Grafting experiments between phytochrome B antisense and wild-type potato (Solanum tuberosum L. subsp. andigena [line 7540]) plants provide evidence that phytochrome B is involved in the production of a graft-transmissible inhibitor of tuberization, the level of which is reduced in the antisense plants, allowing them to tuberize in noninducing photoperiods.In the 1950s grafting experiments suggested that a tuberinducing stimulus is produced in the leaves of potato (Solanum tuberosum L.) plants that were grown in inductive short photoperiods (Gregory, 1956). In these experiments a leaf from an induced plant grown under SD conditions grafted onto a plant grown in noninducing, LD conditions was capable of promoting tuberization in the noninduced plant. These results were repeated with different varieties and species of potato (Chapman, 1958). In S. tuberosum L. subsp. andigena, it has been shown that the grafttransmissible signal can move acropetally as well as basipetally (Kumar and Wareing, 1973).There are many similarities between the tuber-inducing signal and the flower-inducing florigen, and in fact there is evidence that common signaling factors are involved in daylength-induced flowering and tuberization. This evidence also comes from grafting experiments. A leaf from a tobacco (Nicotiana tabacum L.) plant induced to flower grafted onto a potato stock induced the potato stock to tuberize, whereas a leaf from a noninduced tobacco plant did not induce tuberization. This was true even if a leaf from LD-requiring tobacco (Nicotiana sylvestris L.) was used, in which case tuberization of the potato stock was induced under LD conditions and did not occur under SD conditions as it normally would. Similar results have also been obtained in other interspecific grafting experiments between sunflower and Jerusalem artichoke and are thus not unique to tobacco and potato (Nitsch, 1965; Chailakhyan et al., 1981; Martin et al., 1982; for review, see Ewing, 1995).Grafting experiments between LD and day-neutral tobacco species (Lang et al., 1977) and between different flowering time mutants of pea (Taylor and Murfet, 1996) have also indicated the existence of a floral inhibitor or "antiflorigen." There appears to be a direct relationship between the inducing signal and an inhibitor in tobacco. When shoots with differing numbers of leaves from the SD plant N. tabacum L. cv Maryland Mammoth and the LD plant N. sylvestris were grafted together onto a receptor cv Maryland Mammoth plant and kept in SD conditions, flowering of the receptor was advanced with increasing numbers of leaves from the SD plant and delayed with increasing numbers of leaves from the LD plant (Lang, 1980). The response to daylength thus appears to be determined by the relative levels of the inducing and inhibitory substances, the levels of at least one of which is affected by photoperiod. The involvement of inducing and inhibitory substances in a multifactorial control is one reason that the isolation and identification of these factors still has not be...

Up to three gibberellin (GA) 20-oxidase genes have now been cloned from several species including Arabidopsis, bean (Phaseolus vulgaris), and potato (Solanum tuberosum). In each case the GA 20-oxidase genes exhibit different patterns of tissue expression. We have performed extensive northern analysis on one of the potato GA 20-oxidase genes (StGA20ox1), which is the only one that shows significant transcript levels in leaves. We show that levels ofStGA20ox1 transcript are elevated in transgenic antisense plants that have reduced levels of phytochrome B (PHYB) compared with wild-type plants, implicating PHYB in the control of GA biosynthesis. We show that StGA20ox1 transcript levels vary in leaves of different age throughout the plant and cycle throughout the day, furthermore they are up-regulated by light and down-regulated in the dark. The degree of the response to the light-on signal is similar in potato plants deficient in phytochrome A or PHYB and wild-type plants. The induction ofStGA20ox1 by blue light raises the possibility that a blue light receptor may be involved in the control of this gene by light.

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A cDNA library has been constructed from cauliflower curd in which floral development had been initiated. Two cDNAs (pBOFH3 and pBOFH8) have been isolated using the Antirrhinum flo gene as a heterologous probe. The two clones were sequenced and found to contain introns. Comparison of the deduced cDNA sequence of bofh with flo and the Arabidopsis homologue lfy reveals extensive homology. An mRNA transcript of 1.6 kb appears on northern RNA blots. This transcript can be detected, at low levels, before any obvious signs of floral differentiation, reflecting the role bofh plays in determining floral meristem identity.

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