1Plant Biology Division, Samuel Roberts Noble Foundation, Ardmore, OK 73402, USA; 2Department of Entomology, University of Arkansas, Fayetteville, AR 72701, USA; 3;Plant Biology Laboratory, Salk Institute for Biological Studies, La Jolla, CA 93182, USA

Background and objectives
Plant natural products are believed to play important roles in resistance against microbial pathogens and insect predators. Both constitutive and inducible pathways contribute to the plant's metabolic arsenal. Recent advances in our understanding of the biochemistry and molecular biology of the pathways leading to bioactive phenylpropanoids, flavonoids, isoprenoids and alkaloids [1,2,3] provide the necessary information for rational modifications of these pathways by genetic engineering.

Results and conclusions
The phenylpropanoid pathway provides a model system for discussion and evaluation of the strategies that could be used to improve resistance through metabolic engineering. These include:
Increasing overall pathway flux by constitutive over-expression of early pathway enzymes. For example, altering expression of phenylalanine ammonia-lyase in transgenic tobacco affects the levels of chlorogenic acid, the major leaf phenylpropanoid compound, and salicylic acid, an important regulatory molecule for expression of local and systemic resistance [4,5]. PAL-suppressed plants are more susceptible to fungal infection, and do not establish systemic acquired resistance. However, they appear to be exhibit increased systemic resistance to insect attack. We will describe the corresponding phenotypes of PAL overexpressing plants.
Global regulation of pathway expression by altering levels/activities of transcriptional regulators. Several of the genes in the flavonoid/isoflavonoid branch of the phenyipropanoid pathway share common regulatory elements involved in stress response signal transduction. Direct cloning of genes encoding transcription factors that bind to these elements and their protein modifiers [7], or indirect approaches such as activation tagging, open up the possibility of modifying expression of coordinately regulated sets of enzymes.
Modification of existing pathways to lead to products with increased bioactivity or stability
Introduction of novel pathways from other species. Present limitations in transformation technology have restricted the applicability of this approach to cases where only one or two new gene products effect the necessary metabolic transformation. Possible genetic linkage of some natural product pathway enzyme genes [8], and strategies for transformation of BAC clones [9], lends hope to the possibility that transformation with individual isolated genes may not always be the only applicable approach.


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