ABSTRACT
The activities of several enzymes of C and N metabolism in the cytosol, mitochondria and plastids of plant cells are regulated by rapid changes in protein phosphorylation in response to environmental and metabolic stimuli. Recent experiments using transgenic plants have highlighted the practical importance of understanding the impact of these regulatory mechanisms on patterns of metabolic flux in plants. For example, over-or under-expressing SPS and NR in plants was found to have minimal effects in some cases because compensatory changes in the activation (phosphorylation) states of these enzymes took place that restored their activities towards the levels in wild-type plants (e.g. Sonnewald et al., 1994). In contrast, plants that constitutively over-expressed ‘unregulated’ SPS showed large increases in biomass (Chapter 6). Testing the physiological effects of mutating the regulatory phosphorylation sites on metabolic enzymes, and altering the activities of the relevant protein kinases and phosphatases by genetic and/or ‘pharmaceutical’ means are likely, therefore, to be important contributions to future developments in engineering plant metabolism.
