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Murder in the vineyard
One of the most famous crop losses in recent history was the devastation of French vines by the aphid-like parasite phylloxera (Daktulosphaira vitifolia), introduced into France with vines from the USA. The latter are resistant to phylloxera, but in the French vineyards, the parasite encountered a largely susceptible population. The name phylloxera suggests dry leaves, but the above-ground symptoms result from the disruption of root function. Infestation of roots by phylloxera leads to the formation of gall-like structures called nodosities, the formation of which has been investigated by Alison Kellow and colleagues (Glen Osmond, Australia, pp. 581-590). These nodosities form in the root elongation zone indicating that the parasite is able to modify the normal patterns of root growth and differentiation. An interesting feature of this disruption is the failure to form a suberized endodermis in this region of the root, although vascular tissue differentiation appears normal. The nodosities provide an abundant source of nitrogen (via amides and amino acids) and carbon (via sugars derived from starch that accumulates at these sites). That the parasite does indeed use these structures as feeding sources was confirmed by the observation of stylet tracks. After the feeding phase, the nodosities eventually become necrotic, contributing further to the disruption in root function. One key question still to be resolved is why particular vine cultivars are susceptible. The authors report that there is no immediate localized response to the parasite, such as occurs in resistant varieties and, furthermore, there is no induction of the expression of a range of ‘defence genes’. Indeed, the authors suggest that the failure to up-regulate these genes is a direct consequence of the absence of an immediate response. In susceptible cultivars the parasite is somehow ‘invisible’ to the host defence mechanisms and it will be very intriguing to discover how it puts on this ‘cloak of invisibility’.
j.a.bryant{at}exeter.ac.uk
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