Video Transcript
Imagine that a plant has a serious
infection in a part of a leaf. How could it prevent this infection
from spreading? (A) The plant can kill the cells
that surround the infected tissue. (B) The plant can release
specialized immunoglobulins that target the infected tissue. (C) The plant can generate the
production of an epidermis around the infected tissue. Or (D) there is no method the plant
can use to stop the infection from spreading.
Let’s start by listing some of the
methods that plants use to prevent infections from spreading. Once a plant detects some type of
internal infection, which would generally be by receptors on the plant’s internal
immune cells detecting an antigen or a surface protein from a foreign cell, one of
the first things that happens is that the detecting cells will release chemicals,
such as salicylic acid, to alert the rest of the plant’s immune system. The plant may then release
antimicrobial chemicals to inhibit the pathogen’s growth or spread, as well as
detoxifying enzymes to break down any toxins released by the pathogen.
Likewise, when the parenchyma cells
that line the xylem — the xylem are vessels that carry water from the roots to the
leaves — when these cells are alerted to an infection, they will protrude and create
tyloses, or outgrowths, to block the xylem to prevent the movement of fluid through
the plant from spreading the infection. Other cells, especially those on
the surface of the plant, the epidermis, will swell to become more difficult for
pathogens to penetrate, or enter, as well as using tougher building blocks, such as
lignin and callose, to construct stronger cell walls. Lastly, in some cases, the plant
will destroy its own infected tissues to destroy the pathogen and keep it from
spreading.
So, there are multiple methods that
plants use to prevent the spread of a detected pathogen inside of the plant. Now, let’s imagine the particular
circumstance that this question provides us. There is an infection in part of a
leaf. Blocking the xylem in this case
likely wouldn’t be much help since the xylem flows toward the leaves and not from
the leaves toward the rest of the plant. We could therefore predict that the
plant would respond by, one, releasing antimicrobial chemicals and detoxifying
enzymes; two, swelling epidermal cells and adding reinforcements to the cell walls
to make it more difficult for the pathogen to enter; and three, destroying infected
tissue to prevent the spread of this pathogen.
We can look through the answer
choices to see what most closely matches one of these. The correct answer must be (A). The plant can kill the cells that
surround the infected tissue.
