fatty acid biosynthetic pathway. Two adjacent MYB cis-regulatory elements are essential for the binding and activation of the BCCP2 promoter by MYB92. Overexpression of MYB92 or WRI1 in Nicotiana benthamiana induces the expression of fatty acid biosynthetic genes but results in the accumulation of different types of acyl lipids. In the presence of WRI1, triacylglycerol biosynthetic enzymes coded by constitutively expressed genes efficiently channel the excess fatty acids toward reserve lipid accumulation. By contrast, MYB92 activates both fatty acid and suberin biosynthetic genes; hence, the remarkable increase in suberin monomers measured in leaves expressing MYB92. These results provide additional insight into the molecular mechanisms that control the biosynthesis of an important cell wall-associated acylglycerol polymer playing critical roles in plants.