The Molecular Basis of Vernalization in Different Plant Groups

Abstract

Timing of flowering is key to the reproductive success of many plants. In temperate climates, flowering is often coordinated with seasonal environmental cues such as temperature and photoperiod. Vernalization, the process by which a prolonged exposure to the cold of winter results in competence to flower during the following spring, is an example of the influence of temperature on the timing of flowering. In different groups of plants, there are distinct genes involved in vernalization, indicating that vernalization systems evolved independently in different plant groups. The convergent evolution of vernalization systems is not surprising given that angiosperm families had begun to diverge in warmer paleoclimates in which a vernalization response was not advantageous. Here, we review what is known of the vernalization response in three different plant groups: crucifers (Arabidopsis), Amaranthaceae (sugar beet), and Pooideae (wheat, barley, and Brachypodium distachyon). We also discuss the advantages of using Brachypodium as a model system to study flowering and vernalization in the Pooids. Finally, we discuss the evolution and function of the Ghd7/VRN2 gene family in grasses.

Interaction of Photoperiod and Vernalization Determines Flowering Time of Brachypodium distachyon

“…flowering is less clear. VRN2L, the closest homolog to wheat/barley VRN2 identified to date in Brachypodium spp., shares some features in common with wheat/barley VRN2 (Ream et al, 2012). Specifically, VRN2L is expressed more highly as days get longer ( Fig.…”

“…We refer to this gene as VRN2-like (VRN2L) because Bayesian phylogenetic studies using the CCT domain place Bradi3g10010 within the VRN2 clade; however, the VRN2 and Ghd7 (a photoperiod response regulator) clades are unresolved but may in fact be orthologous (for additional discussion, see Ream et al, 2012). Additionally, BLAST searches using VRN2L in the most recently released versions of the wheat (Brenchley et al, 2012) and barley genomes (Mayer et al, 2012) do not reveal a closer homolog to Bradi3g10010 than …”

Evolution of VRN2/Ghd7-Like Genes in Vernalization-Mediated Repression of Grass Flowering

“…The attainment of flowering competence in response to vernalization has evolved multiple times independently across major lineages of angiosperms and is hypothesized to be a key adaptation facilitating niche shifts from the tropics to the temperate zone (Ream et al, 2012;Preston and Sandve, 2013). One such niche transition occurred in the grass subfamily Pooideae, members of which are distributed primarily in the northern temperate zone (Hartley, 1973;Edwards and Smith, 2009) and are heavily relied upon for grain, turf, and fodder.…”

“…One posits a single gene duplication event before the base of grasses, giving rise to a VRN2/Ghd7-and a CO9-containing clade and implying an orthologous relationship between pooid VRN2 genes and rice Ghd7 (Ream et al, 2012). The other postulates two duplication events before the base of grasses, with the first giving rise to the VRN2 and Ghd7/CO9 clades and the second producing the Ghd7 and CO9 clades, followed by loss of Ghd7 genes at the base of pooids (Cockram et al, 2012).…”

“…Evolutionary History of Pooid VRN2/Ghd7 and CO9 Genes Two hypotheses about the evolutionary history of VRN2-like genes have been previously proposed (Cockram et al, 2012;Ream et al, 2012). One posits a single gene duplication event before the base of grasses, giving rise to a VRN2/Ghd7-and a CO9-containing clade and implying an orthologous relationship between pooid VRN2 genes and rice Ghd7 (Ream et al, 2012).…”