Previous analyses of the circadian clock in the plant
Arabidopsis thaliana have turned up just a few genes regulated
by the clock. In the 15 December Science, Harmer et al. use
oligonucleotide-based arrays to find a vast new collection of
clock-regulated genes (Science 2000, 290:2110-2113). Using
probes derived from tissue harvested every four hours, and an
array representing 8,200 different genes, Harmer et al. find
that 453 genes (6%) fit a cosine test wave with a period between
20 and 28 hours. The day begins with induction of photosynthetic
genes, and genes for making photoprotective pigments. Genes for
the energy-intensive process of nitrate assimilation are induced
only during the daytime. As night approaches, genes for chilling
resistance are induced, and metabolic and sugar transporter
genes are induced, so that energy resources can be mobilized for
the long night ahead. Energy management is further aided by the
nighttime induction of genes for starch mobilization. Cell
elongation genes peak late in t