Approaches to achieving extended shelf-life have centred upon the manipulation of a few key regulatory pathways, such as cytokinin biosynthesis. However, it is clear that if tight control of senescence is to be achieved, without any associated detrimental effects on growth and fertility, the manipulation of elements downstream of cytokinin biosynthesis and other controlling factors needs to be addressed (McCabe et al., 2001). Indeed, in order to produce functional 'stay-green' leaves, modification will probably be required of several regulatory pathways (Wingler et al., 1998). Similarly, it has been proposed that studies of the differences in the expression of possible candidate genes in PSAG12-IPT transformed and non-transgenic plants of lettuce or, indeed, in other species, may reveal alternative pathways for genetic manipulation in order to achieve more efficient strategies for delaying senescence (McCabe et al., 2001). To date, there is little evidence of attempts to delay senescence being applied to root crops, with the exception of the generation of potatotransformedwiththe ipt gene(Macha-kova et al., 1997). Certainly, the effects of altered carbohydrate partitioning in vegetable crops, such as potato andcarrot,couldproveinteresting.
Rapid advancements in genomics and the application of microarray technology should facilitate the evolution and refinement of new approaches to manipulating and extending shelf-life. Thus, bymonitoringdifferentialgeneexpression during senescence or fruit ripening, newtargetsmaybeidentifiedforgenetic manipulation in the context of extendingshelf-life.In A.thaliana,thesteady-state mRNA levels of over 800 genes havebeenstudiedsimultaneouslyusing high-density arrays (Desprez et al., 1998). The number of genes that can be assessed using this technology has increased substantially, with arrays containing 7000-10000 non-redundant expressed sequence tags (ESTs) representing about 7500 genes, being made available through the Arabidopsis Functional Genomics Consortium, involving Michigan State University, The University of Wisconsin, Yale University and the Carnegie Institute of Washington at Stanford University. This figure is expected to increase towards the goal of 20000 genes in the near future. Undoubtedly, such anapproachwillproveextremelyusefulin guiding reverse-genetics technology to identify the key genes of relevance in extending the shelf-life of a rangeofvegetablecropspecies.
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