Virus-induced gene silencing for Asteraceae--a reverse genetics approach for functional genomics in Gerbera hybrida

Virus-induced gene silencing (VIGS) is a natural defence mechanism in plants which leads to sequence-specific degradation of viral RNA. For identifying gene functions, Tobacco rattle virus (TRV)-based VIGS has been applied for silencing of endogenous genes in many plant species. Gerbera hybrida (Asteraceae) has emerged as a novel model for studies in flower development and secondary metabolism. For this highly heterozygous species, functional studies have been conducted through reverse genetic methods by producing stable transgenic lines, which, however, is labour-intensive and time-consuming. For the development of TRV-based VIGS system for gerbera, and for the first time for an Asteraceaeous species, we screened several gerbera cultivars and optimized the agroinfiltration methods for efficient silencing. Gene fragments for gerbera phytoene desaturase (GPDS) and Mg-chelatase subunits (GChl-H and GChl-I), expressed from a TRV vector, induced silencing phenotypes in leaves, scapes, and involucral bracts indicating their feasibility as markers for green tissues. In addition, robust silencing symptoms were achieved in gerbera floral tissues by silencing the anthocyanin pathway gene for chalcone synthase (GCHS1) and a gerbera B-type MADS-box gene globosa (GGLO1), confirming the phenotypes previously observed in stable transgenic lines. Unexpectedly, photobleaching induced by GPDS and GChl-H or GChl-I silencing, or by the herbicide norflurazon, resulted in silencing of the polyketide synthase gene G2PS1, which has no apparent connections to carotenoid or chlorophyll biosynthesis. We have shown feasibility of VIGS for functional studies in gerbera, but our results also show that selection of the marker gene for silencing must be critically evaluated.