Summary: The Arabidopsis genes CONSTANS‐LIKE 1 (COL1) and CONSTANS‐LIKE 2 (COL2) are predicted to encode zinc finger proteins with ∼67% amino acid identity to the protein encoded by the flowering‐time gene CONSTANS (CO). We show that the circadian clock regulates expression of COL1 and COL2 with a peak in transcript levels around dawn. We analyzed transgenic plants misexpressing COL1, COL2 and CO. Unlike CO, altered expression of COL1 and COL2 in transgenic plants had little effect on flowering time. However, analysis of circadian phenotypes in the transgenic plants showed that over‐expression of COL1 can shorten the period of two distinct circadian rhythms. Experiments with the highest COL1 over‐expressing line indicate that its circadian defects are fluence rate‐dependent, suggesting an effect on a light input pathway(s).

Medicago flowering, like that of Arabidopsis, is promoted by vernalization and long days, but alternative mechanisms are predicted because Medicago lacks the key regulators CO and FLC. Three Medicago SOC1-like genes, including MtSOC1a, were previously implicated in flowering control, but no legume soc1 mutants with altered flowering were reported. Here, reverse transciption–quantitative PCR (RT–qPCR) indicated that the timing and magnitude of MtSOC1a expression was regulated by the flowering promoter FTa1, while in situ hybridization indicated that MtSOC1a expression increased in the shoot apical meristem during the floral transition. A Mtsoc1a mutant showed delayed flowering and short primary stems. Overexpression of MtSOC1a partially rescued the flowering of Mtsoc1a, but caused a dramatic increase in primary stem height, well before the transition to flowering. Internode cell length correlated with stem height, indicating that MtSOC1a promotes cell elongation in the primary stem. However, application of gibberellin (GA₃) caused stem elongation in both the wild type and Mtsoc1a, indicating that the mutant was not defective in gibberellin responsiveness. These results indicate that MtSOC1a may function as a floral integrator gene and promotes primary stem elongation. Overall, this study suggests that apart from some conservation with the Arabidopsis flowering network, MtSOC1a has a novel role in regulating aspects of shoot architecture.

Flowering time is an important contributor to plant productivity and yield. Plants integrate flowering signals from a range of different internal and external cues in order to flower and set seed under optimal conditions. Networks of genes controlling flowering time have been uncovered in the flowering models Arabidopsis, wheat, barley and rice. Investigations have revealed important commonalities such as FT genes that promote flowering in all of these plants, as well as regulators that are unique to some of them. FT genes also have functions beyond floral promotion, including acting as floral repressors and having a complex role in woody polycarpic plants such as vines and trees. However, much less is known overall about flowering control in other important groups of plants such as the legumes. This review discusses recent efforts to uncover flowering-time regulators using candidate gene approaches or forward screens for spring early flowering mutants in the legume Medicago truncatula. The results highlight the importance of a Medicago FT gene, FTa1, in flowering-time control. However, the mechanisms by which FTa1 is regulated by environmental signals such as long days (photoperiod) and vernalisation (winter cold) appear to differ from Arabidopsis.

The workshop was held at the John Innes Centre, Norwich, UK, July 11–14, 2001. http://www.jic.bbsrc.ac.uk/events/embo/. Workshop Administrator: Dee Rawsthorne, John Innes Centre, Norwich Research Park, Colney, Norwich NR4 7UH, Norfolk, UK (Tel: +44 1603 450527; Fax: +44 1603 450025; E‐mail: dee.rawsthorne@bbsrc.ac.uk). ![][1] Successful sexual reproduction in plants depends on the recognition of favourable environmental conditions and the integration of that information with endogenous developmental cues. Flowering in higher plants involves the transition of a vegetative meristem, producing leaves and stems, into a floral meristem, producing flowers. Most of our understanding of the regulation of the floral transition has been developed in the past 10 years using molecular genetic approaches in the model plant Arabidopsis thaliana , a rosette plant in which the initiation of flowering is followed by the elongation of the main stem. The two main environmental factors that promote flowering in Arabidopsis are long days (Figure 1) and exposure to low temperatures (vernalization). Genetic and physiological analysis of flowering time in Arabidopsis has led to the identification of a large number of flowering‐time genes (>80) that regulate flowering time in response to environmental and endogenous cues (reviewed by Simpson et al ., 1999). Regulation occurs through a complex network of genetic pathways, with two main pathways mediating environmental responses (the long‐day and vernalization pathways) and two pathways that function independently of environmental cues: the autonomous pathway, which promotes flowering under all conditions, and the gibberellin (GA) pathway, which is needed for flowering under non‐inductive short‐day conditions. These pathways converge in the induction of floral meristem identity genes and the floral transition (Figure 2). Many flowering‐time genes have been cloned recently, and the primary aim of the EMBO workshop in Norwich, and this report, was to highlight the most recent advances in our understanding of the molecular mechanisms regulating … [1]: pending:yes