1 Zoology and Genetics Department, Iowa State University, Ames, IA 50011,
USA
2 Agronomy Department, Iowa State University, Ames, IA 50011, USA
3 Molecular Cellular and Developmental Biology Program, Iowa State University,
Ames, IA 50011, USA
* Author for correspondence (e-mail: becraft{at}iastate.edu)
Accepted 23 August 2002
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SUMMARY |
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Key words: Maize, Endosperm, Aleurone, Embryo, Epidermis, Cell fate
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INTRODUCTION |
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Despite the apparent simplicity of the mature tissue, endosperm development
is complex and shows several unique innovations in the regulation of cell
cycle, cytokinesis and cytoskeletal functions (reviewed by
Becraft, 2001;
Becraft et al., 2001a
;
Olsen, 2001
;
Olsen et al., 1999
). Endosperm
development is surprisingly plastic, with aleurone cell fate decisions
occurring dynamically throughout the course of kernel development
(Becraft and Asuncion-Crabb,
2000
). Positional cues are required throughout endosperm
development to specify and maintain aleurone cell identity, and starchy
endosperm cells in the peripheral region remain competent to respond to these
cues.
The crinkly4 (cr4) gene is important for the aleurone
cell fate decision because mutations in this gene disrupt aleurone
development. In regions where aleurone fails to differentiate, the peripheral
cell layer of the endosperm is composed of starchy endosperm cells. This cell
fate switch indicates that Cr4+ is important for the
aleurone cell fate decision (Becraft and
Asuncion-Crabb, 2000; Becraft
et al., 1996
; Jin et al.,
2000
). The cr4 gene encodes a receptor-like kinase
(Becraft et al., 1996
)
suggesting that CR4 might function in the perception of the positional cues
that specify aleurone cell identity. As such, understanding the CR4 signal
transduction system will be essential for understanding cell fate acquisition
in endosperm development.
Maize dek1 mutants lack aleurone cells, demonstrating that
Dek1+ is also critical for aleurone development
(Becraft and Asuncion-Crabb,
2000; Lid et al.,
2002
). Late reversion of the transposon-induced dek1-PIA
mutant allele allows peripheral starchy endosperm cells to switch fate to
aleurone, while late loss of Dek1+ gene function causes
aleurone cells to transdifferentiate to starchy endosperm
(Becraft and Asuncion-Crabb,
2000
). Thus, the Dek1+ gene product is
required in endosperm cells for the perception and/or response to the
positional cues that specify aleurone identity. The weak dek1-Dooner
(dek1-D) allele shows a mosaic aleurone, with the pattern of
mosaicism reflecting the germinal-abgerminal polarity that is typical of
mutants, including cr4, that act early in aleurone differentiation
(Fig. 1) (Becraft and Asuncion-Crabb,
2000
; Becraft et al.,
1996
).
|
Dek1 encodes a large protein of 2,159 amino acid residues
(Lid et al., 2002). It is
predicted to contain an N-terminal integral membrane domain with 21
membrane-spanning regions. The cytoplasmic carboxyl terminus is similar to the
calcium-dependent cysteine protease, calpain. The phenotypic similarities
between cr4 and dek1 mutants suggest that DEK1 might act in
the CR4 signal transduction pathway. How these molecules may function within
the context of a pathway is not yet clear. The dek1 mutant phenotype
was suggested to be non cell-autonomous
(Neuffer, 1994
;
Neuffer, 1995
) which is the
expected property for a diffusible signal. The molecular identity of DEK1
makes it unlikely to function as a diffusible ligand for the CR4 receptor
(i.e. the positional cue for aleurone cell fate), however, the possibility
that DEK1 regulates production of the CR4 ligand remained open. Here we
present a genetic analysis of dek1, testing this hypothesis. The
phenotypes of dek1 mutants were examined in detail, cell-autonomy was
tested with genetic mosaics, and double mutants with cr4 were
analyzed. We conclude that DEK1 is a good candidate for a component of the CR4
signal transduction system but that it is not likely to function in the
production of a diffusible ligand.
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MATERIALS AND METHODS |
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Scanning electron microscopy (SEM)
Tissue was fixed overnight in FAA, dehydrated through an ethanol series and
critical point dried. Samples were sputter coated with palladium in a Denton
sputter coater and examined with a Jeol 5800LV SEM operating at 10 kV
accelerating voltage. Images were digitally recorded.
Genetic mosaic analysis
Fig. 3A shows the general
strategy for generating genetic mosaics. The mutant dek1-792 allele
was marked by the linked vp5 mutation, which confers carotenoid
deficiency, causing a white endosperm and albino leaves because of chlorophyll
photobleaching. Chromosome breakage results in the loss of the homologous arm
carrying the wild-type alleles of both loci. This uncovers the mutant alleles
and the cells derived from such an event form an albino, dek1 mutant
sector in an otherwise normal plant. Two methods were used to induce
chromosome breakage. In the presence of an Ac element, the
chromosome-breaking Ds element, present on chromosome 1S in
the Ds1S4 stocks, causes chromosome breakage through an aberrant transposition
(Weil and Wessler, 1993).
Using the Ds-induced breakage, every plant showed sectors. Leaves
from approximately 70 plants were examined for infrequent sectoring. Leaves
from 23 plants were used for the analysis. Alternatively,
-rays were
used to induce chromosome breakage in double heterozygotes. Seeds were
irradiated at the University of Iowa Radiation Laboratory (Iowa City, IA) with
a 137Cs rod source. Bags were laid flat beneath the source so the
seed formed a single layer. The seed received a dose of 20 gray over a period
of 20 minutes. Approximately 800 seeds were irradiated and 14 sectors
obtained.
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Sectors were examined by observing fresh hand-cut sections with an Olympus BX-60 fluorescence microscope equipped with a narrow violet filter (excitation 400-410 nm, dichroic mirror and barrier filter, 455 nm). Wild-type cells can be identified by the red fluorescence of chlorophyll, which is absent in albino mutant cells. The epidermal genotype is inferred from the guard cells, the only epidermal cells to contain chloroplasts. All photomicrography was performed with an Olympus PM-20 photography system using Ektachrome 160T film (Kodak).
Reverse transcription-polymerase chain reaction (RT-PCR)
Endosperm tissues of W22 wild-type and dek1 mutant kernels were
collected 12 to 24 days after pollination (DAP). Samples were pooled for each
genotype and total RNA extracted with the `hot-phenol' method. Briefly, tissue
was ground in liquid nitrogen and suspended at 1 weight:2 volumes in 1:1
phenol:buffer (0.1 M LiCl, 0.01 M EDTA, 1% SDS, 0.1 M Tris pH 9.0) at
80°C. After 30 minutes incubation at room temperature, the mixture was
centrifuged and the RNA precipitated from the supernatant with 2 M LiCl. The
pellet was washed in 2 M LiCl, then ethanol, air dried and dissolved in water.
mRNA was purified from the total RNA using PolyATract mRNA isolation systems
(Promega, Madison, WI, USA). First strand cDNA was synthesized from 2.0 µg
mRNA with AMV reverse transcriptase and Oligo(dT) (T-17) primers at 42°C
for 1.5 hours. PCR was performed on 1 µl of first strand reaction products
using Taq DNA polymerase (Promega), and cr4 gene-specific primers
CR2007 (5'-GGGAATTGAGTACTTGCATGG-3') and CR2792
(5'-AGTCCGTCACCTATGCTGCT-3'). Twenty-six cycles of PCR were
conducted with denaturation at 92°C for 1 minute, annealing at 55°C
for 1 minute and extension at 72°C for 2 minutes. Seven µl of each PCR
reaction was analyzed on 1% agarose gel.
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RESULTS |
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The dek1-D and dek1-928A alleles are leaky, sometimes
causing complete loss of the aleurone layer but often causing only partial
loss of the aleurone layer. Partial loss causes mosaic aleurone development,
with some regions showing a nearly normal aleurone layer and others completely
lacking aleurone cells (Fig.
1B). The pattern of mosaicism is non-random with the region
surrounding the silk scar being most likely to form an aleurone layer and the
abgerminal crown region most likely to lack aleurone
(Becraft and Asuncion-Crabb,
2000). There is also variability in other aspects of the endosperm
phenotype with color ranging from normal yellow to nearly white and texture
ranging from normal vitreous to opaque and floury. The severities of these
different aspects of the endosperm phenotype are correlated.
Dek1 is required for axial pattern formation in
embryogenesis
Mutations in dek1 have dramatic effects on embryogenesis. Mutant
kernels often appear germless as a result of the early arrest of embryo
development. Early embryo development up to the globular (proembryo) stage
appears normal. During transition, when normal embryos begin to elongate,
growth of strong dek1 mutants becomes disorganized. In strong
dek1 mutants (Table
1), only a limited amount of growth continues beyond the globular
stage and mature kernels contain a small necrotic mass. Embryos carrying the
intermediate dek1-792 allele develop into spherical bodies, often
with a root primordium (Fig. 1)
(Lid et al., 2002;
Neuffer et al., 1997
).
Sometimes the root can grow in germinated seeds (not shown). Shoot primordia
do not form and there is no scutellar structure. The mutant phenotype in these
seeds also first appears during the transition stage of embryogenesis. In our
greenhouse grown material, this occurred at approximately 6DAP, which is
considerably earlier than in most published descriptions
(Abbe and Stein, 1954
). When
normal embryos elongate from the globular (proembryo) stage, mutant embryos
undergo symmetrical growth. By 7 DAP, normal embryos had reached coleoptilar
stage (Fig. 1F), while mutant
embryos from the same ear were much smaller and the embryo proper was
spherical in shape (Fig. 1H).
No evidence of cell death was observed in mutant embryos, suggesting that the
defects are caused by disrupted pattern formation rather than death of apical
cells. By 12 DAP, normal embryos had established a shoot apical meristem and
initiated 2-3 leaves (Fig. 1G).
A scutellum was present and the root primordium well organized. Mutant embryos
showed a center of meristematic activity at the site of root primordium
initiation but no indication of a shoot or scutellum
(Fig. 1I). These characters
persisted throughout seed maturation (not shown).
The weak dek1-D allele alters cell fate in the leaf
epidermis
In dek1-D mutants there is a range of effects on embryogenesis,
from the small spherical clusters of cells seen in stronger alleles, to nearly
normal embryos. Kernels with well-formed embryos can produce a viable, albeit
abnormal, plant. dek1-D plants have crinkled leaves, shortened
internodes and nodes that bend alternately back and forth
(Fig. 2A). Inflorescences
commonly show a barren region of rachis midway along their length, with
florets formed at the proximal and distal ends
(Fig. 2B).
|
Normal maize leaves contain a variety of epidermal cell types. Bulliform
cells form distinct files approximately 4 cells wide on the adaxial surface of
adult leaves (Fig. 2) (reviewed
by Becraft, 1999). Bulliform
rows are typically bordered by prickle hairs and contain periodic macrohairs
within the rows. Ground cells of the adult leaf epidermis are rectangular with
a smooth surface and crenulations that interlock adjoining cells along the
lateral walls (Fig. 2I). In
transverse section, they appear cuboidal with a thick outer wall and stain
turquoise with Toluidine Blue (Evans et
al., 1994
). Bulliform cells show a reticulate pattern of ridges on
their surfaces (Fig. 2G). In
section, they appear bulbous and are approximately 2.5 to 4 times as thick as
most epidermal ground cells. Bulliform cells stain deep purple-blue with
Toluidine Blue. Occasionally, angular interlocking walls among neighboring
bulliform cells are observed (Fig.
2C).
In the dek1-D mutants, epidermal cells on both sides of the leaf are intermediate in size and shape between epidermal ground cells and bulliform cells, when viewed in section. The cells stain dark purple-blue with Toluidine Blue, and show the angular junctions between cells similar to bulliform cells. Cells of the dek1-D epidermis have the reticulate ridges on their surfaces, like bulliform cells, when viewed by SEM (Fig. 2H,J). Macrohairs and prickle hairs do not always occur in discrete files (Fig. 2F). Stomata are misshapen and smaller than normal (Fig. 2I,J) but usually occur in files, similar to normal.
The effects of the dek1-D mutant are most pronounced in the epidermis but there are clear effects on internal cells too. Vascular bundles are often flattened or oblong in section (Fig. 2D). Mesophyll cells sometimes show an unusual degree of lobing and often appear elongated radially around the vascular bundles (not shown). Sometimes, multiple cell layers with epidermal cell characteristics are observed (not shown).
There appears to be an environmental influence on the phenotype of dek1-D mutant plants. In winter crops grown in Juana Diaz, Puerto Rico or in the greenhouse, the phenotype was ameliorated and performing pollinations with homozygotes was feasible. When the same lines were grown during the summer in Ames, Iowa, the phenotype was strong and it was rare to find a plant that produced functional anthers. The basis of these effects is unknown.
Genetic mosaic analysis of Dek1 function
Aspects of the phenotype of dek1 mutant sectors in endosperm
tissue appeared non cell-autonomous
(Neuffer, 1994;
Neuffer, 1995
). A genetic
mosaic analysis was conducted to rigorously test the cell autonomy of
Dek1+ function. This also afforded the opportunity to examine the
phenotype of strong dek1 mutant leaf cells. dek1 mutant
sectors were generated in otherwise normal plants, as shown in
Fig. 3 and described in
Materials and Methods. The dek1-792 allele was marked with the linked
viviparous5 (vp5) mutation, which confers albinism due to
carotenoid deficiency. Loss of the chromosome 1S arm carrying the
wild-type Dek1+ and Vp5+ alleles simultaneously uncovers the
recessive mutant dek1-792 and vp5 alleles, resulting in a
clone of albino dek1 mutant cells in an otherwise normal leaf or
endosperm. Endosperm sectors were not considered because the exact cellular
boundaries were equivocal. In leaves, mesophyll sectors were albino and the
cellular boundaries were ascertained by examining hand-sectioned fresh tissue
by fluorescence microscopy. Albino cells lack red-fluorescing chloroplasts
(Fig. 3B-D). It was also
possible to determine sector boundaries by the presence or absence of
chloroplasts in stained sections of fixed and embedded tissue
(Fig. 3F). The epidermal
genotype was determined by examining guard cells, which are the only epidermal
cell type to contain chloroplasts.
Sectored plants contained small protruding ridges in the epidermis
(Neuffer, 1995). These are
formed by files of distended cells (Fig.
3F-I) that are not seen on normal leaves. These cells have some of
the attributes of bulliform cells: in transverse section they are large,
bulbous cells with angular junctions between neighbors and stain deep
purple-blue with Toluidine Blue. They have reticulate ridgework on their
surfaces, however the ridges are less pronounced than on normal bulliform
cells. Fig. 3H shows a direct
comparison of mutant cells (lower part of figure) and normal bulliform cells
in the same leaf. Several sectors produced unusual cup-shaped structures on
epidermal hairs (Fig. 3I).
The mutant phenotype of internal cells was less dramatic than that of the epidermis. The morphology of vascular elements and bundle sheath cells was indistinguishable from normal. Some mesophyll cells also had nearly normal morphology but in other cases the cells were highly lobed (Fig. 3E). The lobing was more extensive than that observed in dek1-D leaves.
In most cases, the mutant phenotype appeared cell-autonomous (Table 2); that is, genetically wild-type cells showed a normal phenotype and neighboring mutant cells showed a mutant phenotype. Fig. 3B shows a sector where only epidermal cells were mutant. The wild-type internal cells could not rescue the mutant epidermis. Fig. 3C,D shows a sector with a large area of mutant adaxial epidermis (upper surface), overlying wild-type internal cells, marked by red fluorescence. Again, the wild-type mesophyll did not rescue the mutant epidermis. It also appeared that wild-type epidermal cells cannot rescue adjacent mutant cells. The enlarged view in Fig. 3D shows the boundary between the wildtype and mutant epidermis on the abaxial side. A wild-type guard cell was directly adjacent to a cell showing the mutant phenotype. Fourteen such examples were observed. Fig. 3G shows a mutant cell surrounded on three sides by cells with normal phenotypes and Fig. 3H shows a normal cell surrounded on 3 sides by abnormal cells. In both cases the phenotypes show sharp cellular boundaries. In Fig. 3E, mutant mesophyll cells directly under a wild-type epidermis show a mutant phenotype indicating that the epidermis could not rescue mesophyll cells. Mutant mesophyll cells showed a mutant phenotype even when in direct contact with wild-type mesophyll or bundle sheath cells (Fig. 3F).
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Several instances showed subtle evidence of non cell-autonomy and in all cases it appeared that mutant cells influenced the phenotype of wild-type neighbors (Fig. 4). Eight examples were found where the surface ridge-work typical of mutant cells affected only part of a cell's surface. Fig. 4A,B shows an example where partially affected cells occur neighboring files of mutant cells. Fig. 4C,D shows an isolated wild-type mesophyll cell surrounded by mutant epidermis and mesophyll cells. The wild-type cell is larger than normal. In Fig. 4E, the wild-type mesophyll cells beneath a mutant epidermal sector show an elongated appearance similar to that described for dek1-D mutant leaves. One leaf formed an ectopic margin in the middle of the lamina (Fig. 5). It occurred in wild-type tissue just proximal (toward the midrib) to the mutant sector.
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Genetic interactions between dek1 and cr4
The phenotypic similarities between dek1 and cr4 mutants
suggested that they might function in the same developmental process or
pathway. To test this, double mutants were produced between dek1 and
cr4 using all the EMS-induced dek1 alleles, dek1-D
and 5 EMS induced cr4 alleles (cr4-1231, -624, -647, -25,
-651,). These cr4 mutants range from mild (cr4-1231) to
strong (cr4-651) (Jin et al.,
2000). All strong alleles of dek1 were completely
epistatic to cr4. The endosperm was white, chalky and lacked an
aleurone layer, and the embryo developed according to the dek1
allele.
The double mutants between dek1-D and cr4 were more
complex. cr4 mutants did not have any consistent effect on the
dek1-D endosperm phenotype. No viable double mutant seedlings were
produced with the stronger cr4 alleles when seeds were germinated in
sand; double mutant plants were only obtained between dek1-D and the
weak cr4-1231 allele. In the segregating family, both
cr4-1231 and dek1-D single mutants showed relatively mild
phenotypes (Fig. 6A-C). Double
mutant plants were highly contorted, similar to strong cr4 mutants,
although leaves were less adherent. Most epidermal cells showed the
bulliform-like surface features of dek1-D
(Fig. 6D) but were enlarged
similar to strong dek1-792 mutant cells in genetic mosaics
(Fig. 6H,I). These cells did
not show the irregular morphology of cr4 mutants. A novel cellular
phenotype of straight sidewalls lacking the interlocking crenulations between
epidermal cells was observed in some areas
(Fig. 6E). The surfaces on
these cells showed less prominent ridges than on dek1-D single
mutants or wild-type bulliform cells, being more similar to the surfaces of
dek1-792 epidermal cells. Scattered groups of epidermal cells showed
a highly irregular morphology that was stronger than cr4-1231 single
mutants (Fig. 6F) and even
unusual for strong cr4 alleles
(Jin et al., 2000). Double
mutants also showed a greater propensity for apparent multiple-cell-layered
epidermis than either single mutant in this line.
|
The epistasis of dek1 to cr4 suggested that Dek1 might function upstream of Cr4. To test whether Dek1+ is required for Cr4 transcription, dek1-792 mutant kernels were assayed for Cr4 transcripts by RT-PCR. Fig. 6J shows that Cr4-specific primers amplify a fragment of the expected 785 base pair size from both wild-type and mutant endosperm, indicating that Cr4 transcript is present in dek1 mutant kernels. Therefore, Dek1+ is not required for Cr4 gene transcription.
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DISCUSSION |
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The weak dek1-D mutants are capable of completing embryogenesis and producing a viable, albeit highly abnormal, plant. Most ground cells of the leaf epidermis possess characteristics of bulliform cells suggesting their fate may be altered. The patterns of other cell types such as trichomes are irregular. Mutant leaf epidermal cells in genetic mosaics of the stronger dek1-792 allele appear highly distended but also show features of bulliform cells. Mutant mesophyll cells develop an unusual lobed morphology with no evidence for cell fate changes. Thus, Dek1 appears to participate in different developmental processes in different contexts.
dek1 functions overlap with cr4
The phenotype of the weak dek1-D allele is similar to cr4
mutants. Both change the fate of peripheral endosperm cells from aleurone to
starchy endosperm (Becraft and
Asuncion-Crabb, 2000; Becraft
et al., 1996
). In both cases, this effect is seen in a mosaic
pattern with the germinal face of the kernel most likely to produce aleurone.
The prevalence of this pattern in several mutant genes suggests that it
reflects pattern-forming information in endosperm development
(Becraft and Asuncion-Crabb,
2000
). dek1-D and cr4 mutant plants also have
similar but distinct phenotypes with shortened internodes and crinkled leaves
(Fig. 1)
(Becraft et al., 1996
;
Jin et al., 2000
). Both affect
the epidermis more strongly than mesophyll (Figs
3,
4)
(Becraft et al., 1996
;
Becraft et al., 2001b
;
Jin et al., 2000
). Epidermal
cells are enlarged and sometimes the epidermis appears more than one cell
thick. cr4 mutant epidermal cells also often show characteristics
including surface ridges (Fig.
6A,B) (Jin et al.,
2000
) that could indicate a partial transformation to bulliform
cell fate.
dek1 was completely epistatic to cr4 in double mutant endosperms, consistent with both genes functioning in the same developmental process. In plants, the double mutant phenotype was more difficult to interpret. Only plants double mutant for dek1-D and the weak cr4-1231 allele were recovered, suggesting that stronger alleles of either gene confer synthetic lethality. At the cellular level, some aspects of the phenotype appeared epistatic, while others appeared additive or synergistic.
Both dek1 and cr4 appear to perform different
developmental functions, depending on cellular context (this study)
(Jin et al., 2000).
Considering the similar, but distinct mutant phenotypes and the results of the
double mutant analysis, it is likely that the Dek1 and Cr4
gene products function in overlapping regulatory systems. How the
membrane-localized calpain-like DEK1 protein
(Lid et al., 2002
) and the CR4
receptor kinase (Becraft et al.,
1996
) molecules might fit into a pathway is not yet known.
Dek1 is not required for Cr4 transcript accumulation. CR4
could regulate DEK1 because in animal systems, calpain activity can be
regulated by phosphorylation of either calpain or its substrate
(Nicolas et al., 2002
;
Shiraha et al., 2002
).
Alternatively, DEK1 could regulate CR4 or a component of CR4 signal
transduction system through proteolytic processing. Proteolytic steps appear
important for other RLK signaling systems. The BRS1 carboxy peptidase seems to
function upstream of the Arabidopsis BRI1 brassinolide receptor
kinase through an unknown mechanism (Li et
al., 2001
).
Dek1 function is primarily cell-autonomous
With the similarity in mutant phenotypes, a report that Dek1
appeared non cell-autonomous (Neuffer,
1994; Neuffer,
1995
) suggested that DEK1 could be a diffusible signal ligand for
the CR4 receptor kinase, or could regulate the production of such a signal.
The recent identification of DEK1 as a large membrane protein decreases the
likelihood that it would act as a ligand, although such mechanisms exist in
animal systems. For example, in Drosophila eye development, Boss is a
7-transmembrane protein that functions non cell-autonomously in the R8 cell to
activate the Sevenless receptor tyrosine kinase in the neighboring R7
precursor cell, triggering differentiation of the R7 photoreceptor
(Cagan et al., 1992
;
Hart et al., 1993
). Presumably
cell walls would prevent such a mechanism in plants. Non autonomous function
could also have occurred if DEK1 regulated the production of a diffusible
signal. The calpain protease domain had potential to provide non autonomy,
because peptide signals are often produced as proproteins that are
proteolytically processed to release the active signal molecules. However,
such processing proteases are generally extracellular and the calpain domain
is predicted to be cytoplasmic (Lid et
al., 2002
).
With the carotenoid-deficient vp5 mutation as a marker for
dek1 mutant cells, we were unable to discern the exact cellular
boundary of endosperm sectors and so could not analyze cell-autonomy in the
endosperm. However, the presence of revertant or mutant sectors as small as a
single cell connotes cell-autonomy (Becraft
and Asuncion-Crabb, 2000). In leaves, the marker was unequivocal
in chlorophyll-containing cells. The vast majority of leaf sectors showed a
mutant phenotype that appeared strictly cell-autonomous in all tissues
(Fig. 3,
Table 2). Some sectors showed
non cell-autonomous effects that were probably due to secondary effects. The
formation of an ectopic margin alongside a mutant sector suggests that
Dek1-regulated cellular functions may be required to propagate
spatial information across the leaf.
Several sectors showed hints of non cell-autonomy that appeared directly related to Dek1 function. Because all known dek1 mutant alleles are recessive loss-of-function mutations, it would be predicted from the hypothesis that Dek1 promoted the production of a diffusible signaling ligand, that wild-type cells should rescue the phenotype of neighboring mutant cells. Contrary to this prediction, it appeared that mutant cells could sometimes impose a mutant phenotype on neighboring wild-type cells (Fig. 4). One model that would account for this would be if DEK1 were a negative regulator of an inhibitory signal that blocked cell differentiation (perhaps by inhibiting CR4). Thus DEK1 would promote cell differentiation by blocking the inhibitor, and loss-of-function dek1 mutants would lack normal differentiation because the inhibitor was unchecked. Why such a signal would usually appear cell-autonomous but occasionally act non-autonomously is not known.
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ACKNOWLEDGMENTS |
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