factors such as PIF3 to influence gene expression. Many of the gene products required to
construct an active photosynthesizing chloroplast are controlled by the presence of light,
and thus are most likely under the control of phytochrome-mediated signal transduction
pathways. Phytochrome itself is encoded by five different Phy genes called PhyA
throughPhyE.
Mutants defective in photomorphogenesis have been instrumental in identifying genes
required for this process. There are two general categories of photomorphogenesis
mutants: (1)hyand (2)copanddet. Thehy(hypocotyl elongated) mutants look partially
etiolated even when grown in the light, indicating that the HY gene products function in
the perception of light. These screens identified some of thePhygenes and other positive
regulators of photomorphogenesis such as HY5, a key transcription factor. In contrast, the
cop(constitutive photomorphogenesis) anddet(deetiolated) mutants were identified by
virtue of their light-grown phenotypes when grown in the dark. Many of the cop
mutants encode proteins that form a large complex called theCOP9 signalsome(CNS),
a nuclear complex that is similar to the 26Sproteasome proteolytic complex that degrades
ubiquitinated proteins (Rockwell et al. 2006).
The lack of etiolation in somecopanddetmutants can be reversed by adding the plant
steroid hormonebrassinolide(Br), suggesting a role for Br signal transduction in photo-
morphogenesis. Thedet2mutant ofArabidopsishas sequence homology with mammalian
steroid 5a-reductases. This suggests that the DET2 gene product participates in Br syn-
thesis. Thus, light may control photomorphogenesis by downregulating Br production.
Blue light is another important stimulus for photomorphogenesis and for phototropism
(growth toward light). Blue light is perceived by two types of flavin-containing proteins,
crytochromeandphototropin.Both cryptochrome (CRY) and phototropin are encoded
by two genes inArabidopsis. CRY proteins appear to function in the nucleus, although
there are indications that there may be some CRY functions in the cytoplasm as well.
Evidence suggests that phytochrome and cryptochrome physically interact. CRY protein
can be phosphorylated in vitro by the protein kinase activity of PHY. In addition, PHYB
and CRY2 interact in plant extracts. CRY1 and CRY2 also appear to directly interact
with COP1, the negative regulator of photomorphogenesis in the dark.
4.3 Meristems
Plant meristems are dynamic structures whose functions are to renew themselves and to
give rise to new cells with a different identity. There are three types of meristems: apical
meristems including the shoot and root apical meristems (SAMs and RAMs); the lateral
meristems, including the vascular and cambialmeristems responsible for secondary
growth; and theintercalary meristems, common to the grasses that occur at the bases of
nodes. The common function of these meristems is regulation of cell division that
creates new cells specified to become different cell types and renewal of the meristem itself.
4.3.1 Shoot Apical Meristem
Apical meristems are extremely important in terms of growth regulation of plants. As
alluded to previously, the SAM gives rise to the aerial parts of higher plants by conti-
nuously initiating new organs. The basis of this activity is its ability to maintain a pool
of pluripotent stem cells, which are the ultimate source of all tissues of the shoot. The
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