In our environment, we are surrounded by plants,
our most abundant renewable resource.
My name is Debra Mohnen,
and in this presentation,
Melani Atmodjo and I
from the Complex Carbohydrate Research Center
at the University of Georgia
will summarize our current understanding
of plant cell wall structure and synthesis.
Each year approximately 100 billion tons
of carbon dioxide are fixed into biomass
with roughly one-third of that
produced by marine plants and microorganisms
and two-thirds from land plants.
plants use carbon dioxide and water
taken up from the environment
and energy from the sun to produce carbohydrates.
The bulk of that fixed carbon ends up in the plant cell walls.
Plant cell walls are the carbohydrate-rich
extracellular matrix that surrounds all plant cells.
Plant cell walls and plant cells vary in shape and structure
depending upon the cell type,
as can be seen in these micrographs.
Note the difference in shape and surface structure
of the epidermis of Arabidopsis sepals,
the leaf-like structure that supports petals,
on the upper left versus the epidermis
of petals on the upper right.
Furthermore, note the intricate branch structure
on the bottom left of single cell trichomes
versus the elongated and pitted structure
of pollen cells on the bottom right.
Finally, note the round structure
of suspension-cultured cells grown in liquid culture
on the bottom middle
versus the rigid and thicker wall structure
of the large xylem cells,
which make up water transporting vascular cells
and cells in wood.
Plant cell walls have diverse and critical functions in the plant
including providing structure to the plant and plant cells,
being involved in plant growth,
giving the plant flexibility,
as you can see when plants sway in the wind.
They provide hydration, as you may see when a seed germinates.
They are a reservoir of defense and signaling molecules,
and they provide cell adhesion between adjacent cells
and are involved in development.
If you look at the picture,
what you will see is the wild-type Arabidopsis plant on the left
compared to a mutant plant
that is mutant in one of the plant cell wall biosynthetic enzymes,
clearly showing that a knockdown in expression
of a wall biosynthetic enzyme
can lead to dwarfism.
Plant cell walls also have many uses for humans and animals.
They provide clothing, wood, and lumber products.
They are used for biomaterials
including nanocomposites, nanocellulose fibers,
biofuels, and chemicals.
They are also used for animal and human food and fiber
and as gelling and stabilizing agents
in the food industry.
And finally, cell walls have uses
as nutraceuticals and pharmaceuticals.
So what are plant cell walls?
Plant cell walls are extracellular matrices
that are comprised of 80% to 90% carbohydrate
and approximately 10% protein.
Some cell walls also contain lignin.
There are generally 2 types of plant cell walls,
the primary wall versus the secondary wall.
In the middle picture here,
you can see an example of a secondary wall.
This is a cross section of wood.
The wall is thicker.
If you look on the right, however, you see the primary wall.
The primary wall is the first wall laid down
around all dividing and growing cells.
After that wall is produced,
some cells go on to produce a secondary wall,
and that primary wall is pushed outward.
On the right-hand side, you see seedlings
underneath callus culture
and then finally suspension-cultured cells.
All of these cells consist largely of cells
made up of primary wall.
There are 2 types of primary wall,
dicots and non-grass monocots
have a so called type 1 wall,
as Arabidopsis on the bottom left.
The grasses have a so called type 2 wall.
Now finally, if you look at the bottom picture,
you'll see a cross section of 4 contiguous cells.
You have the plasma membrane,
and outside the plasma membrane is the primary wall.
And if you look at this primary wall,
already at this level of viewing,
you can see that it is fibrous.
The wall that surrounds plant cells
is cell type, developmental state,
and to some extent species specific.
So let's review,
primary walls are the first wall laid down,
they surround meristematic and growing cells,
they're prevalent in the succulent parts of the plant,
they're found at the junction between cells,
and they form the outer layer
even of cells that have secondary walls,
they're composed of roughly 90% carbohydrate and 10% protein,
and there are 2 types—
type 1 and type 2 primary walls.
Secondary walls surround cells
that differentiate to form specialized functions
like wood cells, xylem cells, and fiber cells.
They have a different polysaccharide composition than primary walls,
and they are often lignified.