竹嵌紋病毒TGBp2與TGBp3 C-端保留性cysteines對此兩蛋白協助病毒在植物中移動之重要性

Abstract

The TGBp1, TGBp2 and TGBp3 encoded by the triple gene block (TGB) of Bamboo mosaic virus (BaMV) genome are required for virus cell-to-cell movement. Both TGBp2 and TGBp3 are transmembrane proteins which have two conserved Cys residues at their C-terminal tails, respectively. In the first chapter of this study, I investigated the importance of the two conserved cysteine residues (Cys-109 and Cys-112) of TGBp2 on the movement of BaMV. Three mutant BaMV, each having either one or both of the conserved cysteine residues in TGBp2 being replaced with alanine, were constructed and then inoculated onto the leaves of Chenopodium quinoa and Nicotiana benthamiana. Infectivity of each of the BaMV mutants was confirmed by the presence of disease symptoms and accumulation of CP in the leaves. However, the infectivity of the mutant BaMV was lower than that of the wild-type (Wt). To investigate the reason (s) causing the discrepancy, I analyzed the replication efficiency and the cell-to-cell movement activities of the Wt and mutant BaMV. The results indicated that the activities of cell-to-cell movement of mutant BaMV are at least two folds lower than that of the Wt, although they replicate with similar efficiency. Thus, the reduced infectivity of the mutant BaMV is attributed to their defects in virus cell-to-cell movement. In addition, I examined the effects of Cys-to-Ala substitutions in TGBp2 on systemic movement of BaMV and found that the three mutant BaMV are unable to move efficiently in the phloem of the petiole or from phloem of vein to the phloem of petiole. Taken together, the two conserved Cys residues (Cys-109 and Cys-112) at the C-terminal tail of TGBp2 are required for TGBp2 to assist both the cell-to-cell and systemic movement of BaMV. In the second chapter of this study, I investigated the importance of the conserved cysteine residues (Cys-31 and Cys-46) of TGBp3 on the movement of BaMV. Three BaMV mutants, each having either one or both of the conserved cysteine residues in TGBp3 being replaced with alanine, were constructed and then inoculated onto the leaves of C. quinoa and N. benthamiana. No disease symptoms were observed on the leaves inoculated with each of the mutant BaMV. To investigate the reason (s) causing the defect of infection, I checked the replication efficiency and the cell-to-cell movement activities of the Wt and mutant BaMV. Our results revealed that replacement of either one or both of the conserved Cys residues with Ala at the C-terminal tail of TGBp3 inhibits the cell-to-cell movement of BaMV severely but not the replication efficiency. Thus, the two conserved Cys residues (Cys-31 and Cys-46) at the C-terminal tail of TGBp3 are required for the functioning of TGBp3 in assisting the cell-to-cell movement of BaMV.