Primed for battle: helping plants fight off pathogens by enhancing their immune systems

Civilization as it is known today could not
have evolved, nor can it survive, without an
adequate food supply. – Norman Borlaug
Most people have never heard of Norman Borlaug. He is, thus far, the only agricultural scientist ever to win the Nobel Peace Prize. His
work in the development of high-yielding and
disease-resistant cereal crops saved more than
one billion (yes, billion) people from starvation. Though he uttered these words nearly 50 years
ago, his message could not be more relevant
today. We live in a world that is expected to
exceed nine billion people by around 2050, and currently, some 800 million people do not have enough food to live a healthy and active life. The United Nations Food and Agriculture
Organization projects that we need to increase food production by at least 70% to accommodate this surge in population growth. This is a
daunting task, made even more difficult by the
fact that nearly 20% of the global harvest is lost to plant diseases. One of the most efficient ways to combat these diseases is through chemical
control – the application of pesticides. However,
pathogens can quickly develop resistance to
pesticides, which can then require ever higher
usage to maintain production. There are also environmental and health concerns associated with the application of potentially toxic chemicals
to fields. The demand is urgent for safer and more
sustainable methods of crop protection. That’s
where we, the plant pathologists, step in. A plant
pathologist specializes in plant health in the
same way a physician specializes in human
health, and we work tirelessly to protect our food supply.

A plant heavily colonized by a bacterial pathogen. Jeannette Rapicavoli/UC Riverside, CC BY-ND    

A novel area of research in the war against
pathogens focuses on enhancing the plant’s
natural immune system. If a plant can fight off an
infection on its own, we can reduce the amount
of pesticides needed. Similar to how children are
vaccinated to protect against future diseases, plant pathologists are using the same
methodology to “immunize” plants against
pathogens, with the goal of strengthening their
immune defenses against invaders. This method
of priming plants' immune systems could be a
safe and effective way to save some of the global harvest currently lost to diseases.

A cassava specialist inspects a diseased crop in northeastern Thailand.

Understanding the plant immune system
Plants are naturally exposed to a variety of
pathogenic microbes, such as bacteria, fungi and
viruses. In contrast with human beings, who have
the ability to physically evade infections, plants
are immobile. Therefore, every cell in the plant
must defend itself against attack. Plants have a multi-tiered immune system that helps them
fight off these microorganisms. It works in a
manner very similar to the human immune
system.

Plants learn to recognize bacteria like this one
based on defining characteristics, or patterns,
such as their flagella. AJ Cann, CC BY-SA


Plants detect pathogens by recognizing microbial
“patterns.” These are unique characteristics of the
type of microbe (think bacterial flagella) that the
plant has evolved to recognize as “non-self.” We
can equate this capability to the recognition of
antigens by the human body, which induces an immune response. Unfortunately, pathogens
continuously evolve to evade recognition,
typically by shielding or disguising these patterns.
This ability allows them to colonize a plant’s cells
before it can mount an effective immune
response.

Defense priming is like vaccination
One of our major research goals is to harness
these patterns to prime the plant immune system, creating enhanced protection against pathogenic microbes, in lieu of traditional
chemical control methods.

A vaccine protects against future disease. hdptcar, CC BY
The principle of “defense priming” is very similar to how we develop vaccines to treat human
diseases. A vaccine works by acting as a
pathogen impostor. It tricks the immune system
into thinking it’s being attacked, which
stimulates defense responses, such as the
production of antibodies. This creates a defense memory, allowing the immune system to
remember a particular pathogen if the body
encounters it in the future. It can then respond
swiftly and robustly, thanks to its primed memory
from the vaccine. We can apply this same principle to a plant-
pathogen relationship. For example, once we’ve
identified a pathogen’s pattern of interest, we
work to isolate and purify it. This step is like
manufacturing the vaccine. We can then
inoculate the plant with the purified pattern – for instance, by injecting it into the stem or leaves
with a syringe. The goal is to stimulate the plant’s
natural immune response, resulting in a faster
and/or stronger defense response the next time
the plant encounters that pathogen.

We’re basically ensuring that the plants are
prepared for battle before the enemy attacks. Primed plants display enhanced tolerance to infection, which is often characterized by fewer
symptoms and reduced pathogen populations
within the plant. Although primed plants haven’t
yet been implemented on a large-scale basis in
commercial agriculture, scientists are actively
conducting research on the use of defense priming in both greenhouse and field settings for protection against bacteria, viruses and fungi. My own research focuses primarily on the use of
defense priming for protection against a bacterial
pathogen called Xylella fastidiosa that affects the
multi-billion-dollar wine, table and raisin grape
industries. It causes Pierce’s disease, which costs
the state of California over US $100 million annually in crop loss expenses and efforts to treat it. There is currently no cure for the diseases
caused by this plant pathogen, but our goal is to
utilize defense priming to vanquish it.

Plants get sick too! Stem rust fungus on wheat. Yue Jin, Agricultural Research Service

Potential in commercial agriculture
In contrast with the human immune system, in
which defense responses are specific to a
particular germ, the effects of priming in plants
are broad-spectrum, protecting the plant against
a wide range of diseases and insect pests.
Another major benefit of defense priming is that
there is little to no reduction in plant fitness –
plants still grow and reproduce normally. This is a
critical advantage in commercial agriculture,
where success is contingent upon high yields. Furthermore, the primed state is durable and can
be maintained long after the initial stimulus.
Current research has also shown that plants can
pass on this defense memory to their progeny,
providing multigenerational protection without any genetic modification. Further research is needed to improve our
understanding of the molecular mechanisms
behind this phenomenon, but defense priming
looks likely to be a valuable and promising tool in
the future of sustainable agriculture.