To return to Chapter 12 of Volume 7 ‘‘Secondary Plants Products’’ in the series The Biochemistry of Plants (Waller and Dermer, 1981) authored by George Waller
and Otis Dermer, the authors concluded at the time that the important characteristics
of alkaloid biosynthetic enzymes should be catalytic properties, regulation,
intracellular localization, and tissue distribution. The authors were arguably not
satisfied with the progress that had been made in these areas up until 1979. Since
that time, astonishingly much progress has been made in our understanding the
nature of the enzymes that synthesize alkaloids in plants. With the advent of the
application of molecular genetic methods to the field, we have sophisticated tools
at our disposal to study the regulation of enzyme biosynthesis as well as the
cellular and subcellular localization. We still purify enzymes with ever more
refined instrumentation, but we can also identify biosynthetic enzymes with
genetic approaches. The first reports of successful crystallization of enzymes of
alkaloid biosynthesis, a requisite to X-ray crystallographic structure determination,
have now appeared (Ma et al., 2004a,b). Notably, the first alkaloid biosynthetic
enzyme for which a crystal structure has been determined is also the first
one for which a cDNA was isolated, strictosidine synthase from R. serpentina (Ma et al., 2006). A topic not touched upon at all by Waller and Dermer was the
possibility to alter alkaloid metabolism in plants and plant tissue and cell cultures.
In 2006, we are just beginning to metabolically engineer alkaloid metabolism in
plants and in in vitro culture. Multicellular compartmentation of alkaloid pathways must be considered if meaningful metabolic engineering experiments are to be
designed. We will need to use promoters that drive transgene expression in the
correct cell types. Regulation of these pathways at the gene and enzyme level
is complex, and there is still much to be learned about metabolite levels and
pathway interconnections as we systematically overexpress and suppress gene
transcription. Today, pathway engineering in plants remains highly variable.
When we perturb cellular physiology, metabolite homeostasis and intra- and
intercellular partitioning can be affected in unpredictable ways. Another aspect
that needs attention is the development of efficient transformation and regeneration
protocols for alkaloid-producing plants that do not belong to the Solanaceae.
All told, there is still much to be achieved.