An engineered citrus tristeza virus (T36CA)-based vector induces gene-specific RNA silencing and is graft transmissible to commercial citrus varieties.

ABSTRACT

A protein-expressing citrus tristeza virus (CTV)-based vector construct, pT36CA-V1.3, obtained from a California isolate of the T36 strain (T36CA), was retooled into a virus induced gene silencing (VIGS) system intended for use with studies of California citrus. VIGS constructs engineered with a truncated Citrus macrophylla (Cm) PHYTOENE DESATURASE (PDS) gene sequence in the sense or anti-sense orientation worked equally well to silence the endogenous CmPDS gene. In a parallel effort to optimize vector performance, two non-synonymous nucleotides in open reading frame 1a of pT36CA-V1.3 were replaced with those conserved in the reference sequences from the T36CA cDNA library. The resulting viruses, T36CA-V1.4 (with one amino acid modification D760N) and T36CA-V1.5 (with two amino acid modifications D760N and P1174L), along with T36CA-V1.3 were individually propagated in Nicotiana benthamiana and C. macrophylla plants. Enzyme-linked immunosorbent assay (ELISA) measurements of extracts of the newly emerged leaves suggested that all three viruses accumulated to similar levels in N. benthamiana plants at 5 week-post-inoculation. ELISA values of T36CA-V1.4- and -V1.5-infected C. macrophylla samples were significantly higher than that of T36CA-V1.3-infected samples within an 8 to 12 month-post-inoculation (mpi) window, suggesting a higher accumulation of T36CA-V1.4 and -V1.5 than T36CA-V1.3. However, at 36 mpi, the ELISA values suggested that all three viruses accumulated to similar levels. When C. macrophylla plants infected with each of the three viruses were grafted to commercial citrus varieties, a limited number of receptor plants became infected, demonstrating a weak but nonetheless (the first) successful delivery of T36CA to California-grown commercial citrus.