Barley Flowering is Delayed by the Overexpression of VRN2 Under a General Promoter but not Under a Phloem-Specific Promoter

The duration of days and nights (photoperiod) and the extended exposure to cold temperatures during the winter (vernalization) are two major environmental signals used by plants to coordinate flowering.  These signals control the FLOWERING LOCUS T (FT), which up-regulates meristem genes in the shoot apical meristem initiating the transition to flowering.  In the temperate cereals, VRN2 is a CCT domain transcription factor that acts as a long-day repressor of FT, while in Arabidopsis, FLC is the main flowering repressor.  In Arabidopsis, CONSTANS (CO) is the main activator of FT.  Ectopic expression of CO induces early flowering when expressed by a phloem-specific promoter, but not when driven by an apex specific promoter, suggesting the regulation of FT in the phloem is critical in controlling flowering.  To test if the regulation of FT in the phloem was also important in the temperate grasses, we performed a similar experiment.  However, this experiment could not be done with the CO homologues CO1 and CO2 because a null-co1-co2 mutant in wheat shows a very limited effect on flowering time.  Instead, we decided to test if the expression of the FT-repressor VRN2, in the phloem, would delay flowering.  Preliminary results indicated that this hypothesis was possible.  First, the overexpression of VRN2 under a UBIQUITIN promoter showed delayed flowering.  Additionally, RNA in situ experiments showed VRN2 to be expressed broadly in the leaves.  In this study, we show the expression of the VRN2 gene under a phloem promoter to have a limited effect on barley flowering-time.