Protocols for Micropropagation of Woody Trees and Fruits

MICROGRAFTING GRAPEVINE FOR VIRUS INDEXING 265
Root initiation on the rootstock can be expected 2–3 weeks after grafting
(Figure 2C). Successful grafts first produce callus at the point of grafting before the
vascular connections are established.
2.2.4. Detection of Viral Symptoms in Grafted Rootstock
Tissue culture is an ideal environment for monitoring the presence of GRLaVIII
because symptoms of this virus are ideally expressed at temperatures between 22
and 25°C. When an infected scion is micrografted on to an uninfected rootstock of
the indicator variety (e.g. Cabernet sauvignon), the basal leaf of the rootstock turns
red, at first at the edges, within 2–3 weeks (Figure 2D). There appears to be no
difference in the rate of symptom appearance in the rootstock when different scions
are used. However, the degree of expression of the symptoms may vary depending
on the indicator rootstock. Other serotypes of leafroll virus may also produce
symptoms easily when micrografted, but experimental evidence is lacking to date.
Three months following micrografting, shoots can be expected to produce 4–8
leaves and further general symptoms of leafroll may be observed. These include
rolling of lower leaves, stunted growth and small, underdeveloped leaves (Figure
2E, F). Seventy to ninety percent of micrografts can be expected to display at least
one visual symptom of leafrolling disease by this time if the scions used are all
infected.
3. CONCLUSION
The described micrografting technique for grapevine is a robust and reliable method,
providing actively growing plant material of a uniform nature established in tissue
culture. Micrografting success appears to be most reliant on the good fit of scion to
root stock and conditions of high humidity in the tissue culture system. Wrapping of
the graft union is not necessary.
The technique is particularly useful as a rapid method for detecting viral
contamination in grapevine and has been applied successfully to detect corkybark
and GRLaVIII viruses within 8–12 weeks (Tanne et al., 1993; Pathirana and
McKenzie, 2005b). Within the limited amount of experimental evidence available to
date, it appears that the incompatibility seen in other methods of grafting applies less
to micrografting. However, selection of the proper indicator rootstock is important
to maximize the chances of detecting the viruses because not all the rootstocks used
in woody grafting produce similar results. Validation of this method for other
viruses that infect grapevine could support the grapevine industry by increasing the
rate and ease of detection of contaminating viruses that have a serious economic
impact on both imported and domestic grapevine material.
Acknowledgements. We thank the New Zealand Ministry of Agriculture and Forestry for
funding our research (MAF project code MBS 320), and Tony Baker and the staff of New
Zealand Grapevine Improvement Group Inc. who supplied the initial plant material.