The molecular mechanisms of vernalization induced flowering in temperate cereals

Plants coordinate development with environmental cues to ensure flowering occurs under optimal seasonal conditions. Many plants from temperate regions only flower after exposure to prolonged cold: vernalization. While the molecular mechanisms of the vernalization response have been studied extensively in the model plant Arabidopsis, this seasonal flowering response has probably evolved independently in other plants. For instance, no homologues of FLOWER LOCUS C (FLC), the gene central to the vernalization response in Arabidopsis, have been found in economically important crops such as barley and wheat. Instead VERNALIZATION1 (VRN1) is central to the vernalization response in these plants. This highlights a need to study the vernalization response directly in cereal crop species. Previous studies have identified VRN1 as a master regulator of the vernalization response in cereals, but the extent to which other genes contribute is unclear. To identify genes that are potentially involved in regulating the vernalization response the Barley1 Affymetrix chip was used to compare gene expression in barley seedlings during short or prolonged cold treatments. Additionally, gene expression was assayed when plants were shifted to normal growth temperatures following prolonged cold treatment. This identified genes that show lasting changes in transcriptional activity, which might contribute to vernalization-induced flowering. Only a small group of genes showed a lasting change in activity when plants were shifted to warm conditions following prolonged cold. These included VRN1 and another MADS box gene, ODDSOC2 (OS2). OS2 belongs to a group of MADS box genes only found in grasses. Expression analysis showed that OS2 is down-regulated by cold and that long term repression of OS2 is dependent on VRN1. Overexpression of OS2 in barley delayed flowering and caused dwarfing. These phenotypes could be due to the down-regulation of FLOWERING PROMOTING FACTOR1 (FPF1)-like genes. In Arabidopsis FPF1 promotes flowering and elongation. Thus down-regulation of OS2 likely contributes to the acceleration of flowering through the de-repression of FPF1-like genes as daylength increases in spring. To determine if any components of the vernalization response pathway are conserved between Arabidopsis and temperate cereals, barley homologues of the Arabidopsis MADS box gene SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) were identified and characterised. In Arabidopsis SOC1 promotes flowering and is positively regulated by vernalization, long-days, and development. Similar to SOC1, the expression of a barley SOC1-like genes increases during development. Unlike Arabidopsis, the expression of a barley SOC1-like gene, HvSOC1-like1 was similar regardless of daylength suggesting that photoperiod does not regulate the expression of these genes. The regulation of HvSOC1-like genes by vernalization is also different. The expression of HvSOC1-like genes was lower in the leaves and crown tissue of vernalized plants versus non-vernalized plants. Over-expression of one of the barley SOC1-like genes, HvSOC1-like1, delayed flowering and caused dwarfing in barley plants. The phenotypes of these plants are different SOC1-like genes. Overall these data suggest that cereal SOC1-like genes have evolved differently and have different regulatory functions than SOC1 from Arabidopsis. Overall the findings from this thesis extend our understanding of the vernalization response in temperate cereals and highlight the difference of how this seasonal flowering response is regulated in temperate cereals and Arabidopsis. Since VRN1 is one of only a limited number of genes that initiates flowering in temperate cereals, understanding how VRN1 is regulated and identifying the targets of VRN1 will be of critical importance to understanding the mechanisms of seasonal flowering responses in these plants