It is known for a long time that the plant hormone auxin is transmitted from
the top to the bottom of a plant, and that the local concentration of auxin
is important for the growth direction of stems, the growth of roots, the
sprouting of shoots.
To name a few things; auxin is also relevant to, for instance, the ripening
of fruit, the clinging of climbers and a series of other processes.
Thousands of researchers try to understand the different roles of auxin.

In many instances the distribution of auxin in the plant plays a key role,
and thus the transport from cell to cell. At the bottom of plant cells,
so-called PIN proteins are located on the cell membrane, helping auxin to
flow through to the lower cell. However, no one thoroughly understood why
the PIN proteins only showed up at the bottom of a cell.

Endocytosis

An international group of scientists from labs in five countries, headed by
Jirí Friml of the VIB-department Plant Systems Biology at Ghent University,
revealed a rather unusual mechanism. PIN proteins are made in the protein
factories of the cell and are transported all over the cell membrane.
Subsequently they are engulfed by the cell membrane, a process called
endocytosis. The invagination closes to a vesicle, disconnects and moves
back into the cell. Thus the PIN proteins are recycled and subsequently
transported to the bottom of the cell, where they are again incorporated in
the cell membrane. It is unclear why plants use such a complex mechanism,
but a plausible explanation is this mechanism enables a quick reaction when
plant cells feel a change of direction of gravity, giving them a new
'underside'.

Gene technology

To see the path of the protein, the researchers used gene technology to make
cells in which the PIN protein was linked to fluorescent proteins. (This
technology was rewarded with the Nobel Prize 2008 for chemistry.)
Subsequently they produced cells in which the endocytosis was disrupted in
two different ways. The PIN proteins showed up all over the cell membrane.
When the researchers proceeded from single cells to plant embryos, the
embryos developed deformations, because the pattern of auxin concentrations
in the embryo was distorted. When these plants with disrupted endocytosis
grew further, roots developed where the first leaflet should have been.
www.checkbiotech.org