Bacteria that cause tumours in plants modify plant genomes by
skilfully exploiting the plants' first line of defence. Utilising the
plant's own proteins, bacterial genes infiltrate first the nucleus then the
plant genome, where they reprogramme the plant's metabolism to suit their
own needs. This process was published in Science.
The genetic manipulation of plants is both, a subject of great
controversy in Europe and a tactic already practiced by certain bacteria.
The soil bacterium known as crown-gall bacterium (Agrobacterium) manipulates
the genetic make-up of plants by inserting its own DNA into the nuclei and,
consequently, into the genetic material of the plant cells.

The genetically modified plants are then reprogrammed to ensure
uninhibited cell division and produce nutrients to feed the bacteria. What
was not previously understood is exactly how bacteria genes infiltrate the
cell's nucleus - particularly as the defence mechanisms of plant cells react
so rapidly to bacterial invasion.

Weak defences

A surprising detail of this process has now been uncovered by the team
of Prof. Heribert Hirt working at the Max F. Perutz Laboratories at the
University of Vienna and the URGV Plant Genomics Institute near Paris which
Hirt joined as future director earlier this year. VIP1, a plant cell
protein, is at the heart of their research. It was already known that this
protein supports the transport of bacterial DNA known as T-DNA into the
nucleus, and yet the exact role of VIP1 was unclear. Prof. Hirt explains:

"We were able to show that VIP1 is a protein that regulates various
genes designed to defend against bacterial invasion. However, VIP1 only
occurs initially in the cytoplasm of cells and - in order to fulfil its role
as a regulator - it then needs to migrate into the nucleus. It is precisely
this movement that the bacterium exploits in order to inject its T-DNA into
the nucleus." Prof. Hirt compares this strategy, which inevitably means that
the plants own defences cause its downfall, to the famous Trojan Horse.

Friend & foe

Prof. Hirt explains further - "Plants have an immune defence mechanism
that is triggered when the plant detects certain molecules of the invader
and works by activating genes in the nucleus." Once the invader has been
detected, specific protein kinases in the cytoplasm are activated. These are
enzymes that regulate the activity of other proteins by adding phosphate
groups to them. One of the proteins phosphorylated by these protein kinases
is VIP1, which is only granted access to the nucleus after this
phosphorylation, so that it can activate the relevant defence genes there.

The following model illustrates the early processes in an infected
plant cell. The invasion of T-DNA and the identification of the bacterium as
an invader occur simultaneously. While protein kinases phosphorylate VIP1 in
the cytoplasm, the bacterial T-DNA adheres to VIP1, thereby enabling it to
infiltrate the nucleus unnoticed.

The result is the joint infiltration of both friend and foe. Once
inside the nucleus, the T-DNA is inserted into the plant genome and the
process of tumour formation begins while the activated defence genes
simultaneously organise the plant cell's defence mechanisms. It is too late
though - the cell has already been transformed.


[www.sciencedaily.com]