Roots play an essential role in plant growth by absorbing water and nutrients from soil. Microorganisms in the soil, such as bacteria and fungi, form symbiotic relationships with plant roots that are beneficial to both parties. The microbes help plants uptake nutrients and water, while the plants provide carbohydrates to the microbes. Two important examples are rhizobia bacteria that fix nitrogen for legume plants, and mycorrhizal fungi that extend the root system and help access additional soil nutrients. These symbiotic relationships between plant roots and soil microbes strengthen plants and increase crop yields.

1. PLANT ROOTS SYMBIOSIS
Grade Level: Grade 3
Agricultural Background: Plant Roots and Their Symbiotic Partners
Roots play an essential role in the growth and vigor of a plant. They provide the anchor that keeps
it from blowing or washing away. The roots support the stem and the vascular tissues that are
continuous throughout the root to the stem. Roots absorb water and dissolved nutrients from the soil,
transporting it through the vascular system to the rest of the plant for photosynthesis and transpiration.
Roots also store sugars and carbohydrates that are used by the plant to survive winter and carry out
other functions. Some roots can produce new plants.
One teaspoon of soil contains more microorganisms than there are people on Earth. However, the
narrow region of soil affected by root secretions, know as the rhizosphere, can have 1000-2000 times
that number. This root zone is a dynamic region in which numerous bio-geo-chemical processes take
place driven diversity of chemicals released by the plant root and mediated by soil microorganisms.
These micro-organisms are vital to the plant, providing a valuable symbiotic
relationship. This association is beneficial to both the plant and the micro-organisms.
The beneficial microbes colonize in the root zone and on the
roots, preventing infections from pathogenic fungus. They also produce
enzymes that act as a catalyst increasing the plant’s ability to take in
water and nutrients. In return the microbes benefit by utilizing
the carbohydrates (sugars) that are produced in the leaves
during photosynthesis and then translocated to the
roots. The microorganisms use these carbohydrates
for energy, which allows them to function and
reproduce.
There are many plant rhizosphere -microbe
relationships. The two that have received the most
attention, and are important to farmers and nursery
growers are Rhizobia bacteria and mychorrhizal fungi.
Bacterium and Legumes - Nitrogen Fixation
Nitrogen fixing bacteria call Rhizobium form symbiotic relationships with plants in the legume (pea)
family. Each is able to survive independently, but life together is beneficial to both. The bacteria
cannot fix atmospheric nitrogen until they invade the roots of the appropriate
legume.
Lesson and Agricultural Extensions supported by a grant from the Massachusetts
Society for Promoting Agriculture.

2. Once the bacterium invaded the roots it
multiplies. The plant supplies the nutrients
and energy. A nodule forms within a week.
Young nodules are white or gray inside and
do not fix nitrogen. As they age, they turn
pink or reddish in color, indicating the start of
nitrogen fixation. Nodules on perennials are
long-lived and fix nitrogen through the entire
season, so long as conditions are favorable.
Nodules on annuals are short-lived and will be
replaced constantly.
Some legumes are better at fixing nitrogen
than others. Common beans are poor fixers,
alfalfa and clover are better. Almost all of
the fixed nitrogen goes directly into the plant.
Little leaks into the soil for a neighboring
non-legume plant. However, nitrogen eventually returns to the soil for use by other plants when
legumes die and decompose, if the legumes are turned into the soil.
The relationship between rhizobaceteria and plant roots was first discovered in 1978. Since then two
dozen rhizobacteria have been identified. Farmers regularly plant legumes that have been inoculated
with rhizobacteria as a cover crop in this fields. The cover crop keeps weeds from growing and
protects the soils from blowing away. Once the plants have grown they are tilled into the soil. This
improves the soil by adding organic material and also increases the soil nitrogen content, all of which
increases crop yields.
Mycorrhizal Fungi and Nutrient Acquisition
Mycorrhizae (from the Greek words for fungus and root) is a general term describing a symbiotic
relationship between a soil fungus and plant root. These associations occur on approximately 80%
of angiosperms and in all gymnosperms. The fungi form a dense covering around the root, growing
into intercellular spaces and sometimes even penetrating the root cortex. There is then a transfer of
nutrients to the plant and carbon to the fungus occurs. These fungus are obligant symbiants, and are
unable to grow without their plant hosts.
Lesson and Agricultural Extensions supported by a grant from the Massachusetts
Society for Promoting Agriculture.

3. These specialized fungi also serve as extensions
of the root system, stretching far into the depths
of the soil. In fact, mycorrhizal fungi can
increase the absorbing surface area of a plant’s
roots by 100 to 1000 times, which greatly
improves access to soil resources. Mycorrhizae
release enzymes that dissolve otherwise hard
to capture nutrients, such as organic nitrogen,
phosphorus and iron. The small size of the
fungi allows them to grow into small soil pores
and cracks that the plant root would otherwise
not be able to access. The network of fungi from
the plant roots also has a tremendous impact on
soil quality, reducing soil erosion, increasing
aeration and allowing for water filtration.
When farmers and nursery growers utilize this
symbiotic relationship to strengthen plant roots
and increase the size of their plants. They may
provide carbohydrate supplements to the root
zone. This accelerates the reproductive rate of beneficial microorganisms while allowing the plant to
retain a good portion of the carbohydrates normally secreted through its roots. The carbohydrates that
remain in the plant can be used for other purposes. Some formulas have also been developed to add
additional microorganisms into the growing medium. Hydroponic growers bypassing the microbes
themselves and directly placing specific enzymes in the root zone.
Lesson and Agricultural Extensions supported by a grant from the Massachusetts
Society for Promoting Agriculture.

4. MICROBE ROOT RELATIONSHIPS
Follow-up Lesson to: “Soil Beast”
Grade Level: Grade 3
Season: Any
Lesson/Activity Description
In this lesson, students will learn about the relationships between
plants and the microbes in the soil and how microbes in the soil
enhance the growing of plants.
Adapted from “Friend of Foe,” A Kids Gardening Lesson
Guiding Question
What organisms that live in the soil are beneficial for plants to grow?
Big Idea
Plants and microorganisms in the soil have a symbiotic relationship where both parties benefit.
Learning Objectives
- To understand the symbiotic relationship between the microbes in soil and plants.
Materials
• Pots
• Bean or pea seeds
• Potting soil
• Rhizobia bacteria (can be found at a local garden center or through a scientific catalog)
• Pencils and paper
Preparation
Review Background material above.
Introducing the Lesson
Talk about microorganism in the soil, especially those that are important to
plants.
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Massachusetts Society for Promoting Agriculture.

5. Activate prior knowledge
Ask students whether organisms in soil are a good
or bad thing. What would be some benefits or
consequences?
Engage Student Interest:
Explain that you are going to do an experiment. Half
of the class will plant a bean with a bacteria while the
other half of the class will plant the seed normally.
What do they think will happen?
Procedure
Total time approximately 30 minutes
1. Split the class into two groups.
2. Have the first group fill their pots with potting soil. Then give them each a seed to plant. Help the
students to plant the seed and then water it.
3. Have the second group fill their pots with potting soil. Then give them each a seed to plant. Treat the
seats in the second group with rhizobia. Help the students to plant the seed and then water it.
4. Place pots from group one and group two in different areas so you can tell the difference between
the two.
5. Encourage the students to observe the plants every day and record the data
6. When the plant has matured, remove the plants from the soil and observe the differences between
the two groups of plants. What are the differences between the roots?
Wrap Up
Help students to understand that though they may not be able to see them, there are plenty of
microorganisms living in the soil that helps plants to thrive. How did the rhizobia help the plant to
grow?
Assessing Student Knowledge
As a class, discuss what farmers need to do if the soil does not provide all of the necessary nutrients.
What would happen if there were too many nutrients in the soil
Extensions
Have the students explore different microorganisms and what they do for
plants.
Lesson and Agricultural Extensions supported by a grant from the Massachusetts
Society for Promoting Agriculture.

6. * Some of the Massachusetts Department of Education Standards in this lesson *
Earth Science 4:
Explain and give examples of the ways in which soil is formed.
Books and Resources
Soil Beasts
Lesson to be used before this one.
http://aginclassroom.org/School%20Gardens/School_Gardening_Lesson_Plans/School_Gardening_
Lesson_Grade%203%20Soils.htm
“Friend or Foe” http://www.kidsgardening.org/activity/friend-or-foe
Lesson and Agricultural Extensions supported by a grant from the
Massachusetts Society for Promoting Agriculture.

7. Massachusetts Agriculture in the Classroom
P. O. Box 345 Seekonk, MA 02771
www.aginclassroom.org
Please Visit the Massachusetts Agriculture in the Classroom Website
to tell us how you used this resource. Thanks!
Lesson and Agricultural Extensions supported by a grant from the
Massachusetts Society for Promoting Agriculture.