METABOLIC PATHWAYS OF THE DISEASED POTATO
Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, Scotland, UK
Information on secondary metabolism in the diseased potato has been published over many years, most of it associated with infection by Phytophthora infestans, Erwinia carotovora, or the application of resistance elicitors. Compounds such as the glycoalkaloids are preformed and may have a role in resistance to some fungi, whilst the sesquiterpene phytoalexins are formed only in response to infection or elicitors, and are absent from healthy tissue. Some pathways will be common to many plant families, e.g. phenylpropanoid metabolism, whilst others, e.g. sesquiterpene phytoalexin biosynthesis involving rishitin, will be unique to the Solanaceae. Enzymes associated with these metabolic pathways may be products of 'response' genes and activated after receptors (products of resistance genes) have detected the presence of the pathogen. Many coordinated responses are involved in the resistant reaction.
Results and conclusions
Combining information in the format of a metabolic pathways chart  has a number in of advantages. It draws attention to the complexity of the plant's response to infection, and highlights some of the responses which may not be so obvious when reading primary publications on individual enzymes. The chart also clearly highlights those areas where information is lacking, e.g. enzymes involved in the biosynthesis of the sesquiterpene phytoalexins. This contrasts with the greater knowledge about phenylpropanoid metabolism, although this is still incomplete. There is little firm data on signal transduction pathways in potato and hence the chart includes some generalizations. For example, the MAP kinase pathway has been poorly described in plants but is much better described in animal signalling systems.
There may be some common elements in the manner in which a plant responds to infection, but a plant's response may vary between plant pathogens, suggesting that plant signals can discriminate between different pathogens, e.g. not only between fungi and viruses but between biotrophs and necrotrophs. The metabolic pathways chart should be viewed as the potential response to infection; it does not imply that all such processes are, or can be, activated by every pathogen. In addition, not all of the pathways shown will be up-regulated after infection. For example, glycoalkaloid accumulation is suppressed after infection in favour of sesquiterpene accumulation. Importantly, one should also consider the intracellular localizations of proteins. Such compartmentalization plays a crucial role in enabling the plant to differentially up-regulate gene products to give a different wound- or pathogen-induced response.
With the recent sequencing of a number of disease resistance genes in plants, and in consequence the potential to analyse the signal-transduction pathway leading to induced resistance, there will be increased interest in engineering resistance cascades. This can be carried out effectively only if the complexity of the plant's responses is better understood. Genetic manipulation of response genes has a number of important considerations, not only the primary one in relation to disease resistance but also the toxicological implications, and whether such manipulation can alter the nutritional status of the plant. For example, Laudla et al.  showed that new glycoalkaloids are present in potatoes derived from a cross between S. tuberosum and S. brevidens which are not present in either parent.
The chart  summarizing the metabolic pathways of the diseased potato is accessible on the Internet as a portable document format file which can be read using Adobe Acrobat Reader 3.0. Acrobat Reader can be downloaded free from the lnternet.
1. Laudla J, Laakso I, Valkonen JPT et al., 1996. Plant Science 118, 145-155.
2. Lyon GD, 1997. Metabolic pathways of the diseased potato. htp://www.scri.ac.uk/bpp/charttxt.htm