How do plants "know" it is time to flower? For a long time, no one has
identified the cells which produce the small protein called Flowering Locus
T (FT). A new study conducted by researchers from Cornell University
uncovers exactly where the key protein forms before it triggers the
flowering process in plants.

Flowering in many plants begins when the leaves perceive day-length. It was
previously known that in Arabidopsis plants, long day-length starts a
process where leaves synthesize and transmit FT in the plant's vascular
tissue, which carries sugars and nutrients from leaves to the rest of the
plant. FT travels to the shoot apex, the highest point of new leaves and
stems, where it promotes the formation of flowers. Flowering regulation is
complex as the release of FT is controlled by more than 30 proteins in
interacting cascades. Identifying FT-producing cells was difficult because
leaf veins are very small and covered by photosynthetic cells rich in
chlorophyll.

The researchers discovered that FT was also produced in the same type of
companion cells in the phloem of Maryland Mammoth tobacco. When these
companion cells were killed, it delayed flowering in both Arabidopsis and
the tobacco plants. When the researchers looked more closely at the pathways
that lead to flowering, they found that killing these companion cells
stopped FT's downstream process, but not its upstream, confirming that FT
originates in these cells and that the synthesis of FT is regulated by an
extensive intercellular signaling system.

[news.cornell.edu]
lowering