| Summary: | 碩士 === 國立中興大學 === 生物科技學研究所 === 102 === Plants infected by geminiviruses usually show typical leaf curling symptoms with severe growth retardation and yield reduction, leading to serious damages to agricultural economy. Specific geminivirus species could inflict leaf curling towards unique direction on a specific host. Previous studies in our lab have revealed that the C4 proteins of geminiviruses are involved in the determination of the directions of leaf curling symptoms. However, the underlying mechanisms remained to be explored. Therefore the objectives of this study are to identify the key amino acid residues involved in the determination of leaf curling directions, and to test the possible applications in disease attenuation or therapy. The C4 protein of Ageratum yellow vein virus Nan-Tou Strain (AYVV-NT), which caused sever upward leaf curl symptom on Nicotiana benthamiana, was used as the primary material for mutational analyses. Multiple sequence alignment of the amino acid sequences of C4 protein of AYVV-NT and those of Tomato leaf curl virus Tai-Nan Strain (TLCV-TN) and Squash leaf curl virus (SqLCV), which cause downward leaf curling, revealed three possible regions that might be involved in curling direction determination. Accordingly, infectious clones of three mutants, designated A-, B-, and C-mut, were constructed and assayed in N. benthamiana. The result revealed that two amino acids in A-mut could significantly attenuate the upward leaf curling symptoms. Cross protection assays demonstrated that A-mut could serve as a mild strain to provide effective protection against wild type AYVV-NT infections. Thus, the results elucidated key amino acids in C4 proteins that are involved in the modulation of leaf curling symptoms, and provided proof of concept for the possible application in disease management. Further studies include the analyses of other mutants to dissect the underlying mechanisms involved in the determination of leaf curling directions.
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