Efficacy of Barley Stripe Mosaic Virus to Induce Gene Silencing of a Gene Repressing Flowering in Sugar Beet

Sugar beet (Beta vulgaris) is a biennial plant that requires a cold period in order to induce flowering, a process called vernalization. The period of vernalization required to induce flowering can vary by genotype (between 90-120 days). Holding beets in cold storage (4C) at high humidity (95%) is expensive and labor intensive, requiring specialized equipment and multiple fungicide applications. Therefore it would be advantageous to both breeders and seed producers to develop a system that would require less than the 90 day minimum vernalization period in order to initiate uniform flowering of sugar beet plants for seed production. The sugar beet gene, BvFL1, which is a homologue of FLC in Arabidopsis, has been proposed to act as a repressor of flowering in sugarbeet (Reeves et al., 2007). In that study, it was shown that four mRNA variants are created from a single genomic locus via alternate splicing. Virus-induced gene silencing (VIGS) can be used to study the effect of turning off or down regulating gene expression of a specific gene (Meister and Tuschl, 2004). A viral vector is used to deliver dsRNA corresponding to a gene of interest into the cells, which triggers the plant’s natural ability to destroy that sequence of RNA (Baulcombe, 2004). This technique has been shown to be effective in diverse plant species such as wheat (Triticum aestivum), California poppy (Eschscholzia californica), and Arabidopsis (Burch-Smith et al., 2006; Scofield et al., 2005; Wege et al., 2007). Delivery of a specific region of the BvFL1 sequence and subsequent silencing of this gene could potentially induce flowering without vernalization. Such a procedure could expedite breeding efforts without alteration of the genotype. Barley stripe mosaic virus (BSMV) has been used previously to affect VIGS in sugar beet (Larson and Weiland, 2005). BSMV is a tripartite RNA virus with all three subunits (alpha, beta, and gamma) required for infection. BSMV does not naturally infect sugar beet. The objectives of this research were to develop dsRNA sequences that would suppress BvFL1 expression in sugar beet and potentially induce floral development without vernalization. The VIGS silencing system has an approximate 21-28 day window of effectiveness (Tai et al., 2005). Induction of flowering within this time frame would be a dramatic improvement over the current 90–120 day vernalization requirement.