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Passive movement pierces plant defences—and demolishes dogma
For growers of grapevines, Xylella fastidiosa spells trouble; it is the causative agent of Pierce’s disease, a condition that is almost inevitably lethal. Chatelet et al. (Davis, California, USA, pp. 483–494) describe X. fastidiosa as typifying those pathogenic bacteria that are limited within the host plant to the xylem, within which they move passively. Several of the symptoms of Pierce’s disease relate to blockage of the xylem vessels, not just by the pathogen but also by substances produced by the host in response to infection. Other symptoms, including necrosis of leaf margins and loss of leaf laminae, imply that the bacterial cells can move out into the leaves. As the authors state, accepted dogma is that the bacteria can move between vessels by digestion of the membranes of pits that connect the vessels. However, this group has previously obtained evidence to counter this view. In particular, lux-expressing cells of bacteria that lack any ability to digest pit membranes were shown to move through the plant in a similar way to X. fastidiosa, indicating that there is an open route from the stem into the leaf with no discontinuities. In the present paper, the authors present an extensive range of results that confirm this idea. For example, the movement of air, latex paint particles and GFP-tagged X. fastidiosa cells from stem to leaf clearly indicates the presence of ‘open conduits’ out into the leaves. The stem–leaf connection does not occur in the youngest 12 nodes and, further, even when developed the ‘conduit’ does not reach the leaf margin: bacteria only reach approx. 50–60 % of the distance from the leaf base to the margin. Anatomical studies show that perforation plates are absent from tracheary elements beyond the basal 50–60 % of the leaf blade, thus preventing further outward movement of bacteria—but even at a distance they can cause necrosis at the leaf margins.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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