Other SAM regulatory genes are known to be expressed in the SAM. Theshoot meris-
temless(Stm) homeodomain transcription factor gene is required for normal SAM function,
asArabidopsis stmmutants lack a functional meristem. Further, transgenic tobacco plants
expressing an extra copy of the cornKN1(STM-related) gene develop superficial SAMs on
leaves, suggesting strongly thatKN1expression directs SAM formation. TheMgogenes
also play a role in SAM function. Themgo1andmgo2mutants contain disorganized
SAMs and fewer leaves 10 days after germination, suggesting that the SAMs of the
mutants delegate fewer cells to the leaf primordia. The Mgogenes encode proteins
similar to asymmetric cell division regulators in animal cells, suggesting a key role for
the MGO proteins in meristematic cell divisions. Finally, thePhantastica(Phan) genes
help specify adaxial leaf identity, and thus are involved in leaf primoridia differentiation
(Traas and Bohn-Courseau 2005; Shani et al. 2006).
4.3.2 Root Apical Meristem and Root Development
Root development is illustrated in Figure 4.6. Organization of the root apical meristem
(RAM) involves fewer cells than does development of the SAM. The basic organization
of the SAM and RAM are similar in terms of having a central region of slowly dividing
cells surrounded by cells with a higher cell division rate. Recall that specification of the
RAM occurs during the embryonic heart stage. Thus at the heart stage the radial organiz-
ation of tissues is in place and the RAM initials and central cells that will generate and
maintain the root in the seedling are specified. Thequiescent center(QC) is the region
of slowly dividing cells within the RAM. The QC is involved in RAM activity and main-
tenance. In bindweed (Convolvus arvense) the QC cell cycle is 430h, whereas in other cells
it is about 13h. Therefore, the QC must be viewed as the ultimate source for new cells, but
not the factory that produces them.
Figure 4.6.Root development. Arrangement (a) and division plane (b) of cell types within the devel-
oping root. [Reprinted from Di Laurenzio et al. (1996), with permission from Cell Press.]
94 PLANT DEVELOPMENT AND PHYSIOLOGY