Articles
THE ASSOCIATION OF CHERRY LEAF ROLL VIRUS WITH BIRCH AND WALNUT POLLEN
In Southern England, cherry leaf roll virus (CLRV) infects about half of the mature walnut trees (Cooper, 1980) and locally, wild birches (Betula spp.) are infected to a similar extent (Cooper & Atkinson, 1975). Controlled pollinations have shown that CLRV is transmitted through both male and female gametes (Cooper, 1976) but the relationship of virus with the gametes and the effects of CLRV on the reproduction behaviour of birch and walnut was not known until recently.
This abstract describes some anatomical and epidemiological features of pollen-borne CLRV.
The association of CLRV with the pollen of birch and walnut was studied by examining sections (c 100nm thick) of anther tissue or shed pollen in a Jeol 100S electron microscope at 100KV. Anatomical comparisons were made between healthy and infected tissue: CLRV being detected by bioassay or ELISA.
In cells forming the anther walls, virus-like particles (c 25nm diameter) were observed as close packed paracrystalline arrays in the cytoplasm and within tubules that were often near cell walls.
Paracrystalline arrays of VLPs were observed in the vegetative cell cytoplasm of both birch and walnut pollen, and in some instances these arrays appeared to be bounded by a membrane within which the particles were aligned in rows.
Small arrays of VLPs were observed in the sperm cells of some pollen grains.
Only very occasionally were tubules containing VLPs observed in the cytoplasm of the vegetative cells and neither tubules nor paracrystalline arrays were seen in cell nuclei.
VLPs were observed only in pollen grains or anthers from trees in which CLRV was detected.
Although male catkins (that originate in one growing season and complete their development the following spring) contain ELISA detectable CLRV during most of the period from initiation, significant accumulations of VLPs were not seen until anther tissue collected in February (birch) or May (walnut) was examined.
The outer surfaces (exine) of pollen never appeared to be contaminated by VLPs and when mature (shed) pollen grains containing CLRV were washed in a range of solutions differing in pH and ionic strength, there was no evidence of virus elution into the aqueous media.
Thus, CLRV infects pollen grains and tests were done to determine whether and to what extent the virus influenced reproductive behaviour.
Male catkins from CLRV-infected birches are indistinguishable from their healthy counterparts.
On the other hand, CLRV-infected walnuts