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April 4, 2008

How plants resist attack by disease

More of the mystery of immunity solved with insights on the proteins needed for resistance in plants

Figure 1: Proposed model for the interaction between HSP90–RAR1–SGT1. SGT1 (light blue) binds HSP90 (light green) and RAR1 (pink) on opposite sides of the CS domain.

© The Plant Cell/American Society of Plant Biologists/19/3801 (2007)

An international research team led by a RIKEN molecular biologist has revealed a significant part of the complex mechanism of plant disease resistance. The team has found that the interaction of a highly conserved protein, SGT1, with the molecular chaperone HSP90 (heat shock protein 90) is essential for plant immunity. Understanding the molecular details of plant immunity is not only important for the protection of crop plants against disease, but can also provide insight into the human immune system where many of the same compounds and mechanisms are used.

Previous work showed that disease organisms or pathogens are detected in the cells of higher plants by the interaction of compounds they secrete with resistance (R) proteins. These interactions somehow activate a system which results in cell death, thereby limiting the spread of the pathogen. Recent research demonstrated that R proteins require HSP90 to function properly. It is also known that HSP90 interacts with the proteins SGT1 and RAR1 to regulate plant disease resistance. These two proteins can also interact with each other, and are required to stabilize many R proteins.

In order to find out more about the complex interplay between all these elements, a research team with members from RIKEN’s Plant Science Center in Yokohama, research institutes in France and the UK and two British universities investigated the structure and function of SGT1 and its interaction with HSP90 in particular. The team published their results recently in The Plant Cell1.

The researchers generated a series of random point mutations in the SGT1 gene. They found that the gene essential for the resistance of the thale cress, Arabidopsis thaliana, to Potato virus X, and that all of the mutations resulting in loss of resistance were located in two key domains of the SGT1 protein that interact with other proteins. One of those domains, CS, binds both HSP90 and RAR1. On the basis of nuclear magnetic resonance-based surface mapping, the researchers determined that the two compounds were bound on opposite sides of the domain (Fig. 1). But only interaction with HSP90 was required for Potato virus X resistance. Other experiments suggested that RAR1 may enhance the interaction with HSP90.

“We would now like to know how this complex interacts with R proteins,” says project leader Ken Shirasu. “Our next step is to investigate the structure of a complex of the three proteins. The more challenging task is to solve the structure of the R proteins.”

References

  1. Botër. M., Amigues, B., Peart, J., Breuer, C., Kadota, Y., Casais, C., Moore, G., Kleanthous, C., Ochsenbein, F., Shirasu, K. & Guerois, R. Structural and functional analysis of SGT1 reveals that its interaction with HSP90 is required for the accumulation of Rx, an R protein involved in plant immunity. The Plant Cell 19, 3791–3804 (2007). |  |
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