The Use of Transgenic Plants to Study the Regulation of Plant Carbohydrate Metabolism

Transgenic plants with decreased expression of specific enzymes provide a powerful new tool to investigate metabolic regulation. Their use is discussed in the context of theories of metabolic regulation. It is argued that an enzyme is a key site for regulation, in the strict sense, when (i) natural mechanisms exist to alter the activity of the enzyme in vivo ('regulatability'), and (ii) a change in the activity of the enzyme is able to lead to a change in flux through the pathway ('regulatory capacity'). Previous approaches to the study of regulation allow the identification of enzymes with high 'regulatability', but they do not provide consistent or valid criteria to assess their 'regulatory capacity'. They therefore do not distinguish between enzymes which actually control metabolic fluxes, and enzymes which just respond to changes initiated elsewhere in the pathway. They may also underestimate the contribution of enzymes that catalyse reversible reactions. In contrast, mutants and transgenic plants can be used to directly test the importance of different aspects of an enzyme's regulatory properties in vivo. Even more importantly, they provide a method to determine flux control coefficients which provide a quantitative measure of an enzyme's 'regulatory capacity'. Recent results are surnrnarised, and potential practical problems in measuring control coefficients are reviewed.