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PR boost for compost claims
The current emphasis on more sustainable agriculture and horticulture has stimulated research on integrated management of pests and diseases. In such systems, the grower relies less on control by agrichemicals and more on biological control. Further, there is a growing literature to support the many claims that certain composts can provide protection against plant pathogens. Seedlings grown in such composts are much less prone to disease. The frequency of the diseases is very much lower and, if infection does occur, the severity of the symptoms is reduced. Much of the research has focused on the soil-borne pathogens themselves, but Kavroulakis et al. (Kalamata and Athens, Greece, pp. 555–564) have turned their attention to the plants grown in disease-suppressive composts. Using the known sequences of plant proteins that play a role in defence (a group of pathogenesis-related or PR proteins), they have designed primers to detect, by reverse transcription–polymerase chain reaction (RT–PCR), the expression of genes encoding these proteins. Their results clearly show that, even in the absence of any pathogens, growth of tomato (Solanum lycopersicum) in compost induced the expression of two PR genes: PR1 and PR5. This did not happen in plants grown in a peat-based medium. A third PR gene (PR69, a member of the PR7 gene family) was expressed in peat-grown plants, but expression was strongly up-regulated by growth in compost. They then carried out in situ hybridization to examine the spatial patterning of the expression of these genes: PR1 was expressed in the root pericycle and PR5 in the phloem of both roots and stems. The expression of PR69 was less predictable, occurring sporadically in root parenchyma and vascular tissue. These results provide clear evidence that the compost can induce the synthesis of proteins involved in plant defences against pathogens: the plant is thus primed with its defence system ready to go, should an attack occur.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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