Identification of the mechanisms
by which plant cells monitor and respond to their
basic metabolic requirements will rely on the integration of
transcript, protein and metabolite profiles of different cell
types . Such studies must also consider
the effects of changes in the cell cycle and circadian
rhythms. For the enzymes that are encoded by multiple
genes, the expression, subcellular localization and activity
of each member of the family will need to be evaluated
since they are often expressed in a tissue specific manner.
Unfortunately the modest expression levels of housekeeping
genes complicate their analysis since small differences
in activity (2-3 fold) that may have large metabolic impacts
are difficult to reproducibly detect. Also techniques such
as in situ hybridization for monitoring transcript abundance
in different tissues are not easily applied to constitutively
expressed genes since cell density and volume changes
affect the intensity of the signal. However, as more sensitive
techniques for measuring the levels of proteins and
transcript are applied to the study of purine and pyrimidine
nucleotide metabolism, the functional significance of these
housekeeping genes in plant development will become
more evident.