CaMV gene VI protein, a virulence factor and the host responses in Arabidopsis

The gene VI protein (P6) of Cauliflower mosaic virus (CaMV) functions as a virulence factor in crucifers by eliciting a chlorosis symptom in infected plants. The ability to induce chlorosis has been associated previously with P6 through gene-swapping experiments between strains and through the analysis of transgenic plants that express P6. In this study, I characterized a variant of P6, derived from CaMV strain D4, which does not induce chlorosis upon transformation into Arabidopsis thaliana, and two other variants (W260 and CM1841) that induce strong chlorosis and stunting. The asymptomatic D4 P6 was proven to be fully functional. This work demonstrated that the virulence function of CaMV P6 was not related to its function as a Translational Transactivator (TAV), it also had implications for the CaMV gene expression strategies, and enabled us to analyze the functions of other CaMV genes not related to P6 in the disease expression.; To characterize the host response to the CaMV P6, I profiled the host global gene expression in both the symptomatic W260 P6 and the asymptomatic D4 P6 transgenic Arabidopsis by a cDNA microarray. The microarray analysis revealed that the expression of 69 genes was affected by the more virulent W260 P6, whereas the expression of only 22 genes was affected by the D4 P6 in transgenic Arabidopsis. In general, genes involved in transcription, translation and stress response were induced, whereas genes involved in photosynthesis, hormone response, metabolism, and transport were suppressed in transgenic plants that expressed W260 P6. A cluster analysis coupled with a search of upstream regions of clustered genes, revealed two putative transcription factor binding motifs that may function in the induction of stress responses in transgenic plants that express P6.; To visualize the subcellular location of CaMV P6, the GFP gene was fused to the C-terminus of P6 of the CaMV W260 isolate (G6GFP). The G6GFP fusion protein was able to form fluorescent inclusion bodies when expressed in agroinfiltrated N. edwardsonii leaves and in transgenic Arabidopsis. Furthermore, G6GFP appeared to have full TAV activity in an agroinfiltration assay. However, the G6GFP did not elicit HR in agroinfiltrated N. edwardsonii or elicit virus-like symptoms in transgenic Arabidopsis, likely due to the instability of the G6GFP transcript. Preliminary experiments indicated that co-infiltration of the wild type W260 P6 construct with pG6GFP could stabilize the G6GFP transcript.