Q32 (e)

Agrobacterium tumefaciens is a rod-shaped bacterium found in soil. Its flagella allow it to move through the soil in response to chemicals released by wounded plant cells. Once it enters its plant host, it injects a section of its DNA called the T-DNA which is derived from its Ti (tumour inducing) plasmid into its host causing the host to form undifferentiated (tumour) tissue termed a crown gall. Figure 1: Crown gall The interaction between a plant and Agrobacterium is detailed below: (A) Agrobacteria cells produce the hormones cytokinin and auxin, which increase host susceptibility and inhibit H2O2 production and hypersensitive response (HR) upon infection. (B) After integration of the bacterial T-DNA into the plant genome, cytokinin and auxin are synthesized by T-DNA encoded enzymes and accumulate inside the tumour. (C) This causes massive changes in the gene expression pattern resulting in metabolomic and morphological adaptations that are necessary for tumour growth and differentiation. (D) Loss of water is minimized by an increase in the levels of the stress hormone ABA. Evaporation of water from the disrupted crown gall surface drives the flow of water and minerals into crown galls. (E) Because photosynthesis is downregulated, the O2 levels are low, the tumour produces C and N compounds heterotrophically and gains energy mainly anaerobically by alcoholic fermentation. (F) The developing tumour becomes a metabolic sink for the host plant, which accumulates metabolites produced by source leaves and minerals taken up by the roots. (G) Auxin and cytokinin also cause an increase in ethylene (ET) which together with salicylic acid (SA) inhibits agrobacterial virulence. (H) ABA induces DNA methylation of the plant genome, regulating gene expression of drought- stress responsive genes. The oncogenes of the T-DNA remain unaffected by methylation of the plant genome.

Identify a mechanism by which the plant maintains homeostasis when infected by agrobacterium.