Wolfgang Lukowitz1,2
and Chris Somerville2
1Cold
Spring Harbor Laboratory, Cold Spring Harbor, NY; 2 Department of
Plant Biology, Carnegie Institution, Stanford University, Stanford, CA
Early development of Arabidopsis proceeds in an almost invariant sequence of cell division and expansion. Axis formation is apparent by the early globular stage, when cells in the lower tier of the proembryo expand along the apical/basal axis. Root formation is initiated at about the same time and involves the uppermost suspensor cell, termed hypophysis, which gives rise to the quiescent center of the meristem. Both processes are dependent on the establishment of polar auxin transport.
Mutations in the HAN gene severely disrupt root and axis formation in the early embryo. Cells at the base of the proembryo divide aberrantly producing fewer and irregularly arranged daughters. The cells of the suspensor, including the hypophysis, stop dividing at the 8-cell stage. In contrast to wild type, growth of the mutant embryos is dominated by the upper tier. A variable fraction of the mutants overcomes these early defects and develops into complete and viable seedlings indicating that HAN function is not generally required for axis and root formation.
Genetic evidence suggests that HAN is a negative regulator of auxin responses. PIN4, a presumptive auxin efflux carrier implicated in the formation of an auxin sink, is expressed ectopically in the center of the proembryo. Furthermore, han mutations act as suppressors of monopteros mutations. MONOPTEROS is a positive regulator of auxin responses, and loss of MONOPTEROS function results in seedlings without root or hypocotyls. Double-mutant embryos, if they germinate, have a hypocotyl and root.
The HAN gene product is similar to transcriptional regulators of the GATA family. In the early embryo, HAN transcripts are specifically expressed in the cells of the proembryo. We propose that HAN functions in temporarily inhibiting a subset of auxin responses, such as the formation of an auxin sink, to facilitate the establishment of polar auxin transport.
MAP-kinase
signaling in the Arabidopsis embryo
Wolfgang Lukowitz1,2,
Adrienne Roeder2,3, Dana Parmenter2 and Chris Somerville2
1Cold
Spring Harbor Laboratory, Cold Spring Harbor, NY; 2 Department of
Plant Biology, Carnegie Institution, Stanford University, Stanford, CA; 3University
of California, San Diego, CA
Upon fertilization, the Arabidopsis zygote elongates more than 2-fold and divides
transversely into a small apical and a large basal cell. The daughter cells of
the zygote subsequently establish two lineages with fundamentally different
developmental fates. Cells of the basal lineage continue to elongate and to
divide transversely producing the mostly extra-embryonic suspensor. Cells of
the apical lineage switch to a more isodiametric growth pattern and produce the
proembryo.
We have identified mutations in three genes that
affect this process in similar ways, and have characterized one of these genes,
designated YODA, in detail. Mutant
zygotes elongate only about 30% before dividing. While development of the
apical cell is initially not affected, cells of the basal lineage are much
smaller than in wild type and their division pattern is irregular. No visible
suspensor is formed, and a molecular marker for suspensor development is absent
in 80% of the mutants. Despite these defects, yoda embryos can occasionally form all pattern elements of
a seedling and develop into dwarfed and sterile plants.
YODA encodes a
MAPKK kinase expressed throughout development in all tissues analyzed. All nine
alleles we recovered are predicted to abolish or reduce kinase activity.
Manipulations in the presumptive regulatory domain of the protein result in
artificial gain-of-function alleles which have opposite phenotypic effects as
loss-of-function alleles. Growth of the suspensor is exaggerated, and growth of
the proembryo suppressed. In the most severe cases, the zygote develops into a
file of cells without recognizable proembryo.
We propose that YODA identifies
a MAP-kinase signaling pathway that modulates a fundamental cell-fate decision
in the early embryo. YODA signaling
promotes a developmental program characterized by elongation, transverse
divisions and, ultimately, extra-embryonic fate. After the asymmetric division
of the zygote, YODA activity is
down-regulated in the apical lineage which becomes free to adopt an embryonic
fate.