Video Transcript
Which plant structural defense is
not preexisting and is only formed as a result of infection by a pathogen? A waxy cuticle, hairs, thorns,
cellulose cell walls, or tyloses.
Let’s look at some of the key terms
that have been used in the question so we can identify which of the structural
defenses is not preexisting. The preexisting structural defenses
of a plant help to limit the entry of a pathogen. The diagram here shows some of the
key cells in a leaf. In some cases, the plant epidermis
produces a waxy cuticle to coat it. The waxy cuticle is a
water-resistant layer. And if water cannot easily settle
on the surface of the epidermis, nor can waterborne pathogens. The epidermis can also be covered
with hairs or thorns. These features mostly function to
deter herbivores, like this rabbit, from consuming the whole or part of the plant
which would damage it and make it vulnerable to pathogens.
In this diagram, we have magnified
one of the cells in the leaf so we can see their contents more clearly. A major defense present in the cell
and all other plant cells is a cell wall. Plant cell walls are made of a
carbohydrate polymer called cellulose. By magnifying this image of the
plant cell wall further, we can see the cellulose forms a strong mesh and a physical
barrier between the cell’s contents and its external environment. This restricts the entry of
pathogens attempting to access the cell from outside the cell wall.
All of these features that we have
discussed so far are preexisting, which means they’re always present. But some pathogens can make it past
these preexisting structural features. If this occurs, physical plant
structures can be induced to help prevent the spread of infection once the pathogen
has entered the plant. One example of an induced
structural defense is the development of tyloses. Tyloses are outgrowths of the
tissues surroundings xylem vessels, which are the tissues responsible for
transporting water around a plant. So, let’s have a closer look at the
xylem to see what this would look like.
You can see here that tyloses grow
into and block up the xylem vessels. This is really helpful as it occurs
after the pathogen has already been detected by the plant and means that the
infection is less likely to spread through the xylem to damage other cells. The structural defense feature that
is not preexisting but is induced by an infection is, therefore, tyloses.
