such as "Introduction", "Conclusion"..etc
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.
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
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.
Enter the code exactly as it appears. All letters are case insensitive.