Functional and predictive structural characterization of WRINKLED2, a unique oil biosynthesis regulator in avocado

Authors: BEHERA, JYOTI - East Tennessee State University; RAHMAN, MD - East Tennessee State University; BHATIA, SHIHA - East Tennessee State University; Shockey, Jay; KILARU, ARUNA - East Tennessee State University

Submitted to: Frontiers in Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/3/2021
Publication Date: 6/8/2021
Citation: Behera, J.R., Rahman, M.M., Bhatia, S., Shockey, J., Kilaru, A. 2021. Functional and predictive structural characterization of WRINKLED2, a unique oil biosynthesis regulator in avocado. Frontiers in Plant Science. 12:648494. https://doi.org/10.3389/fpls.2021.648494.
DOI: https://doi.org/10.3389/fpls.2021.648494

Interpretive Summary: Much has been learned about the basic enzyme reactions that control vegetable oil production in plants over the past 25 years. Yet, most of the details regarding how plants fine-tune the balance between production of sugars, proteins, and oil in their storage tissue remains unresolved. A novel protein called Wrinkled1 (WRI1) that helps to control this balance was recently discovered. WRI1 is part of a small gene family, typically made up of four members in most plants. In some higher plant species, some of the WRI family members have lost their function, and now seem to play no direct role in vegetable oil metabolism. However, WRI family members, especially WRI2, have remained active in avocado, which produces about 60-70% oil by dry weight in its fruit. Avocado WRI2 was analyzed in detail at the molecular level, and found to indeed be capable of driving increases in oil content. Its genetic sequence revealed several features that likely explain why it has remained active during the evolution of the plant branch of the tree of life, and may help to explain how avocado maintains such high levels of oil synthesis during the long fruit development period. Avocado WRI2 and other related proteins may provide the plant biotechnology community with the new tools for rational engineering strategies for manipulating oil content and composition.

Technical Abstract: WRINKLED1 (WRI1), a member of the APETALA2 (AP2) class of transcription factors regulates fatty acid biosynthesis and triacylglycerol (TAG) accumulation in plants. Among the four known Arabidopsis WRI1 paralogs, only WRI2 was unable to complement and restore fatty acid content in wri1-1 mutant seeds. Avocado (Persea americana) mesocarp, which accumulates 60-70% dry weight oil content, showed high expression levels for orthologs of WRI2, along with WRI1 and WRI3, during fruit development. While the role of WRI1 as a master seed oil biosynthesis regulator is well-established, function of WRI1 paralogs in non-seed tissues is poorly understood. Through transient expression assays, we demonstrated that both avocado WRI1 and WRI2 are functional and drive TAG accumulation in Nicotiana benthamiana leaves. Comprehensive comparative in silico analyses of WRI1 paralogs from Arabidopsis (dicot), maize (monocot), and avocado (a basal angiosperm) also revealed distinct features associated with their function. Predictive structural analyses of the WRI orthologs from these three species revealed the presence of AP2 domains and other highly conserved features, such as intrinsically disordered regions associated with predicted PEST motifs and phosphorylation sites. However, avocado WRI proteins, in association with their functionality also contained distinct features that were absent in the nonfunctional Arabidopsis ortholog AtWRI2. The unique features and activities of ancestral PaWRI2 were likely lost in orthologous genes such as AtWRI2 during evolution and speciation, leading to at least partial loss of function in some higher eudicots. This study provides us with new targets to enhance oil biosynthesis in plants.