Lessons from the root apex - Intercellular Communication in Plants - page 208

pathway (Jang et al. , 2000; Schrick et al. , 2000). Recently, two more mutants in
the BS pathway have been isolated, cephalopod (cph) and hydra1 (hyd1) . CPH
encodes a C-24 sterol methyltransferase and HYD1 a sterol C-8,7 isomerase. CPH
catalyses the first step in the BS pathway, and FK and HYD1 act sequentially a bit
further in the pathway. All three mutants have a similar phenotype in which cell
division is disturbed and/or polarity decisions are randomized. This results both
in embryos where the early decisions fail and in very severe seedling phenotypes
(Schrick et al. , 2002). Souter et al. (2002) showed that both hyd and fk mutants are
defective in ethylene- and auxin-mediated gene expression and cell differentiation,
while the auxin transporter PIN is normally localized. The auxin influx component
AUX1 can be blocked with NPA, yet the mutants showed auxin responses after two,
4-D treatment. axr1 and axr3 mutations, which block some auxin responses, were
partially able to rescue the hyd mutant phenotype. This all suggests that the hyd/fk
mutants are defective in membrane-bound protein activity, and thus indirectly affect
polar auxin transport. This can be interpreted as further support for auxin being the
keyregulator in embryonic axis formation/root meristem formation.
The analysis of various mutations has suggested how auxin action might be exe-
cuted during the patterning of the root meristem. Auxin is thought to initiate a spe-
cific signal transduction pathway. Although the receptor for auxin remains elusive,
the transcriptional control related to auxin action is conceptually understood (Gray
et al. , 2001). It involves auxin response factors (ARFs) that activate the primary re-
sponse genes. These genes are repressed by auxin-inducible, short-lived AUX/IAA
proteins, which are selectively targeted for degradation based on an auxin-regulated
proteolytic activity (Hellmann et al. , 2003). During auxin signalling the conserved
degradation domain II of IAA proteins interacts with the F-box protein TIR1 of the
E3 ubiquitin ligase complex SCF TIR1 , which then results in the degradation of IAA
by the 26S proteasome (Hellmann & Estelle, 2002). The MONOPTEROS ( MP ) locus
was first defined by recessive mutations that disrupted the apical-basal organization
of the embryo and the continuity of the vascular strands (Berleth & Jurgens, 1993).
Molecular cloning revealed MP as one of the first ARFs to be genetically charac-
terized. MP encodes ARF5 (Hardtke & Berleth, 1998) that can bind to conserved
'auxin response' promoter elements of downstream genes (Ulmasov et al. , 1997;
Guilfoyle et al. , 1998). MP is first expressed in a broad domain during embryogen-
esis and its expression subsequently becomes restricted to vascular tissues and the
basal domain of the embryo, indicating its importance as a key factor controlling
auxin-mediated specification of vascular and basal fate. A semi-dominant mutation
in the BODENLOS locus results in an mp- like phenotype (Hamann et al. , 1999).
BODENLOS encodes an AUX/IAA repressor (IAA12) and the mutation (located
in domain II) that prevents interaction with SCF TIR1 is likely to extend its stability.
Since MP and BDL gene expression domains overlap and their products interact,
it is likely that they constitute an activator/repressor module regulating vascular
bundle formation and basal fate (Hamann et al. , 2002). Analogously, the axr6 muta-
tion, located in a gene that codes for the SCF subunit CUL1 and which results in an
mp -like phenotype, also causes stabilization of AUX/IAA proteins (Hellmann et al. ,