Abstract:

To evaluate the resistance of two navel orange varieties，‘Newhall’and its bud mutant
‘Gannanzao’，to Citrus tristeza virus（CTV），and their effect on virus mutation，an isolate of CTV
（YC-3）was inoculated to both varieties by graft. CTV isolates YC-3Z and YC-3N were then re-isolated
from‘Gannanzao’and‘Newhall’，respectively. The HinfⅠrestriction fragment length polymorphism
（RFLP） of coat protein（CP），single-strand conformation polymorphism（SSCP），and the sequence
variation of CP genes of both isolates YC-3Z and YC-3N were analyzed and compared. The replication
level of the each isolate in its corresponding variety and activity changes of the enzymes in each variety，including the superoxide dismutase（SOD） were measured. No difference was found between two isolates
for HinfⅠRFLP analysis and DNA bands of CP/SSCP. Sequence analysis of CP gene indicated that the most
substitution occurred at the third nucleotide in a codon，followed by the first nucleotide，while no
substitution occurred at the second nucleotide. The number of codons transition is higher than that of
transversion，non-synonymous mutation occurred in the YC-3N isolate，but not YC-3Z. The replication
level of CTV in‘Newhall’is 3.3 times higher than that in‘Gannanzao’. The POD and SOD activities
increased by more than 78.2%，PAL and PPO activities also increased，while CAT activity decreased in
both varieties. However，there was no significant difference between two varieties for activities changes of
those five enzymes.

Key words: naval orange, Citrus tristeza virus, sequence variation, disease resistance