Plant molecular geneticist
Sheila McCormick uses a
vacuum to collect pollen
from Arabidopsis (thale cress)
plants genetically engineered
in her laboratory.
Welcome to spring, when flowers burst into bloomand pollen counts
soar. Those soft, crumbly grains of pollen that leave many of us sneezing
and red-eyed are, as we all learned in grade school, essential for fertilizing
flowers so that they will form many of the foods we eat and enjoyplump,
juicy tomatoes or golden ears of sweet corn, for example.
But those early lessons might not have covered some of the more complicated
steps that are part of the carefully orchestrated fertilization process
of our planet's flowering green plants. "Floral fertilization,"
explains ARS plant molecular geneticist
Sheila M. McCormick, "occurs deep inside the flower when the two
male sex cells that are carried in each pollen grain meet, recognize,
and fuse with two different cells in the female part of the flowerthe
embryo sac. One embryo sac cell is the egg, just like in animals. But
the otherthe central cellis unique to plants."
This fertilization of the two female cells by the two male cells, called
double fertilization, is critical to creating the crops that keep us
healthy and well fed. For most of Earth's flowering plants, no fusion
means no crop.
Equally important are the steps that lead to that fusion, such as the
sprouting and elongating of the pollen tube. The slender tube forms
when the pollen grain lands on the top of the flower's columnlike style,
then swells with water. As it extends, the tube transports its passengers,
the male sex cells, down through the style, delivering them to the female
cellswhere fertilization occurs.
Michele Engel, postdoctoral fellow,
observes Arabidopsis pollen with a
microscope. The plants were altered
so that the male sex cells (sperm)
appear as glowing green dots
within each pollen grain.
| What Roles Do Genes Play?
Some of these facts about floral fertilization have been known for
at least a century. But many of the details still remain a mystery.
In particular, researchers know very little about the genes and the
products of those genesproteinsthat are key players or at
least play important supporting roles in the fertilization drama.
Why do we need to know more? Scientists would like to be able to alter
the activity of genes that block fertilization of certain wild species
with their domesticated cousins, for example. Until such barriers are
overcome, wild relatives' prized traits cannot, in many cases, be easily
moved into their cultivated kin. The sought-after genes might enable
tomorrow's plants to thrive with less fertilizer, less water, or perhaps
even less pesticide.
McCormick and colleagues are uncovering new facts about infrequently
researched interactions of plants' male and female cells. She's with
the Plant Gene Expression Center at Albany, California, which is staffed
by scientists from ARS and the University of California, Berkeley.
To learn more about genes
involved in fertilization,
Heping Yang, postdoctoral fellow,
prepares to dissect embryo sacs
from within corn kernel ovules.
| The Albany group is identifying
key genes, sleuthing their exact roles in successful fertilization, and
sharing these discoveries with researchers worldwide by reporting the
findings in leading scientific journals and by posting the sequences of
newfound genes on the publicly available GenBank genome database (www.ncbi.nlm.nih.gov).
Cells Send Secret Messages
The researchers are deciphering some cell-to-cell communication in
which protein molecules send messages to other protein molecules. In
one aspect of this work, the scientists are investigating how such messages
may help the growing pollen tube find its way to the embryo sac.
Two types of protein molecules, known as ligands and receptor kinases,
may assist. Both kinds of molecules are formed according to instructions
carried by genes and are secreted from cells. "Receptor kinases
are already known to bind to ligands," says McCormick. "The
binding of these proteins sends a signal. If this same scheme operates
in pollen tubes, such signaling might help guide the pollen tube."
To learn more, the researchers experimented with receptor kinases that
they had discovered earlier in studies of tomato pollen. The kinases,
which they had named LePRK1 and LePRK2 for tomato's scientific moniker,
Lycopersicon esculentum, served as the lures or baits for partner
moleculesthe ligandsthat might be secreted from cells in
the female part of the flower. "We identified many potential ligands,"
Fusion: Finding the Right Partner
Once the two male cells come into the vicinity of the egg and central
cell, how do they know which cell is their correct fusion partner? Perhaps
signaling molecules, similar to those that may assist the pollen tube's
journey, exist on the female and male cells, McCormick says. To find
out, her team is determining the makeup, or DNA sequence, of genes that
are switched on or off in the male cells and in the embryo sac cells.
This is a first step toward learning which proteins are encoded by those
genes and whether those proteins act as signaling molecules on the surfaces
of these cells.
So far, McCormick and co-researchers have discovered hundreds of new
genes in these cells. (There's more information about this research
on the World Wide Web at www.pgec.usda.gov/McCormick/McCormick/mclab.html.)
Their probing into the puzzle of plants' pollen and eggs has won funding
from the ARS national postdoctoral fellowship program (see Forum,
page 2) and $1.1 million from the National Science Foundation. The pioneering
investigations will likely yield even more clues about the 144-million-year-old
reproductive ballet of the world's flowering green plants.By Marcia
Wood, Agricultural Research Service Information Staff.
This research is part of Plant Biological and Molecular Processes,
an ARS National Program (#302) described on the World Wide Web at www.nps.ars.usda.gov.
Sheila M. McCormick is with the USDA-ARS University
of California at Berkeley Plant Gene Expression Center, 800 Buchanan
St., Albany, CA 94710; phone (510) 559-5906, fax (510) 559-5678.
"Probing the Puzzles of Plants' Pollen and Eggs" was
published in the August
2004 issue of Agricultural Research magazine.