Figure 27.9 The Flowering Signal Moves from Leaf to Bud
Evidence supporting the existence of a flowering hormone in plants comes from several experiments involving cocklebur. This short-day plant will not flower if kept under conditions of long days and short nights. If, however, only one leaf of the cocklebur plant is covered for at least a portion of the day, thus subjecting that leaf to conditions resembling short days and long nights, the plant will flower and produce fruit. This result suggested that leaves measure the length of the night and produce a signal that travels to the flowering parts of the plant. The results of a similar experiment involving plant grafts supported this hypothesis. In this experiment, five cocklebur plants were grafted together and kept under conditions of long days and short nights. As above, a single leaf on one of the plants at the end of the chain was then covered, subjecting only that leaf to conditions resembling short days and long nights. Results showed that as a result of this limited exposure to a long night, all of the plants in the chain flowered and produced fruit. Consistent with earlier results, this experiment suggested that the flowering signal was quite stable and could travel across multiple grafts. In 1937, a Russian plant physiologist, Mikhail Chailakhyan, named this graft-transmissible signal florigen. More recently, Brian Ayre and Robert Turgeon from the University of North Texas and Cornell University, respectively, demonstrated that a transcription factor called CONSTANS plays an important role in transmission of the flowering signal. These researchers generated transgenic Arabidopsis plants that expressed the CONSTANS gene in the smallest veins of the leaves. Prior to the initiation of grafting experiments, it was noted that these plants exhibited accelerated flowering. Ayre and Turgeon then grafted tips of plants that failed to express CONSTANS and flowered very late in their life cycle to the stems of the transgenic plants. Flowering was initiated immediately after the graft junction healed, suggesting that a CONSTANS-derived signal had been transported from the transgenic plants to the leaves of the grafted plant. This result implicates CONSTANS in the transmission of a flowering signal.
Original Papers
Hamner, K. C., and J. Bonner. 1938. Photoperiodism in relation to hormones as factors in floral initiation and development. Botanical Gazette 100: 388–431.
http://www.jstor.org/stable/2471641
See also a more recent review of the various experiments:
Zeevaart, J. A. D. 2006. Florigen Coming of Age after 70 Years. Plant Cell 18: 1783–1789.
http://dx.doi.org/10.1105/tpc.106.043513
Links
Kimball’s Biology Pages: Flowering
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/F/Flowering.html
Universität Hamburg: Botany Online: The Flowering Hormone or Florigen
http://www.biologie.uni-hamburg.de/b-online/e30/30c.htm
Wikipedia: Florigen
http://en.wikipedia.org/wiki/Florigen
Blázquez, M. A. 2005. The Right Time and Place for Making Flowers. Science 309: 1024–1025.
http://dx.doi.org/10.1126/science.1117203
Science Daily: Blossoms Of Maturity: Newly Discovered Signaling Pathway Ensures That Plants Remember To Flower
http://www.sciencedaily.com/releases/2009/08/090820123931.htm