Research Article: Overexpression of MYB115, AAD2, or AAD3 in Arabidopsis thaliana seeds yields contrasting omega-7 contents

Date Published: January 30, 2018

Publisher: Public Library of Science

Author(s): Hasna Ettaki, Manuel Adrián Troncoso-Ponce, Alexandra To, Guillaume Barthole, Loïc Lepiniec, Sébastien Baud, Lam-Son Phan Tran.

http://doi.org/10.1371/journal.pone.0192156

Abstract

Omega-7 monoenoic fatty acids (ω-7 FAs) are increasingly exploited both for their positive effects on health and for their industrial potential. Some plant species produce fruits or seeds with high amounts of ω-7 FAs. However, the low yields and poor agronomic properties of these plants preclude their commercial use. As an alternative, the metabolic engineering of oilseed crops for sustainable ω-7 FA production has been proposed. Two palmitoyl-ACP desaturases (PADs) catalyzing ω-7 FA biosynthesis were recently identified and characterized in Arabidopsis thaliana, together with MYB115 and MYB118, two transcription factors that positively control the expression of the corresponding PAD genes. In the present research, we examine the biotechnological potential of these new actors of ω-7 metabolism for the metabolic engineering of plant-based production of ω-7 FAs. We placed the PAD and MYB115 coding sequences under the control of a promoter strongly induced in seeds and evaluated these different constructs in A. thaliana. Seeds were obtained that exhibit ω-7 FA contents ranging from 10 to >50% of the total FAs, and these major compositional changes have no detrimental effect on seed germination.

Partial Text

Vegetable oils enriched in omega-7 (ω-7) monounsaturated fatty acids (FAs) such as palmitoleic acid (cis-ω-7 C16:1) and its elongation product vaccenic acid (cis-ω-7 C18:1) potentially have uses for a number of applications. First, they are attractive for biodiesel formulations since biodiesel containing FA methyl esters produced from oils rich in ω-7s has superior functional properties [1]. In addition, ω-7s from plant oils can offer the chemist promising starting material for catalytic conversions to renewable platform chemicals. Olefin metathesis constitutes a powerful tool for polymer chemistry, and ethenolytic metathesis of ω-7 FAs from plant oils could potentially provide a competitive source of 1-octene to make linear low-density polyethylene [2]. Finally, after the fast growth of ω-3 polyunsaturated FAs in the nutrition market [3], suppliers and marketers are today calling attention to ω-7 FAs since vegetable oils enriched in ω-7s have been ascribed a number of beneficial health properties. Palmitoleic acid in particular is considered a lipokine used by adipose tissues to communicate with distant organs and regulate systemic metabolic homeostasis [4]. Increasing evidence suggests that palmitoleic acid plays a key role in the physiopathology of insulin resistance in humans, increasing muscle response to insulin [5]. Aside from these nutritional functions, palmitoleic acid is attractive for nonfood uses in the skin care industry because of its antioxidant and antimicrobial properties [6].

Iterative optimization of unusual FA production in seeds not only requires the stacking of multiple traits [36], but also implies the evaluation and comparison of different gene versions for a given trait and that of regulatory elements for driving the expression of selected transgenes. The results presented in this study demonstrate that overexpression of either of the two A. thaliana PADs in seeds is sufficient to partially shift monoene production from ω-9 to ω-7 FAs. This result can be attained both by overexpression of the desaturase-coding sequences placed under the control of the AT2S2 promoter and by overexpression of one of their transcriptional activators, such as MYB115. With regard to the efficiency of the different strategies evaluated, the latter appears to have lower efficiency. The relative ω-7 FA content of ProAT2S2:MYB115 seeds is five time less than that of ProAT2S2:AAD2 seeds on average. Further, increased ω-7 levels are observed only in embryos of MYB115 overexpressing lines when the production of ω-7 monoenes is increased both in the embryo and in the endosperm of ProAT2S2:AAD2 and ProAT2S2:AAD3 seeds. These results suggest that transcriptional activation of PAD genes constitutes the main limiting factor in the control of ω-7 accumulation in A. thaliana seeds. This is in agreement with the QTL approach described by Bryant et al. [11], which led to the identification of a single major QTL determining the ω-7 FA content in seeds, the peak of which colocalized with AAD3. Other actors involved in the biosynthesis (acyl-carrier proteins and ferredoxins), the export, or the acylation of ω-7 monoenes seem to be expressed in seeds at a level sufficient to support an important shift from ω-9 to ω-7 FA production. It is tempting to speculate that the substrate range of the enzymes of lipid metabolism using ω-9 monoenes in a wild-type context is broad enough to efficiently metabolize ω-7 monoenes in a transgenic background. This is probably due to the fact that the structures of ω-9 and ω-7 FAs are similar enough. The structure of cis-ω-12 C18:1 (petroselenic acid) or cis-ω-10 C16:1 (sapienic acid) is more divergent. These monoenes are accumulated at high levels in seeds of Coriandrum sativum and Thunbergia alata, respectively. Specialized AADs catalyzing the synthesis of these unusual monoenes have been identified and characterized [37,38]. Expression of the corresponding cDNAs in seeds of A. thaliana yielded limited accumulation (<15 mol%) of the corresponding FAs [39], suggesting that specialized actors other than the AADs [40–42] may be required for efficient channeling of these unusual monoene species.   Source: http://doi.org/10.1371/journal.pone.0192156

 

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