Experimental Transmission of Pospiviroid Populations to Weed Species Characteristic of Potato and Hop Fields▿

Weed plants characteristic for potato and hop fields have not been considered in the past as potential hosts that could transmit and lead to spreading of potato spindle tuber (PSTVd) and hop stunt (HSVd) viroids, respectively. To gain insight into this problem, we biolistically inoculated these weed plants with viroid populations either as RNA or as cDNA. New potential viroid host species, collected in central Europe, were discovered. From 12 weed species characteristic for potato fields, high viroid levels, detectable by molecular hybridization, were maintained after both RNA and DNA transfers in Chamomilla reculita and Anthemis arvensis. Low viroid levels, detectable by reverse transcription-PCR (RT-PCR) only, were maintained after plant inoculations with cDNA in Veronica argensis and Amaranthus retroflexus. In these two species PSTVd concentrations were 105 and 103 times, respectively, lower than in tomato as estimated by real-time PCR. From 14 weeds characteristic for hop fields, high HSVd levels were detected in Galinsoga ciliata after both RNA and DNA transfers. HSVd was found, however, not to be transmissible by seeds of this weed species. Traces of HSVd were detectable by RT-PCR in HSVd-cDNA-inoculated Amaranthus retroflexus. Characteristic monomeric (+)-circular and linear viroid RNAs were present in extracts from weed species propagating viroids to high levels, indicating regular replication, processing, and circularization of viroid RNA in these weed species. Sequence analyses of PSTVd progenies propagated in C. reculita and A. arvensis showed a wide spectrum of variants related to various strains, from mild to lethal variants; the sequence variants isolated from A. retroflexus and V. argensis exhibited similarity or identity to the superlethal AS1 viroid variant. All HSVd clones from G. ciliata corresponded to a HSVdg variant, which is strongly pathogenic for European hops

Cloning and molecular analyses of hop transcription factors

Expression of hop chs_H1 and other hop chalcone synthase (CHS)-like enzymes was assayed in developing pollen tissue. Both, chs_H1 and valerophenone synthase (vps) are highly expressed in pollen suggesting the existence of common specific regulatory element(s). While vps is highly specific for female and male floral organs, the oligofamily of chs_H1 is expressed in both generative and somatic tissues, suggesting a more universal promoter. To identify novel transcription factors involved in the regulation of chs_H1 genes in lupulin glands, cone-specific ESTs showing homology to bZIP and bHLH were utilized to screen hop cDNA and genomic libraries. Additionally, inverse PCR was used to amplify full length cDNAs of members of two bZIP oligofamilies from hop. Transcription factors H/bZIP1A and H/bZIP2 have predicted molecular masses of 27.4 and 34.2 kDa, respectively. While H/bZIP1A is rather neutral (pI 6.42), H/bZIP2 is strongly basic (pI 8.51). A truncated variant of H/bZIP1 (H/bZIP1B) that is strongly basic but lacks the zipper domain was also cloned from hop. Additionally, the first bHLH transcription factor was cloned from hop. H/bHLH1 is a protein of 33.6 kDa with a predicted pI of 6.08. Based on semiquantitative RT PCR results, this TF is expressed specifically in hop floral and cone tissue. The combinatorial action of H/bZIP2 and other hop TFs that have been cloned, especially H/Myb1 and 3 were analyzed in P. hybrida and N. benthamiana transient expression systems. TF H/bZIP2, as well as both Myb TFs, were found as activators of chs_H1 promoter. In the combinatorial system, H/bZIP2 was found to modulate expression of H/Myb1 and H/Myb3 Trs from hop