386 C. Gutierrez
to pre-RC dysfunction. Thus, (1) mutations in theORC2gene, an E2F target
(Diaz-Trivino et al. 2005), leads to failure in nuclear division control (Collinge
et al. 2004), (2) in thePROLIFERA(PRL) gene, that encodes MCM7, produces
abnormal patterns of division planes (Holding and Springer 2002) and (3) in
theCDC45gene, which acts downstream pre-RC, to sterility (Stevens et al.
2004).
2.3
Other Genes
An increasing number of genes, some of them cited in the paragraphs above,
are reported to affect cell morphogenesis, organogenesis or the balance be-
tween proliferation and endoreplication. While the individual mechanism of
action is not known, it is worth discussing them in the context of this article.
Defects in theTONSOKUgene (Suzuki et al. 2004), expressed in S-phase
and involved in meristem maintenance (Suzuki et al. 2005a), delay cell cycle
progression, cause G2/M arrest and slightly increases the number of 4C cells
(Suzuki et al. 2005b). Whether they are actually endoreplicating cells is not
known, but interestingly, they also have increased levels ofCYCB1;1expres-
sion. Disruption of theRPN1agene, encoding a component of the regulatory
particle of the 26S proteasome, causes embryo lethality by promoting arrest
at the globular stage (Brukhin et al. 2005). Whether and how the high levels of
CYCB1;1maintained in therpn1amutant plants contribute to the phenotype
is unknown.CIA1encodes an amidotransferase (ATase2) involved in the first
step of de novo purine biosynthesis.cia1mutants have leaves slightly smaller
in size than the wt but contains about half the number of cells (Hung et al.
2004).
One of the phenotypes exhibited by theroot hairless2(rhl2)andhyp-
cotyl6(hyp6) mutants is reduction in the ploidy level (Hartung et al. 2002;
Sugimoto–Shirazu et al. 2002). These genes encode the A and B subunits
of topoisomerase VI (TOPVI). ROOT HAIRLESS1 (RHL1), also known as
HYPOCOTYL7 (HYP7), is an essential component of the topoisomerase VI
complex, with similarities to the C-terminus of mammalian topoisomerase
IIα. (Sugimoto–Shirazu et al. 2005). Therhl1mutant, as well as therhl2and
hyp6mutants, has a maximum of 8C of DNA content in their cells, revealing
the importance of topoisomerase function in proper endocycle progression.
Auxin response factors ARF6 and ARF10, which are targets ofmiR160,con-
trol root cap formation. Decrease in ARF10 and ARF6 expression (either by
overexpressingmiR160or in the double mutantarf10 arf6), displays root tip
defects, uncontrolled cell division and a blockage of cell differentiation in the
distal parts. These phenotypes suggest that ARF10 and ARF6 restrict the stem
cell niche in the columella (Wang et al. 2005). A thermosensitive mutant,rpd1
(root primordium defective 1) has been identified on the basis of its impair-
ment in producing adventitious roots from the hypocotyl in response to auxin