TEACHING TOOLS IN PLANT BIOLOGY: LECTURE NOTES
Leaf Development 2: Cell Proliferation and Differentiation
The commitment of a group of cells to form a leaf primordium
initiates a sequence of events that ultimately leads to the development
of a mature, functional leaf. These events include the
establishment of the major axes of the leaf, cell division and
expansion programs in the leaf blade, and patterning and
differentiation of cells including those that make up the vascular
tissues and specialized epidermal cells, such as the trichomes
and guard cells. Throughout the development of the leaf, the
temporal and spatial control of cell proliferation and cell differentiation
is tightly controlled through positional information involving
auxin and microRNAs (miRNAs) and expression of
regulatory transcription factors. Most of our understanding of
the molecular underpinnings of leaf development is based on
studies in Arabidopsis thaliana, rice (Oryza sativa), and maize
(Zea mays) and a few other model organisms, such as Antirrhinum
majus, tomato (Solanum lycopersicum), and pea (Pisum
sativum). Comparative genomics analyses have allowed the
models developed based on these angiosperms to be investigated
in other plants as well, to uncover the evolutionary origins
of the developmental pathways.
LEAF EVOLUTION AND DIVERSITY
Leaves evolved from modiﬁed branches. Branches and shoots
are indeterminate; they are capable of ongoing growth. By
contrast, (most) leaves are determinate; they grow to a genetically
predetermined size and shape. Leaf development requires
a suppression of indeterminate growth, but in some cases this
suppression is incomplete. Leaves have evolved multiple times
independently. Nonseed vascular plants, such as Lycopodium
and Selaginella, make small leaves with a single vascular strand;
these leaves are sometimes called microphylls. The larger leaves
in ferns and seed plants evolved independently and are often
called megaphylls.
WHAT DETERMINES LEAF SIZE AND SHAPE?
Leaf primordia do not differ greatly in shape and size, but their
mature forms vary tremendously. Leaf size and shape is largely
a consequence of differential patterns of cell division and cell
expansion. In long, thin monocot leaves, cell division and
expansion primarily occur with a unidirectional orientation. By
contrast, the more rounded and complex dicot leaf shapes are
formed by more complex patterns of cell proliferation. In most
leaves, cell division stops earlier at the leaf tip than the base, in
a wave of cell cycle arrest. Once cells complete their division
cycles, they begin to expand and differentiate into their ﬁnal
forms.
Several mutations that affect cell cycle persistence have been
identiﬁed. The AINTEGUMENTA (ANT) gene, which encodes an
AP2-type transcription factor, is positively correlated with cell
proliferation. The loss-of-function ant mutant has smaller but
morphologically normal leaves with fewer cells. When overexpressed,
ANT causes enlarged but morphologically normallooking
leaves. ANT is thought to maintain meristematic
competence of cells, in part through its regulation of the cyclin
CYCD3 gene, which is a key driver of the cell cycle.
Other mutations have been identiﬁed that affect cell division
unequally across the leaf blade. For example, mutation of the
CINCINNATA (CIN) gene in Antirrhinum causes prolonged cell
cycling in the margins but not the center of the leaf. Loss-offunction
cin leaves have too much margin, or a negative curvature,
and so buckle and bulge. CIN encodes a TCP-type
transcription factor, named for TEOSINTE BRANCHED1 from
maize, CYCLOIDEA from Antirrhinum, and PROLIFERATING
CELL FACTOR from rice. Each of these proteins has an effect
on cell cycle persistence. Expression of the CIN-like genes is
regulated by miRNA expression; the miRNA miJAW is a negative
regulator of CIN-TCP genes, and overaccumulation of miJAW in
Arabidopsis (jaw-D) causes crinkly leaves, similar to the cin lossof-function
phenotype. These mutants demonstrate that spatial
cell cycle control across the leaf contributes to ﬁnal leaf shape.
Differential persistence of cell divisions similarly controls leaf
margin morphology. Leaf margins can be smooth, serrated, or
lobed. In serrated or lobed leaves, the protruding regions are
separated by sinuses. In smooth leaves, cell divisions persist in
the sinuses, causing a smooth, uniform enlargement of the leaf
margin. By contrast, in lobed or serrated leaves, the cells in the
sinus region arrest earlier than those in the adjoining regions.
Polar auxin transport contributes to leaf margin morphology. The
auxin efﬂux carrier PIN1 orients so that an auxin maximum is
produced at the region of outgrowth, and interfering with this
differential accumulation of auxin produces smooth-margined
leaves. Mutations that convert lobed leaves to smooth and vice
versa have revealed a role for boundary genes, such as CUPSHAPED
COTYLEDONS2 (CUC2), which encodes plant-speciﬁc
transcription factor. CUC2 is part of the large NAM/CUC3
multigene family, of which several members affect developmental
patterns. The expression patterns of CUC2 and related genes
are modiﬁed by the action of regulatory miRNAs.
WHAT DETERMINES LEAF COMPLEXITY?
Angiosperm leaves are derived from a simple ancestral leaf.
Compound leaves have evolved many times within the angiosperms
and may contribute advantages to light harvesting or
airﬂow under certain conditions. Genetic studies in compoundleaf
producing plants, such as pea, tomato, and a compoundleaved
close relative of Arabidopsis, Cardamine hirsuta, indicate
that the same signaling pathways that operate in the development
of simple leaves underlie the development of these more
complex leaves.
The Plant Cell, December 2009, www.plantcell.org ª 2009 American Society of Plant Biologists
Like the outgrowth of leaf lobes or serrations, leaﬂet initiation is
preceded by a local auxin maximum, and interfering with its
formation produces simple leaves. Thus, auxin plays a conserved
role in leaf development, from the speciﬁcation of the site of
primordium outgrowth in the shoot apical meristem to the
speciﬁcation of outgrowths of leaﬂets and lobes.
Similarly, class I KNOX (KNOTTED-like homeobox) genes have
a recurring role in leaf and leaﬂet initiation in most taxa. These
KNOX-1 genes encode transcription factors, many of which
promote indeterminacy. They are expressed in the shoot apical
meristem, which is an indeterminate structure. In most but not all
plants, KNOX-1 genes are inactivated in the cells destined to
form a leaf primordium. In plants with compound leaves, KNOX-1
gene expression resumes in the developing leaves. (Interestingly,
however, some plants whose leaves express KNOX-1
nevertheless produce simple leaves because secondary morphogenesis
causes the leaf blade to grow out). Usually, KNOX-1
expression is strongly correlated with leaf complexity; Arabidopsis
plants that overexpress KNOX-1 genes can produce lobed or
compound-like leaves, due to increased proliferation of cells at
the leaf margins, whereas KNOTTED expression in tomato
stimulates ultracompound leaves, and loss of KNOX-1 function
in plants with compound leaves can cause them to produce
simple leaves.
Pea plants have compound leaves and a rich genetic history.
Many pea mutants with abnormal patterns of leaf development
have been identiﬁed. In pea, the unifoliata mutant produces
simple leaves. UNIFOLIATA encodes a transcription factor, but
not a KNOX-1 type. Instead, in pea and other closely related
legumes, a protein of the FLO/LFY family speciﬁes compound
leaf development. This ﬁnding is consistent with the numerous
evolutionary origins of compound leaves. Interestingly, the subsequent
events in pea leaf development are the same as those of
other compound-leaved plants.
Additional studies suggest that KNOX-1 transcription factors
require a window of morphgenetic potential in which to act. In
tomato, the LANCEOLATE (LA) gene controls this potential.
Overexpression of the LA gene converts the normally compound
tomato leaf into a smooth, simple one, by suppressing outgrowth
of the leaﬂet progenitor cells, even in plants that are overexpressing
KNOTTED.
Boundary genes, such as CUC2 and CUC3, are required to
suppress cell divisions at the boundary between an initiating leaf
primordium and the shoot apical meristem. CUC2 expression is
also required for proper sinus formation in serrated or lobed
leaves. Similarly, boundary genes are required for the formation
of leaﬂets in compound leaves; suppression of their expression
prevents the leaves from achieving their proper dissected form.
Thus, boundary genes have a recurring role in leaf development.
Taken together, these studies reveal that leaﬂet initiation
parallels leaf initiation, with KNOX-1 or UNI expression promoting
indeterminacy, a local auxin maximum specifying the
position of leaf or leaﬂet outgrowth, and boundary genes
restricting cell proliferation as appropriate.
These genes and pathways have been identiﬁed in angiosperms,
whose leaves are derived from a single evolutionary
event. Improved techniques for examining gene function and
the completion of several genome sequencing projects in
nonangiosperms are allowing scientists to investigate developmental
pathways in these plants. Ongoing studies are revealing
that some of the same genes are involved in leaf
development, although in some cases their precise function is
not identical. These kinds of comparative studies help us to
understand the genetic resources available to the plant common
ancestor and how these resources were modiﬁed over
time to produce diverse plant forms.
WHAT CONTROLS CELL DIFFERENTIATION?
Patterns of cell division underlie the development of leaf morphology,
but the development of a functional leaf also depends
on the differentiation of proper cell types. Leaves consist of
mesophyll cells, which carry out the photosynthetic reactions,
vascular tissues, and epidermal tissues, including specialized
epidermal hairs (trichomes) and guard cells. Genetic studies
primarily in Arabidopsis have uncovered some of the developmental
pathways that contribute the proper cell differentiation
during leaf development. These include the activation of transcription
factors and cell-speciﬁc genes and positional information
conferred by auxin or direct cell-to-cell communication.
VASCULAR TISSUES
Vascular tissues are the conduits through which water and
photosynthate move through the plant body. Leaves are extensively
vascularized, so that the photosynthetic activities of the
mesophyll cells are well supported with water from the xylem,
and the photosynthate is readily moved into the phloem. Vascular
tissue differentiation during leaf expansion follows a consistent
developmental program, with cells of the preprocambium
differentiating into procambium cells, which then differentiate
into the cells of the xylem and phloem.
The plant hormone auxin plays an important role in specifying
the pattern of vascular tissue formation, as it does in specifying
the placement of leaf primordia. During leaf initiation, a local
auxin maximum, formed by the polar placement of the PIN1
auxin efﬂux carrier, precedes primordium formation. Once the
primordium is initiated, PIN1 redistributes, directing auxin ﬂow
basipetally. The locally elevated auxin ﬂow in the center of the
leaf primordium causes these cells to differentiate into preprocambium
cells and then into procambium cells. These processes
can be monitored by expression of cell-speciﬁc genes. Preprocambium
cells look like the cells that surround them (roughly
cube-shaped) but initiate expression of the HD-ZIPIII transcription
factor–encoding gene Athb8. These cells subsequently
elongate and divide to form long narrow cells and begin to
express a procambium-speciﬁc marker ET1335. From the procambium,
xylem and phloem cells are produced, with the xylem
forming on the adaxial side of the procambium and the phloem
on the abaxial.
Following the speciﬁcation of the midvein, the direction of
auxin transport in the primordium epidermal layer changes,
causing auxin maxima to form midway along the margins of the
primordium. From these new auxin maxima, the ﬂow of auxin
again shifts to an inward direction, ultimately giving rise to the
secondary veins.
2 The Plant Cell
Monocot leaves have veins that are largely parallel, running
along the long axis of the leaf. Cell divisions occur primarily in
a zone at the base of the leaf, and cells enlarge and differentiate
as they mature, resulting in a gradient of leaf and vein differentiation
from tip to base. Mathematical and computer models
have shown that parallel veins can be generated by a simple
polar auxin transport mechanism. Interfering with auxin transport
interferes with vein patterning, suggesting that vascular patterning
in monocots is probably speciﬁed by auxin transport patterns
as it is in dicots.
TRICHOMES
Trichomes are shoot epidermal hairs. On leaves, trichomes can
reduce transpirational water loss or form a reﬂective surface to
reduce heat or UV light absorption. Trichomes on seeds can aid
in their dispersal; cotton seed trichomes are the most important
natural ﬁber for textile production. Glandular trichomes produce
diverse compounds that contribute to defense against herbivory
or pathogens. Many products of glandular trichomes are useful
chemicals for humans too, including fragrances and ﬂavors (e.g.,
lavender and mint), medicinal compounds (e.g., artemisinin, an
antimalarial), and natural insecticides. Modifying gene expression
levels in trichomes can increase production of these
compounds, facilitating their puriﬁcation or making the plants
themselves more resistant to insects or pathogens.
In Arabidopsis, trichomes are dispensable, facilitating genetic
screens for trichome abnormalities. The mechanisms by which
trichome spacing are controlled in Arabidopsis were identiﬁed
through mutational analysis and the identiﬁcation of plants with
too few, too many, or abnormally clumped trichomes. In cells
destined to produce trichomes, transcriptional complexes form
and induce expression of GL2, a homeodomain transcription
factor that causes cells to differentiate into trichomes. Formation
of these transcriptional complexes in adjoining, nontrichome
producing cells is inhibited by one or more trichome inhibitors
that are produced in the trichome cell and move intercellularly to
repress trichome formation in adjoining cells. In Arabidopsis,
some trichome spacing mutations also affect spacing of epidermal
root hairs, which use a similar patterning mechanism.
However, it has become clear that most plants use other genetic
pathways for trichome production and that the mechanism used
by Arabidopsis is a recent innovation, derived from an anthocyanin
biosynthetic pathway. Even within a single plant, different
types of trichomes are produced via distinct pathways, indicating
that trichomes are a diverse set of analogous but not
homologous differentiated cell types.
STOMATA
A stomatal pore is a hole in the leaf epidermis that is covered by
a pair of guard cells that change size to open or close the pore.
When the Arabidopsis leaf is ;200 mm in length, stomata begin
to appear at the distal end, and stomatal maturation continues in
a wave of differentiation from tip to base. Stomatal development
is regulated by environmental parameters, cell intrinsic information,
and signals emanating from nearby cells, including other
guard cells or guard cell precursors and cells in the mesophyll.
Mature guard cells are formed through a regular pattern of cell
divisions that differ in monocots and dicots. In monocots such as
Z. mays, guard cells form in cell ﬁles in between veins. The ﬁrst
step is an asymmetric division that produces a guard mother cell
that ultimately divides again to produce the two guard cells.
However, the stomatal complex also includes a pair of subsidiary
cells that are formed from cells adjacent to the guard mother cell.
How these cells are induced to divide and differentiate is not fully
known, but a recently identiﬁed mutant that interferes with this
process, pan1, encodes a putative receptor that could be involved
in perceiving a signal from the guard mother cell.
In dicots such as Arabidopsis, the ﬁrst stage of guard cell
development is also an asymmetric division occurring in a protodermal
cell. The two daughter cells of the asymmetric division
have different cell states; the larger retains its protodermal
identity, while the smaller cell becomes a meristemoid mother
cell (MMC). The MMC divides asymmetrically one or more times
to produce a meristemoid. The meristemoid differentiates into
a guard mother cell (GMC), which divides symmetrically to
produce a pair of guard cells precursors, which differentiate
into guard cells.
Mutational analysis in Arabidopsis has identiﬁed several basic
helix-loop-helix transcription factors that are key regulators of
each step in guard cell differentiation. SPEECHLESS (SPCH) is
required for asymmetric division of the protodermal cell into the
meristemoid mother cell; as the name implies, mutant plants that
do not produce SPCH do not produce stomata (they don’t make
mouths, so they are speechless). The MUTE gene is required for
the next step, which is the differentiation of the MMC into a GMC;
loss of function of MUTE leads to the production of stomatal
precursors but no stomata. The GMC divides once more and
then differentiates into a pair of guard cells; these are separable
processes, as shown by the phenotype of the fama mutant, in
which the GMC divides repeatedly but the daughter cells do not
differentiate, producing a row of parallel cells.
In dicots such as Arabidopsis, stomata normally form with one
or more cells between them. This patterning is based on a system
of cell signaling that involves a repression of guard cell differentiation.
The current model suggests that one or more peptide
signals are secreted by stomatal precursors, which are perceived
by adjoining cells and serve to repress their differentiation
via a mitogen-activated protein kinase signaling pathway. Guard
cell density is also affected by environmental conditions, with
lower stomatal densities in conditions of low humidity or high
concentrations of CO2. Environmental signals are perceived by
mature leaves, which transduce information to the developing
leaves via an unknown mechanism. The hic mutant affects
environmental stomatal density responses. HIC is thought to
contribute to the synthesis of the epicuticular wax that coats
leaves and prevents gas and water exchange, but it’s not yet
known precisely how this gene contributes to stomatal patterning
control.
CONCLUSIONS
The outgrowth of a leaf primordium from a tiny bulge of
homogenous cells into a large, complex, and physiologically
functional organ involves the sequential and coordinated
December 2009 3
activities of numerous regulatory genes. Transcriptional regulators
that control cell determinacy, division, and differentiation
across the leaf blade are themselves positionally regulated by
both auxin and small RNAs. Within the expanding leaf, vascular
tissues form, while in the epidermis, trichomes and guard cells
are formed through regular programs of differentiation and
division; analysis of the patterning of these cells reveals a role
for cell-to-cell communication as well as environmental factors.
The sophisticated developmental programs that have evolved
that enable plants to produce leaves are matched only by the
amazing photosynthetic processes that occur within them.
Mary E. Williams
Features Editor
mwilliams@aspb.org
SUGGESTED READING
Reviews
Aida, M., and Tasaka, M. (2006). Genetic control of shoot organ
boundaries. Curr. Opin. Plant Biol. 9: 72–77.
Anastasiou, E., and Lenhard, M. (2007). Growing up to one’s standard.
Curr. Opin. Plant Biol. 10: 63–69.
Barkoulas, M., Galinha, C., Grigg, S.P., and Tsiantis, M. (2007). From
genes to shape: Regulatory interactions in leaf development. Curr.
Opin. Plant Biol. 10: 660–666.
Barton, M.K. (2007). Making holes in leaves: Promoting cell state
transitions in stomatal development. Plant Cell 19: 1140–1143.
Beerling, D.J., and Fleming, A.J. (2007). Zimmermann’s telome theory
of megaphyll leaf evolution: A molecular and cellular critique. Curr.
Opin. Plant Biol. 10: 4–12.
Bergmann, D.C., and Sack, F.D. (2007). Stomatal development. Annu.
Rev. Plant Biol. 58: 163–181.
Champagne, C., and Sinha, N. (2004). Compound leaves: Equal to the
sum of their parts? Development 131: 4401–4412.
Dengler, N., and Kang, J. (2001). Vascular patterning and leaf shape.
Curr. Opin. Plant Biol. 4: 50–56.
De Smet, I., and Ju¨ rgens, G. (2007). Patterning the axis in plants –
Auxin in control. Curr. Opin. Genet. Dev. 17: 337–343.
Gray, J.E., Casson, S., and Hunt, L. (2008). Intercellular peptide signals
regulate plant meristematic cell fate decisions. Sci. Signal. 1: pe53.
Ishida, T., Kurata, T., Okada, K., and Wada, T. (2008). A genetic
regulatory network in the development of trichomes and root hairs.
Annu. Rev. Plant Biol. 59: 365–386.
Nadeau, J.A. (2009). Stomatal development: New signals and fate
determinants. Curr. Opin. Plant Biol. 12: 29–35.
Nadeau, J.A., and Sack, F.D. (2002). Stomatal development in Arabidopsis.
In The Arabidopsis Book, C.R. Somerville and E.M. Meyerowitz,
eds (Rockville, MD: American Society of Plant Biologists), doi/10.1199/
tab.0066, http://www.aspb.org/publications/arabidopsis/.
Niklas, K.J., and Kutschera, U. (2009). The evolutionary development
of plant body plans. Funct. Plant Biol. 36: 682–695.
Pesch, M., and Hu¨ lskamp, M. (2004). Creating a two-dimensional
pattern de novo during Arabidopsis trichome and root hair initiation.
Curr. Opin. Genet. Dev. 14: 422–427.
Rolland-Lagan, A.-G. (2008). Vein patterning in growing leaves: Axes
and polarities. Curr. Opin. Genet. Dev. 18: 348–353.
Schilmiller, A.L., Last, R.L., and Pichersky, E. (2008). Harnessing plant
trichome biochemistry for the production of useful compounds. Plant
J. 54: 702–711.
Serna, L., and Martin, C. (2006). Trichomes: Different regulatory
networks lead to convergent structures. Trends Plant Sci. 11: 274–280.
Tomescu, A.M.F. (2009). Megaphylls, microphylls and the evolution of
leaf development. Trends Plant Sci. 14: 5–12.
Tsukaya, H. (2006). Mechanism of leaf-shape determination. Annu. Rev.
Plant Biol. 57: 477–498.
Tsukaya, H. (2008). Controlling size in multicellular organs: Focus on the
leaf. PLoS Biol. 6: e174.
Piazza, P., Jasinski, S., and Tsiantis, M. (2005). Evolution of leaf
developmental mechanisms. New Phytol. 167: 693–710.
Turner, S., and Sieburth, L. E. (2003). Vascular patterning. In The
Arabidopsis Book, C.R. Somerville and E.M. Meyerowitz, eds (Rockville,
MD: American Society of Plant Biologists), doi/10.1199/tab.0073, http://
www.aspb.org/publications/arabidopsis/.
Wagner, G.J., Wang, E., and Shepherd, R.W. (2004). New approaches
for studying and exploiting an old protuberance, the plant trichome.
Ann. Bot. (Lond.) 93: 3–11.
Seminal Articles
Barkoulas, M., Hay, A., Kougioumoutzi, E., and Tsiantis, M. (2008). A
developmental framework for dissected leaf formation in the Arabidopsis
relative Cardamine hirsuta. Nat. Genet. 40: 1136–1141.
Bayer, E.M., Smith, R.S., Mandel, T., Nakayama, N., Sauer, M.,
Prusinkiewicz, P., and Kuhlemeier, C. (2009). Integration of
transport-based models for phyllotaxis and midvein formation. Genes
Dev. 23: 373–384.
Berger, Y., Harpaz-Saad, S., Brand, A., Melnik, H., Sirding, N.,
Alvarez, J.P., Zinder, M., Samach, A., Eshed, Y., and Ori, N. (2009).
The NAC-domain transcription factor GOBLET speciﬁes leaﬂet
boundaries in compound tomato leaves. Development 136: 823–832.
Bergmann, D.C., Lukowitz, W., and Somerville, C.R. (2004). Stomatal
development and pattern controlled by a MAPKK kinase. Science
304: 1494–1497.
Bharathan, G., Goliber, T.E., Moore, C., Kessler, S., Pham, T., and
Sinha, N.R. (2002). Homologies in leaf form inferred from KNOX1
gene expression during development. Science 296: 1858–1860.
Blein, T., Pulido, A., Vialette-Guiraud, A., Nikovics, K., Morin, H.,
Hay, A., Johansen, I.E., Tsiantis, M., and Laufs, P. (2008). A
conserved molecular framework for compound leaf development.
Science 322: 1835–1839.
Cartwright, H.N., Humphries, J.A., and Smith, L.G. (2009). PAN1: A
receptor-like protein that promotes polarization of an asymmetric cell
division in maize. Science 323: 649–651.
Champagne, C.E.M., Goliber, T.E., Wojciechowski, M.F., Mei, R.W.,
Townsley, B.T., Wang, K., Paz, M.M., Geeta, R., and Sinha, N.R.
(2007). Compound leaf development and evolution in the legumes.
Plant Cell 19: 3369–3378.
Crawford, B.C.W., Nath, U., Carpenter, R., and Coen, E.S. (2004).
CINCINNATA controls both cell differentiation and growth in petal
lobes and leaves of Antirrhinum. Plant Physiol. 135: 244–253.
Donnelly, P.M., Bonetta, D., Tsukaya, H., Dengler, R.E., and Dengler,
N.G. (1999). Cell cycling and cell enlargement in developing leaves of
Arabidopsis. Dev. Biol. 215: 407–419.
Efroni, I., Blum, E., Goldshmidt, A., and Eshed, Y. (2008). A protracted
and dynamic maturation schedule underlies Arabidopsis leaf development.
Plant Cell 20: 2293–2306.
Floyd, S.K., and Bowman, J.L. (2006). Distinct developmental mechanisms
reﬂect the independent origins of leaves in vascular plants.
Curr. Biol. 16: 1911–1917.
4 The Plant Cell
Fujita, H., and Mochizuki, A. (2006). The origin of the diversity of leaf
venation patterns. Dev. Dyn. 235: 2710–2721.
Gray, J.E., Holroyd, G.H., van der Lee, F.M., Bahrami, A.R., Sijmons,
P.C., Woodwardk, F.I., Schuch, W., and Hetherinton, A.M. (2000).
The HIC signalling pathway links CO2 perception to stomatal development.
Nature 408: 713–716.
Hay, A., Barkoulas, M., and Tsiantis, M. (2006). ASYMMETRIC
LEAVES1 and auxin activities converge to repress BREVIPEDICELLUS
expression and promote leaf development in Arabidopsis. Development
133: 3955–3961.
Hay, A., and Tsiantis, M. (2006). The genetic basis for differences in
leaf form between Arabidopsis thaliana and its wild relative Cardamine
hirsuta. Nat. Genet. 38: 942–947.
Hara, K., Kajita, R., Torii, K.U., Bergmann, D.C., and Kakimoto, T.
(2007). The secretory peptide gene EPF1 enforces the stomatal onecell
spacing rule. Genes Dev. 21: 1720–1725.
Hareven, D., Gutﬁnger, T., Parnis, A., Eshed, Y., and Lifschitz, E.
(1996). The making of a compound leaf: genetic manipulation of leaf
architecture in tomato. Cell 84: 735–744.
Harrison, C.J., Corley, S.B., Moylan, E.C., Alexander, D.L., Scotland,
R.W., and Langdale, J.A. (2005). Independent recruitment of a conserved
developmental mechanism during leaf evolution. Nature 434:
509–514.
Hofer, J., Turner, L., Hellens, R., Ambrose, M., and Matthews, P.
(1997). UNIFOLIATA regulates leaf and ﬂower morphogenesis in pea.
Curr. Biol. 7: 581–587.
Hunt, L., and Gray, J.E. (2009). The signaling peptide EPF2 controls
asymmetric cell divisions during stomatal development. Curr. Biol. 19:
864–869.
Hu¨ lskamp, M., Misera, S., and Ju¨ rgens, G. (1994). Genetic dissection
of trichome cell development in Arabidopsis. Cell 76: 555–566.
Koenig, D., Bayer, E., Kang, J., Kuhlemeier, C., and Sinha, N. (2009).
Auxin patterns Solanum lycopersicum leaf morphogenesis. Development
136: 2997–3006.
Lampard, G.R., MacAlister, C., and Bergmann, D.C. (2008). Arabidopsis
stomatal initiation is controlled by MAPK-mediated regulation
of the bHLH SPEECHLESS. Science 322: 1113–1116.
MacAlister, C.A., Ohashi-Ito, K., and Bergmann, D.C. (2007). Transcription
factor control of asymmetric cell divisions that establish the
stomatal lineage. Nature 445: 537–540.
Nadeau, J.A., and Sack, F.D. (2002). Control of stomatal distribution on
the Arabidopsis leaf surface. Science 296: 1697–1700.
Nath, U., Crawford, B.C.W., Carpenter, R., and Coen, E. (2003).
Genetic control of surface curvature. Science 299: 1404–1407.
Nikovics, K., Blein, T., Peaucelle, A., Isida, T., Morin, H., Aida, M.,
and Laufs, P. (2006). The balance between the MIR164A and CUC2
genes controls leaf margin serration in Arabidopsis. Plant Cell 18:
2929–2945.
Mizukami, Y., and Fischer, R.L. (2000). Plant organ size control:
AINTEGUMENTA regulates growth and cell numbers during organogenesis.
Proc. Natl. Acad. Sci. USA 97: 942–947.
Ori, N., et al. (2007). Regulation of LANCEOLATE by miR319 is required
for compound-leaf development in tomato. Nat. Genet. 39: 787–791.
Palatnik, J.F., Allen, E., Wu, X., Schommer, C., Schwab, R.,
Carrington, J.C., and Weigel, D. (2003). Control of leaf morphogenesis
by microRNAs. Nature 425: 257–263.
Pillitteri, L.J., Sloan, B.D., Bogenschutz, N.L., and Torii, K.U. (2007).
Termination of asymmetric cell division and differentiation of stomata.
Nature 445: 501–505.
Poethig, R.S., and Sussex, I.M. (1985). The developmental morphology
and growth dynamics of tobacco leaves. Planta 165: 158–169.
Poethig, R.S., and Sussex, I.M. (1985). The cellular parameters of leaf
development in tobacco: A clonal analysis. Planta 165: 170–184.
Scarpella, E., Francis, P., and Berleth, T. (2004). Stage-speciﬁc
markers deﬁne early steps of procambium development in Arabidopsis
leaves and correlate termination of vein formation with mesophyll
differentiation. Development 131: 3445–3455.
Scarpella, E., Marcos, D., Friml, J., and Berleth, T. (2006). Control of
leaf vascular patterning by polar auxin transport. Genes Dev. 20:
1015–1027.
Schnittger, A., Folkers, U., Schwab, B., Ju¨ rgens, G., and Hu¨ lskamp,
M. (1999). Generation of a spacing pattern: the role of TRIPTYCHON
in trichome patterning in Arabidopsis. Plant Cell 11: 1105–1116.
Shpak, E.D., McAbee, J.M., Pillitteri, L.J., and Torii, K.U. (2005).
Stomatal patterning and differentiation by synergistic interactions of
receptor kinases. Science 309: 290–293.
Wang, E., Wang, R., DeParasis, J., Loughrin, J.H., Gan, S., and
Wagner, G.J. (2001). Suppression of a P450 hydroxylase gene in
plant trichome glands enhances natural-product-based aphid resistance.
Nat. Biotechnol. 19: 371–374.
Wenzel, C.L., Schuetz, M., Yu, Q., and Mattsson, J. (2007). Dynamics
of MONOPTEROS and PIN-FORMED1 expression during leaf vein
pattern formation in Arabidopsis thaliana. Plant J. 49: 387–398.
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