Arabidopsis thaliana, known as thale cress when it grows in our gardens, is a model plant used by researchers in plant biology. Its genome, which was sequenced at the beginning of the 21st century, counts 25,000 genes which code for proteins whose functions are largely unknown. Scientists in the INRA Joint Research Unit for Plant Genomics in Versailles-Grignon thus decided to try and discover the immersed part of this gigantic iceberg, in the context of an international collaborative project.
Scientists have revealed 6200 interactions between 2700 proteins.
Thanks to genomic techniques, the researchers explored the interactions between proteins in A. thaliana. By means of systematic analyses, they identified the protein partners of several thousands of proteins in this plant, and were able to reveal the existence of 6200 interactions between 2700 proteins and then to map them out. This allowed them to propose a global scheme for the organisation of biological processes in A. thaliana, together with models of the functional links between proteins and functional pathways. In addition, they evidenced modifications to this scheme that were dependent on certain characteristics of the genome (e.g. gene multiplication), thus supplying elements in favour of an evolutionary model that governs all these interactions (called the interactome).
Some fifteen conserved proteins targeted by pathogenic agents
The scientists focused in particular on the proteins involved in the plant's defence mechanisms against infection. During this process, infective (or pathogenic) agents inject so-called effector proteins into the plant cell; these proteins then interact with those in the cell, modulating their defence reactions. By studying the infection of Arabidopsis thaliana by two microbial pathogenic agents, the Pseudomonas syringae bacterium and the Hyaloperonospora arabidopsidis mould, which cause disease in numerous target plants, the scientists evidenced a network of interactions between proteins in A. thaliana that did or did not belong to its immune system, and the effector proteins of these two pathogenic agents.
Their analyses revealed that, although these two pathogenic agents have evolved independently during the past million years or so, their proteins target identical plant proteins, some of which occupy key functions in the operation and organisation of the cellular machinery. These findings thus show that pathogenic agents from different kingdoms (bacteria, fungi) express effector proteins which, although they have evolved independently, interact with a limited number of closely connected cell proteins in order to facilitate infection.
Global study of the interactions between A. thaliana proteins thus contributes to clarifying how plants function, by enabling:
- An understanding of the concerted action of proteins (i.e. gene products) in time and space,
- The attribution of potential roles for proteins with unknown functions, and
- An understanding of the organisation and dynamics of the cellular machinery and interactions within cells.
More specifically, understanding how proteins in pathogenic agents interact with the plant cell machinery involved in fighting infection opens the way to improving the immune system of plants or the development of new agents for pathogen control.
Références
M. Shahid Mukhtar et al. Independently Evolved Virulence Effectors Converge onto Hubs in a Plant Immune System Network. Science, 29 July 2011, 333: 596. DOI:10.1126/science.1203659
Arabidopsis Interactome Mapping Consortium. Evidence for Network Evolution in an Arabidopsis Interactome Map. Science, 29 July 2011, 333: 601. DOI:10.1126/science.1203877
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