Ever looked carefully at the leaves on a plant and noticed their
various sizes and shapes? Why are they different? What controls the
size and shape of each individual leaf? Very little is known about
the developmental control of leaf size and shape, and understanding
the mechanisms behind this is a major issue in plant biology.
A leaf's size is determined by a combination of cell number,
cell size, and intercellular space. Michael Marcotrigiano from
Smith College, Massachusetts, wanted to find out what role cell
layers played in regulating leaf size and shape. He utilized a
powerful tool - the synthesis of graft chimeras - that has allowed
him to carefully analyze the developmental regulation of leaf size
and shape in Nicotiana and has published his findings in the
February issue of the American Journal of Botany (http://www.amjbot.org/cgi/content/full/97/2/224).
By grafting plants of different Nicotiana genotypes
Marcotrigiano was able to recover shoots from the graft union that
were chimeras. These shoots were composed of both genotypes.
Eventually he recovered leaves with two genetically distinct cell
layers. He grafted N. tabacum, a large-leaf genotype, and N.
glaucum, a small-leaf genotype, to produce leaves where the
resulting epidermal cell layer was a different genotype than the
mesophyll cell layer - but on only one side of the leaf, allowing
for direct comparison of the growth of the leaf from one side to
the other. Thus, one side of the leaf could act as a "control" for
the other side of the leaf. This enabled him to set up some nicely
designed comparisons where on one side of the leaf the outer cell
layer (the epidermis) differed in genotype from the rest of the
leaf.
"Since leaves generally vary in size along the length of the
stem and leaf size is strongly influenced by environmental factors,
my method allowed me to compare one side of a leaf to the other,
negating the complications that arise when comparing different
leaves on a single plant or leaves on different plants,"
Marcotrigiano said.
Creating these graft chimeras was time-consuming and involved an
element of chance; often the growing tip of the chimeral shoots
reverted back to a non-chimeral shoot rendering the leaves
generated from that point on useless for analysis. However, over
the past decade enough leaves were recovered that were perfectly
bisected, homogeneous on one side of the midvein and with a unique
epidermis on the other. This allowed Marcotrigiano to use them to
examine how leaf cell layer affects leaf size and shape.
Marcotrigiano's most striking finding was the important role
that the epidermal cells played in determining leaf size. He found
that leaves grew asymmetrically when one side of the midvein
contained identical cell layer arrangements and the other side
contained epidermal cells that differed genetically from the
mesophyll cells. When big-leaf epidermal cells surrounded
small-leaf mesophyll cells in an otherwise all small-genotype leaf,
the big-leaf epidermal cells caused that side of the leaf to be
bigger than the other side. In contrast, when small-leaf epidermal
cells surrounded big-leaf mesophyll cells in an otherwise all
big-genotype leaf, the small-leaf epidermal cells caused that side
to be smaller than the other side.
Epidermal cells not only controlled overall leaf size, but also
influenced the number of cells produced in the mesophyll layer. For
example, small-leaf epidermal cells surrounding big-leaf mesophyll
cells caused the mesophyll cells to have many fewer cell divisions
than when they were surrounded by big-leaf epidermal cells.
Interestingly, the epidermal cells did not influence, or change,
the size of the mesophyll cells.
Marcotrigiano concludes that while regulation of leaf size is
complex and influenced by many factors and many genes, his findings
show that communication between adjacent cell layers plays an
important role in determining leaf size. Cells in one tissue layer
can control the rate of division of cells in another tissue layer,
which in turn influences overall leaf size.
SOURCE
