Regulation of stilbene biosynthesis

Stilbenes are polyphenols that play an essential role in the plant defense system, protecting against biotic (e.g., herbivore and microbial attacks) and abiotic (e.g., UV radiation) stress. Stilbene biosynthesis occurs in only a few plant species, with the grapevine being one of the major producers. The basic building block of most stilbenes is trans-resveratrol, which is formed using precursors from the phenylpropanoid pathway. This condensation reaction is catalyzed by stilbene synthase (STS). To date, stilbene synthases from various plants have been isolated and partially characterized; however, the transcriptional regulation of stilbene biosynthesis remained unknown for a long time. As part of this project, two R2R3-MYB transcription factors involved in the regulation of stilbene biosynthesis in Vitis vinifera (the grapevine) were isolated and characterized. The two MYB transcription factors were designated VvMYB14 and VvMYB15. Using transient promoter expression assays in the grapevine and in the heterologous tobacco system, specific activation of the stilbene synthase promoter by the two transcription factors was demonstrated. Crucially, however, ectopic expression of the regulator VvMYB15 in various grapevine model systems (leaf infiltration, “hairy roots,” and in vitro plants) led to the induction of stilbene biosynthesis, which was consequently reflected in an increase in stilbene production. These metabolite data were confirmed by LC-Q-TOF-MS analysis; furthermore, this methodology enabled the identification of a wide variety of simple and oligomeric stilbene derivatives with various modifications.

To gain further insight into the regulation of stilbene synthase by the two transcription factors VvMYB14 and VvMYB15, their behavior was examined in response to various environmental conditions and developmental factors. Both transcription factors, as well as the stilbene synthases tested, exhibited increased expression in response to stress. The analysis included cold stress, hormone application, wounding, and light deprivation. In addition, a comparative gene expression analysis during various growth stages in the grape revealed an increase in the expression of both regulators and the tested stilbene synthases at the onset of ripening (“véraison”) in the berry skin and seeds. This was also reflected in an increased concentration of stilbenes in the ripe berry. Furthermore, a gene expression analysis was conducted in various unstressed grapevine tissues and different grape varieties, revealing a consistent profile of VvMYB14/15 and STS expression as well as stilbene content across all samples examined. These analyses of the influence of developmental changes, environmental factors, and tissue specificity detected a high basal level of expression for both transcription factors as well as stilbene biosynthesis, which can be further increased many-fold under stress conditions. Furthermore, the investigations revealed divergent expression patterns of the two transcription factors under various conditions, which may indicate divergent regulatory functions of VvMYB14 and VvMYB15.

As part of this work, two R2R3-MYB transcription factors capable of inducing stilbene biosynthesis in grapevine were identified and functionally characterized for the first time.

Publications:

Höll et al. (2013): The R2R3-MYB transcription factors MYB14 and MYB15 regulate stilbene biosynthesis in Vitis vinifera. In: The Plant Cell 25. 10.1105/tpc.113.117127.

Höll, Janine (2014): The R2R3 MYB Transcription Factors VvMYB14 and VvMYB15 as Regulators of Stilbene Biosynthesis in Vitis vinifera. Dissertation