New insights into an old story: Agrobacterium-induced tumour formation in plants by plant transformation

Abstract

Agrobacterium tumefaciens causes tumour formation in plants. Plant signals induce in the bacteria the expression of a range of virulence (Vir) proteins and the formation of a type IV secretion system (T4SS). On attachment to plant cells, a transfer DNA (T-DNA) and Vir proteins are imported into the host cells through the bacterial T4SS. Through interaction with a number of host proteins, the Vir proteins suppress the host innate immune system and support the transfer, nuclear targeting, and integration of T-DNA into host cell chromosomes. Owing to extensive genetic analyses, the bacterial side of the plant-Agrobacterium interaction is well understood. However, progress on the plant side has only been achieved recently, revealing a highly complex molecular choreography under the direction of the Vir proteins that impinge on multiple processes including transport, transcription, and chromosome status of their host cells.

Figure 1

Overview of the Agrobacterium–plant interaction. 1. Plant signals induce 2. VirA/G activation and thereby 3. T-DNA synthesis and vir gene expression in Agrobacterium. 4. Through a bacterial type IV secretion system (T4SS) T-DNA and Vir proteins are transferred into the plant cell to assemble a T-DNA/Vir protein complex. 5. The T-DNA complex is imported into the host cell nucleus in which 6. the T-DNA becomes integrated into the host chromosomes by illegitimate recombination.

Figure 2

Molecular details of Agrobacterium and host factors involved in T-DNA transfer and host responses. 1. Nuclear import of the T-DNA complex along with other Vir proteins. 2. VIP1 becomes activated by PAMP-triggered MAPK activation, but undergoes proteasomal degradation through VirF. 3. Manipulation of host gene expression by Vir proteins helps 4. T-DNA integration and tumour development as well as suppression of defence responses.