IEEE Spectrum June, 2013 - 53

growing it

roboTic breeding

robot-assisted genetic analysis helps plant breeders
combine several genetic traits into one plant breed.

Dna is a ladderlike structure with rungs made up
of four types of chemicals. adenine binds only
to thymine, and guanine
binds only to cytosine.

Scientists discover that
corn with a particular
mutation-a change in a single spot-
is better able to tolerate drought.
They build a genetic probe containing
the mutant DNA sequence and a
chemical that lights up when the DNA
sequence binds to its complement.

1

have to grow it from a cell culture. This isn't just time-consuming
and inefficient-there are many plants that scientists don't know
how to grow from cultures. (Hence, no genetically modified pep-
pers.) Equally irritating is that agrobacteria are not too particular
about where their injected DNA goes in a plant's genome. The
injected DNA gets incorporated in semirandom spots, meaning
it can land in a place where the cell's machinery might never put
it to work. Or worse, it might scrunch itself right into the middle
of a gene that the plant actually needs to survive.
But in the past two decades, agricultural biotechnologists have
come up with ways of manipulating a plant's genome much more
precisely. One way puts to work a class of natural nanomachines
collectively called restriction endonucleases. An endonuclease is
effectively a smart pair of molecular scissors. It finds a particular
sequence in a cell's genome and then snips.
illustration by

James Provost

Scientists crossbreed the
drought-tolerant plants with
a high-yield variety. But which of the
thousands of resulting seeds has
both genetic traits? To find out, robots
extract DNA from the seeds and add
the probe. The probe binds only to a
complementary DNA sequence.

2

A breeder can then grow-
and sell-only those seeds
that are both high yield and drought
tolerant. Or the process can be
repeated to add other traits, such
as disease resistance.

3

When a double strand of DNA is cut, the cell's molecular machinery
tries to repair the damage. Snip the DNA in two places and the cell
might join it together without the snipped-out part, disabling or at
least mutating a gene. Do the same in the presence of a new DNA
sequence-say, one that makes baby's breath flowers mauve-and the
molecular-repair machinery might splice it right into the desired
spot. "The cell knows how to put together broken pieces," explains
Alexander Vainstein, a plant biotechnologist at the Hebrew Univer-
sity of Jerusalem. "But it doesn't know what the original pieces were."
In the last few years, made-to-order endonucleases have gone
on the market, and biotechnologists have started using them to
great effect. Dow AgroSciences, for example, used a type called a
zinc-finger nuclease to delete part of a maize gene. The end result
was to reduce the maize's level of phytate, a so-called antinutrient
because it makes it harder for animals to absorb critical minerals.
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Table of Contents for the Digital Edition of IEEE Spectrum June, 2013
