Video Transcript
Which of the following is not an
example of a biochemical defense used by plants? (A) Formation of tyloses, (B)
production of phenols, (C) secretion of antimicrobials, or (D) production of
canavanine.
To answer this question, let us
have a closer look at the mentioned defense mechanisms of a plant.
Plant defense mechanisms can be
roughly divided into physical and biochemical defenses.
Physical defenses defend the plant
against pathogens and potentially harmful substances by preventing them to
enter. If the pathogens or other
potentially harmful substances did manage to enter, physical defenses try to stop
the spread of the pathogen or substance throughout the plant.
Biochemical defenses include
defense mechanisms for which a chemical substance has been excreted by the
plant. In other words, because a chemical
compound is excreted, this defense mechanism is considered a biochemical
defense.
How does this work? Let’s assume a bacterium tries to
infect a plant. The plant might be able to produce
chemical substances that act either by killing the bacterium directly or by
inhibiting its reproduction and growth to prevent the infection spreading.
Now that we have repeated the two
defense mechanisms of a plant, let’s have a look through the given answer choices to
see if we can identify the example that is not a biochemical defense.
Many chemical substances often
increase in concentration following an infection to kill microorganisms or slow down
their spread. Phenols are aromatic organic
chemical compounds which contain the structure of phenol with the molecular formula
C6H5OH within them. Aromatic compounds are composed of
chemical rings as you can see here, which give them special properties. For example, they can neutralize
toxic free radicals that can damage cells. As phenols are chemical compounds
and are involved in the defense against ultraviolet radiation and pathogens, we can
exclude answer option (B).
Canavanine is a potentially toxic
amino acid produced by certain plants. This molecule is also a chemical
substance and helps plants in the defense against predation and pathogens. An antimicrobial is a chemical
substance that either kills a microbe or that inhibits its reproduction and growth
and is therefore by definition a biochemical defense.
Finally, tyloses are outgrowths of
the living tissues surrounding xylem vessels. The xylem is responsible for
transporting water and mineral ions from the plant roots to the rest of the plant
organs that require them. The tyloses close up the xylem
following an infection of a pathogen. This is helpful as the infection is
less likely to spread through the vascular system to other organs. The formation of tyloses acts as a
physical prevention of spreading of pathogens and is therefore a physical defense
mechanism.
Coming back to the question, we can
see clearly now that the answer to “Which of the following is not an example of a
biochemical defense used by plants?” is given by answer choice (A), formation of
tyloses.
