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Casing the joint--lignification at a graft union
Grafting, used for many years in the propagation of shrubs and trees, has until recently been used only rarely with non-woody species. Further, we know little about the formation of the graft union in non-woody plants especially in comparison to our understanding of grafts in woody species. Fernandez-Garcia et al. (Murcia, Spain, pp. 53-60) have therefore studied the formation of the graft union in tomato (Lycopersicon esculentum). Grafts were made between aerial shoots of cv Fanny and rootstocks of cv AR-9704. Establishment of vascular continuity was studied by microscopy and by assaying hydraulic conductivity. The early stages were marked by cell death at the graft site, followed by cell proliferation. After 4 days, there was measurable hydraulic conductivity, concomitant with the early stages of vascular tissue formation. After 8 days, fully formed xylem and phloem traversed the graft junction but hydraulic conductivity continued to increase until day 15. The formation of xylem was, as expected, accompanied by lignification, initially in scion and rootstock but soon traversing the graft site. Increases in peroxidase (an enzyme involved in lignin formation) activity were detectable in both scion and rootstock from day 4 and continued until at least day 15. The activity was thus correlated with the onset and continuation of lignification. Catalase activity also increased between day 4 and day 8 but then declined to baseline levels, these changes coinciding with a rise and fall in the concentration of H2O2, the substrate for catalase. The reason for the peak in H2O2 is not clear but the authors surmise that it represents an excess produced during rapid lignification. It remains to be seen whether this is so. However, it is clear firstly that graft formation is a workable tool in improvement of tomato cultivars and secondly that this system provides a good model for studying the induction of specific developmental pathways.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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