LTR-retrotransposon diversity and transcriptional activation under phytoplasma stress in Ziziphus jujuba

Abstract

Degenerate primers were used to amplify the conserved domains of the Ty1-copia and Ty3-gypsy reverse transcriptase (RT) genes of the long terminal repeat retrotransposons in the jujube (Ziziphus jujuba Mill.) genome. Sequence analysis showed that 18 of the 50 Ty1-copia unique sequences and 28 of the 38 Ty3-gypsy unique sequences possessed stop codons, frameshifts, or both. The overall sequence divergence in jujube was higher in the Ty3-gypsy than the Ty1-copia group, in contrast to the results from other plants. Our results showed that sequence heterogeneity during vertical transmission could be a major influence on the evolution of copia- and gypsy-type retroelements in the jujube genome. In order to elucidate the transcriptional activity under phytoplasma stress of these two classes of RTs, RT-PCR primers were designed to amplify products corresponding to either the Ty3-gypsy or Ty1-copia RT domains from total RNA extracted from leaves or flowers of plants infected by phytoplasma. Amplification products were present for Ty3-gypsy RT in both tissues during infection; however, no RT-PCR products were amplified using the Ty1-copia RT domain primers in phytoplasma-infected leaves or flowers. RT-PCR amplification from total RNA extracted from leaves or flowers of healthy plants did not yield either of the RT fragments. The Ty3-gypsy RT-PCR products were confirmed to originate from the Z. jujuba genome. Our results indicated that the gypsy group of retrotransposons was autonomous retroelements. This is the first report characterizing the Ty1-copia and Ty3-gypsy groups of retrotransposons in the Z. jujuba genome.