The Convenience of Arabidopsis
Elizabeth A.
Kellogg
small, it’s plain, it’s absolutely ordinary, but it’s become one of the most
popular lab plants around. It’s Arabidopsis thaliana, and its most important
characteristic is that it’s handy.
It’s
Scientists have a long tradition of working
with the handy stuff, literally the things
close at hand, to answer questions about
phenomena that are otherwise inaccessible.
Charles Darwin opens The Origin of Species
with a chapter that describes in some detail
breeding experiments
were
on
pigeons. Pigeons
convenient and served
as a
model. If
people could select pigeons for complex
characteristics, Darwin reasoned, then
could select other organisms the
way. Hence, evolution by natural
selection: an all-encompassing theory supported in part by experiments on the very
nature
same
plain, thoroughly homely pigeon.
Using whatever’s handy is of course an art
fully exploited by schoolteachers. If they
want to make a mask or a turkey, they use a
paper plate. An egg carton does fine for an
alligator (or stegosaurus or a bouquet of flowers), and noodles are clearly necklace material. Lab scientists use a lot of those same
items. Just like Charles Darwin and schoolteachers everywhere, we work with whatever’s
handy.
procedure in molecular biology labs, called Southern blotting after the
One
common
who invented it, is carried out in
Tupperware containers. Rubbermaid does
fine, too; the lid just has to be watertight.
man
This same procedure can be done with specialized chemicals like dextran sulfate-dextran sulfate being notable because it costs
about $400 a pound-but for many purposes
ordinary powdered milk from the supermarket works just fine. The standard procedure
then involves washing a bit of high-tech
nylon membrane in powdered milk in a plastic kitchen container. When the procedure
is finished the piece of nylon is wrapped in
Saran Wrap. (In fact, in the standard chemical stockroom Saran Wrap is on the shelf
right next to all the fancy chemicals.) This
procedure is like many others in research; for
some purposes only a very particular tool
will do (like the special bit of nylon membrane), but there are many cases where you
can simply use what’s handy.
This same "principle of handiness" applies
choosing organisms to study, especially in
rapidly growing field of plant molecular
biology. People who study plant molecular
biology are trying to understand exactly how
plants work, down to the details of the DNA
to
the
that make up their genes. Because almost
anything they discover will be new, several
plants are equally good to begin working on.
So why not start with the most convenient?
I currently make my living studying genetic
relationships in the wheat tribe, a group that
Arabidopsis thaliana. Photo and © by Kurt Stepnitz, MSU/DOE Plant Research Laboratory.
14 .
includes
species,
Turkey,
barley and
rye and a lot of other
many of them native to Mongolia,
and the Mideast. It happens, how-
ever, that three wheat-related species are
weeds that grow near the parking lot of the
Harvard Bio Labs, and another grows next to
the playing fields in my Cambridge neighborhood. It’s obvious which ones I looked at
first. As the study has progressed I have had
to seek out the less accessible members of
the group to fill in the story, but the starting
point was arbitrary and determined as much
by convenience as by logic. That’s the major
attraction of Arabidopsis thaliana to molecular biologists.
It does have a common name-mouse ear
cress-but it’s rarely used. The plant has no
horticultural value. It isn’t edible. But for
some purposes it is very convenient, and it
has thus become important because of its
value as a scientific tool, analogous to the
fruit fly [Diosophila).
Arabidopsis thaliana is a tiny relative of
the cabbage and part of the same family,
commonly known as the mustard family and
botanically as Cruciferae or Brassicaceae
(either name is acceptable).
At
maturity it is
about eight inches tall. It can be germinated
by the hundreds on petri plates. Populations
of thousands of plants fit easily on one greenhouse bench. The generation time (seed to
seed) is about three months. Compare this to
that other workhorse of the plant genetics
world, maize. Maize plants are large, well
over eight inches tall, and planting thousands requires acres of land and scientists
with strong backs. The generation time is
about six months, but to get two generations
a year requires that you plant one winter
crop in Hawaii. Arabidopsis clearly has a
logistical advantage, although you do lose
the excuse to winter in Hawaii.
Arabidopsis is also handy in that its internal workings, its genetic machinery, are
unusually simple. No extras, no add-ons, no
window dressing. One scientist, Dr. Jerry
Fink of the Whitehead Institute, has
described it
as
the
plant equivalent
of
a
Hyundai. For many studies this makes it the
plant of choice. The Boston Area Arabidopsis
Group alone includes sixty or so scientists
who specialize in topics such as hrp, GA,
auxin, rubisco activase, and ribozymes.
Most of these formidable-sounding specialities have direct applications to understanding the crop plants that feed humanity.
Hrp, for example, is a gene or set of genes
involved in the plant’s response to a pathogenic fungus or bacterium. If we know
exactly how plants respond to pathogen
attack and what allows some plants to resist
some pathogens, then we might be able to
find ways to reduce our dependence on the
toxic chemicals that are now used to control
damage by plant pests and to engineer resistant crop plants. GA and auxin are plant hormones that control the rate and timing of
growth and development. It’s all basic
research, the fertile ground that fosters
direct applications.
So it is that a modest weed is achieving an
eminence formerly reserved for crop plants.
There are seedbanks that store Arabidopsis
seed, there is an Arabidopsis newsletter and
an Arabidopsis Information Service as well
as an Arabidopsis Research Initiative. It is
becoming, in its own curious way,
cally important.
economi-
There are, of course, questions that A.
thaliana can’t help us with. Since we know
almost nothing about the relatives of
Arabidopsis and very little about its natural
history, it is almost useless in studies of evolution within the mustard family. It has,
however, become a big part of one story that
promises to tell us a lot about the evolution
of flowers and their multiple forms. If you
look closely at a developing flower during
the very early stages when it is best seen
with an electron microscope, you will see
tiny nubbins for all the floral parts. In
Arabidopsis there are four that will be
sepals, four that will become petals, six stamen nubbins, and two carpels that will form
15
Scanning electron micrographs of a developing Arabidopsis flower.
A. The top of the inflorescence and flower
buds. B. A flower bud with one sepal removed to show nubbins that will become stamens; the mound in the
center will become the pistil. C. A flower at a slightly later stage. P petal; LS lateral stamen; MS medial stamen ; G gynoecium (pistil). D. A flower bud shortly before openmg. Some of the sepals and petals have been
removed. Note that the stamens have not yet elongated fully. Bar 10 mm m A, B, and C; 100 mm thick.
Photographs reproduced with permission from Bowman et al. (1 992).
=
=
=
=
=
16
the two halves of the ovary. Scientists in a
California lab headed by Elliot Meyerowitz
have found the chemical signals that tell the
various nubbins how they should develop.
Altering these chemicals can make the stamens turn into petals, or the petals turn into
stamens, or even turn all the flower parts
into leaves. (This last idea-that flower parts
can be viewed as modified leaves-was first
suggested in the eighteenth century by the
German poet Goethe.) Comparing work on
Arabidopsis with studies done on snapdragshows that similar substances appear in
other dicot flowers as well. It is now a tantalizing possibility that these chemicals may
be involved in generating some of the startling diversity of floral form that we enjoy
in our gardens, fields, and forests.
Scientific papers are typically written as
though the scientist had thought of an unanswered question, carefully designed an
experiment, chose a perfect model system,
and concluded with a formal and thoroughly
rational analysis. (This, incidentally, was not
the structure used by Darwin.) Such papers
ons
fairly easy to read once you get the hang
although rarely as pleasurable as The
Origin of Species. The style of the scientific
paper is simply a late-twentieth-century
convention. Unfortunately, it obscures the
way that science actually works, the way
are
of it,
decisions are made and directions taken.
There is a lot of serendipity involved. There
are insights from chance conversations,
opportunities created by particular combinations of people, place, and time. And there
is the very practical tendency to grasp the
tools at hand.
Reference
Bowman, J. L., H. Sakai, T Jack, D. Weigel, U. Mayer,
and E. M. Meyerowitz. 1992. SUPERMAN, a
regulator of floral homeotic genes
Arabidopsis. Development 114: 599-615
in
ol
Elizabeth Kellogg is Research Associate in Organismic
and Evolutionary Biology at Harvard University and an
Associate of the Arnold Arboretum.