Video Transcript
Many species of willow, shown in
the figure, make and distribute salicylic acid to all parts of the plant following
an infection. What is the role of salicylic acid
in defending the willow against disease? (A) It alerts the whole plant that
an infection has occurred. (B) It kills the infecting pathogen
directly. (C) It increases the formation of
food storage organs. Or (D) it promotes widespread cell
death to limit the infection.
Infections in plants are caused by
disease-causing pathogens. They can spread quickly to other
parts of the plant and even to other organisms. Let’s recall some of the defense
mechanisms that plants have to respond to pathogens to slow or stop their
spread.
A plant’s defense mechanisms can be
split into two categories: structural defenses and biochemical defenses. The plant needs to spend energy to
initiate these defense mechanisms. When a plant does not suffer from
an infection, a plant’s energy is used to keep up general life processes, like the
formation of food storage organs, growth, and reproduction.
During a response to an infection,
the plant reduces the energy it is allocating to nonessential processes and instead
diverts this energy to be used on inducing structural and biochemical immunity. But this energy use reallocation is
only done if a pathogen is present. Otherwise, that would be wasted
energy. For defense mechanisms to start,
the plant therefore has to first detect the entrance of a pathogen into its tissues
or cells.
Plant receptors are one of the
first to detect the entrance of a pathogen, as they bind pathogenic molecules. The binding of these molecules to a
receptor activates the receptor. Upon activation, the receptors
cause the release of chemicals, such as salicylic acid, which then alerts the
plant’s innate immune system that something is wrong. This alert tells the immune system
that a potentially dangerous organism has been detected and enables the plant to
respond.
Though one of these responses is a
localized cell death to limit the spread of infection, this is not caused directly
by salicylic acid. Other responses include releasing
chemicals, such as toxins or enzymes, to break down the pathogen. But again, this is not caused
directly by salicylic acid itself. The role of salicylic acid is to
alert the whole plant that an infection has occurred.
So the answer to this question is
given by answer option (A). It alerts the whole plant that an
infection has occurred.
