The effect of the plant growth hormone gibberellin on flowering
The effects of gibberellins on flowering can differ between species. They promote flowering in herbaceous plants and some conifers but inhibit flowering in many woody species, such as apple, citrus and pear (Luckwill, 1980) . The role of the plant growth hormone gibberellin in flowering is therefore complex. Although their role in long-distance signalling or as part of the floral stimulus has not been demonstrated, they are commonly found in the vascular tissue (Xu et al., 1997) . However, as enzymes involved in gibberellin biosynthesis are found throughout the plant, it is possible that translocation of gibberellins may not be a prerequisite for a role in the initiation of flowering (Colasanti and Sundaresan, 2000).
Experiments on pea (Pisum sativa) have provided insight into how mutations can affect the translocation of signals from the shoot to the shoot apex (Howell, 1998) . Murfet and colleagues (1985) determined the site of action of mutations that affect photoperiodic control of flowering by grafting mutant scions onto wild-type or mutant stocks. This allowed tissues in which particular genes function to regulate flowering to be determined. If the shoot apex were mutant for VEGETATIVE (VEG1) gene this totally abolished flowering, even if the stock were veg1 mutant. VEG1 is not involved in the photoperiodic response, but is likely to be a general initiator of flowering, rather than to be involved in the perception of a floral stimulus (Murfet, 1985) . Three genes STERILE NODES (SN), DAY NEUTRAL (DNE) and PHOTOPERIOD RESPONSE (PPD) are required in the leaves and cotyledons and are photoperiod responsive. Mutations in any of the three genes results in flowering under non-inductive short-day photoperiods. These genes therefore appear to control a graft-transmissible inhibitor of flowering (Murfet and Reid, 1993; Weller et al., 1997) . The GIGAS gene, which is required in the leaf, promotes flowering. The gigas mutant flowers late when grown under long-day conditions, and therefore GIGAS is thought to promote the synthesis or export of a floral stimulus that is produced in the leaf and can cross graft junctions to initiate flowering at the apex (Weller et al., 1997) .
Commercial maize varieties are daylength insensitive, flowering with the same number of leaves regardless of photoperiod. Mutations in INDETERMINATE 1 (ID1) delay flowering and affect tassel and ear morphology. Expression of ID1 mRNA is restricted to the leaves, and is absent from the SAM (Colasanti et al., 1998) . Maize does not have a vascular cambium and so is not suitable for grafting experiments. Therefore genetic chimeras were analysed to test whether ID1 gene function in the leaves was sufficient to promote flowering. Chimeric id1 plants that contained sectors of ID1 flowered earlier than mutant id1 plants. This suggests that ID1 is involved in producing a transmissible signal from young leaves that promotes flowering at the apex.
Although Arabidopsis has been an effective model species in which to study genetic flowering-time networks, the involvement of transmissible signals in flowering has only been studied recently. An and colleagues (2004) as well as Ayre and Turgeon (2004) showed that the flowering-time gene CONSTANS (CO) is active in the phloem. A Y-grafted junction could form phloem connections across which radiolabelled sucrose was transferred. Grafting of induced donor shoots to plants grown in non-inductive short-day conditions demonstrated that a floral stimulus was transferred from the long-day grown shoot to induce flowering in the short-day grown plant. Finally, a wild-type donor shoot was grafted to a co-2 mutant and this resulted in accelerated flowering of the mutant. This suggested that CO or downstream targets of CO act in the phloem in response to long days to regulate a signal that induces flowering (An et al., 2004; Ayre and Turgeon, 2004) .
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