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 recently discovered as part of an Austrian Science Fund FWF project and was published today 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.
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.
Original publication: Trojan horse strategy in Agrobacterium transformation - Abusing MAPK-targeted VIP1 defence signalling Armin Djamei, Andrea Pitzschke, Hirofumi Nakagami, Iva Rajh, Heribert Hirt, Science 318, 453 (2007).
Prof. Heribert Hirt
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