For Students
Engineering Design in Oregon Science Classrooms
Page 1 of 2
Reading Handout for Frankenplants
In Mary Shelley’s classic novel Frankenstein, the scientist Victor Frankenstein creates a super creature
from a collection of body parts. Today plant engineers, in the form of botanists and agricultural researchers, are
doing something similar with vegetable plants. It is called grafting, which is when you combine the root system
of one plant (called the stock), with the top portion of another plant with the goal of creating a super plant with
the best qualities of both plants. The plant used for the top portion, and the top portion of the graft, is called a
scion. Even though the grafted tomato plants seem unusual, they have the disease resistance and strong growth
of the rootstock, and the tasty tomatoes of the scion. Grafting is considered a form of asexual reproduction,
because it can be used to propagate a desired variety of plant. But before grafting, you must wait until the plant
has developed beyond just a single cotyledon (an embryonic part of a seed which, upon germination, emerges
from the ground and becomes the first leaf-like part of the seedling.
Figure 1: Grafting an heirloom scion and a
rootstock plant creates a superior tomato plant;
one that produces a lot of tasty fruit, but is also
disease resistant.
If you have studied
genetics, then you already know
that the phenotypes of plants, their
physical characteristics, are in
large part determined by their
genotypes or genetic make-up. You
should also know that different
types of plants, even those of the
same species, have different
genotypes. For instance, one
variety of tomato plant called
heirlooms is known for producing
particularly good tasting tomatoes.
They are called heirlooms because
the varieties have been grown for
many decades for a specific
purpose—in this case, flavor—and
some varieties were unavailable in
certain parts of the world until
recently. Most heirloom tomato
plants produce tomatoes over
several weeks rather than all at
once and some produce tomatoes
in unusual colors and shapes.
These traits make them particularly attractive to home gardeners who only want a few tomatoes at a time
or value having a variety of shapes and sizes of tomatoes available. The trade-off though for these advantages
is that heirloom varieties are often susceptible to root diseases that may kill them or stunt their growth. In
contrast, another variety of tomato, referred to as rootstock, has been bred relatively recently to have a hardy
root system that is resistant to molds and other diseases, but the tomatoes don’t taste good. However, by
grafting the scion of an heirloom plant to the bottom portion of a rootstock plant, engineers are able to create a
superior tomato plant that yields plenty of tasty fruit that will seldom contract a root disease (see Figure 1).
In the lesson that follows, you will design your own tomato grafts using the engineering design
process. Grafting works, because plant tissues called vascular bundles are able to fuse to one another (see
Figure 2). After exploring grafting methods, you will need to determine the criteria (goals) and constraints
(limitations) for the grafts you will create. Next, you will brainstorm ideas and build prototypes, or samples, of
initial grafts. You will then test and collect data on the success of your graft.
For Students
Engineering Design in Oregon Science Classrooms
Page 2 of 2
It’s important to note that neither the
heirlooms nor rootstocks you will use in this
activity occur naturally in the wild. Instead,
both have been genetically engineered
through selective breeding, a process by
which engineers deliberately cross certain
species in order to achieve desirable
characteristics in the offspring. Selective
breeding is a type of genetic engineering that
has been performed scientifically for over 100
years—do not confuse this with gene splicing
or recombinant DNA techniques, which have
become possible in the last 20 years. The
rootstock varieties you will use in this lesson
were created in the Netherlands from a cross
Figure 2: Vascular tissue can be seen at the grafting point. It is lighter in
between S.lycopersicon and S.habrochaites
color than the stems.
and are reported to be resistant to tomato root
rot and nematodes, which are a type of roundworm.
Both of the parent species that produced the disease resistant rootstock plants are pure homozygous
meaning they have two of the same alleles. Thus all the resulting offspring of their cross are hybrids, which
mean they have two different alleles of the same gene (see Figure 3). This is a drawback for growers who want
to propagate their own seeds, because if they let some of the rootstock plants produce tomatoes, perhaps 50% of
the tomatoes would contain hybrid seeds, but it is unlikely that the growers would be able to tell them apart
from the other homozygous tomato seeds (see Figure 4). As a result every year growers must purchase new
seeds instead of making their own. In contrast most heirloom species are homozygous for the traits associated
with the particular variety so the seeds they produce result in similar offspring.
.
Homozygous
Genotype (alleles)
RR
Phenotype
(characteristics)
Resistance to
rot root
Homozygous
+
NN
Resistance
to
nematodes
Hybrid
=
RN
Resistance to
root rot and
nematodes
Figure 3: The cross between S.lycopersicon and S.habrochaites is an example of codominance.
Resistance to root rot and nematodes are both dominate traits so the hybrid offspring display both
phenotypes.
Figure 4: The
offspring of two
hybrid rootstock
tomatoes would
likely be 25% RR,
50% RN, and 25%
NN, but all the seeds
would look the same.