Saturday, November 18, 2017

Chestnut Redemption

In April, 
I submitted the following essay 
to a Science and Faith writing contest. 
Recently I found that I had not placed, 
but no worries-- 
it was fun to integrate a childhood memory 
with college botany memories 
and a current breakthrough. 
Skip to the end 
if there is too much science for you. 
Appreciation to Lori Mulligan Davis 
and Steve Zimmerly 
for their Very Useful Comments. 
(...and Lori, I reinstated a bit of 
not-the-best-choice-for-publishing stuff 
and added slightly distracting pictures. Ha.)

Genetic Engineering in the American Chesnut

I crawled out of the back seat of my grandfather’s Volkswagen Bug
and walked, hand in hand,
with my grandma through the overgrown field
to a corner of the woods.
It was Grandma Maud who taught me the creation story at VBS,
using visual aids she had made with paper fasteners and magazine cutouts,
photos of skies and waters and lions and monkeys and trees
and drawings of Adam and Eve’s sin that doomed the earth to imperfection.
However, it was my Grandpa Bernie, a nonbeliever,
who most appreciated the Creator’s details.
His basement barbershop, a source of side income since the Great Depression,
contained turtle shells,
snake eggs in jars,
and furniture made from the burls he’d found on old trees.
When we arrived at the edge of the woods,
Grandpa Bernie spoke.
“Look carefully and remember, Susie.
You’ll never see one of these again.”
“What is it, Grandpa?”
“It’s an American chestnut tree, the last one in these parts.”
It was enormous.
 The three of us holding hands did not stretch around even half the trunk.
And it was dead.

The corner of the same field today

A Polaroid of Grandpa Bernie's barber shop in the 60s

When Grandpa Bernie was a boy,
there were billions of huge American chestnut trees.
Chestnut was the species that made up the largest part of the forest canopy.
One of every four trees in the forests of the Appalachians
was thought to be a chestnut;
many of them had diameters of eight to ten feet.
Some even reached fourteen feet,
with no branches for the first fifty feet up.
It was thought that a very energetic squirrel
could start in Maine
and adventure to Alabama
solely on the branches of American chestnut trees.

Bernie and his dogs in the time of chestnuts

The American chestnut has had a place in literature for hundreds of years,
beginning with explorer Hernando de Soto’s travel journals in 1540:
“Where there be mountains, there be chestnuts.”
Longfellow’s poem “The Village Blacksmith”
immortalized the spreading chestnut tree.
Henry David Thoreau and Louisa May Alcott were familiar with chestnuts
and wrote of them.
Each Christmas while driving alone,
millions of us sing quite loudly,
“ChestNUUUUUUUTS roasting on an open fire...”
but most of us do not share lyricist Bob Wells’s memory of chestnuts.

"Under the spreading chestnut tree the village smithy stands..."
A Paul Detlefsen painting from my childhood kitchen

The American chestnut tree, Castanea dentata,
was a keystone species,
 a species that makes a huge contribution to its environment.
The chestnut’s mast, or nut crop,
was reliable each year
and supported many kinds of wildlife,
as opposed to the present-day oaks whose nut crops vary from year to year.
Passenger pigeons
and many other creatures
were dependent upon chestnuts.
Farmers would turn their pigs loose in the forests
in the autumn to fatten them on fallen chestnuts.
The bark was used to produce tannic acid for making leather.
Chestnut wood was strong and straight,
easy to work and resistant to rot;
this near-perfect construction material was used in houses
and barns
and fences
and furniture.
The branches that fell into streams provided long-lasting homes for fish,
and still can be pulled from waters today.

We inherited Bernie's chestnut drawers

In 1904,
when Grandpa Bernie was a toddler,
a fungal blight was discovered in an American chestnut tree
in the New York Botanical Garden.
It’s thought that a NYC nurseryman
accidentally imported the fungus Cryphoectria parasitica in 1876
with a shipment of Asian chestnut trees.
The nation took action to save the lives of its chestnut trees.
In the 1930s,
my father-in-law Walt worked for the Civilian Conservation Corps,
trudging acres of woods with a mattock
to dig out gooseberry bushes that were thought to host the fungus.
It was too late.
By the start of WWII,
it’s estimated that four billion chestnut trees were gone.

A few of the Windfall Run Civilian Conservation Corps boys
near Cross Fork, PA.
Walt and his grin are front and center 

I learned more about the American chestnut in my college botany classes.
The fungus spores were wind-borne,
so pockets of chestnuts at least ten miles from infected trees
survived longer.
C. parasitica entered a tree at a wound site
and began to produce oxalic acid,
which formed a canker,
damaging the bark as well as the inner cambium layer,
interrupting the nutrient-water uptake.
As C. parasitica’s fingery mycelium fanned out in the cambium layer,
the damage widened
and the canker grew bigger,
circling the tree
and cutting off nutrient flow to anything above the wound.
The chestnut tree died slowly, from top to bottom,
or weakened until reproduction no longer occurred,
and without reproduction there is no next generation.
Oaks, maples, staghorn sumac, and shagbark hickories
also were susceptible to C. parasitica;
they got cankers but did not die,
which allowed the fungus to survive
and attack other American chestnut trees.
In Senior Biology Seminar,
we discussed the possibility of genetic engineering,
but it was still mostly at the theoretical stage.

Canker on our only remaining American chestnut tree

When my husband Denny and I moved to our Pennsylvania mountain,
we noticed prickly hulls
beneath an ugly-but-interesting understory tree behind the outhouse,
like miniature porcupines on the forest floor,
and so found our first American chestnut.
A new sprout had grown from a surviving rootstock
to a diameter of eight inches.
Its nuts were non-viable,
making it functionally extinct,
but we were thrilled to find the feeble chestnut tree.
When it died some years later,
we used chestnut wood to panel the bathroom in its memory.
I show its hulls to my first graders
and remember Luther Burbank, the pioneering horticulturalist
whose quote hangs in my classroom:
            "Every child should have mud pies,
            water bugs,
            mud turtles,
            wild strawberries,
            trees to climb ...
            any child who has been deprived of these
            has been deprived of the best part of education."

The now-deceased chestnut tree's view from behind the outhouse

American chestnut husks are Very Prickly,
just like little porcupines

Resistance is the only way to control blight,
and Burbank’s method of hybridization is one way to achieve this.
Botanists crossbred American chestnuts
with their much smaller, resistant cousin,
the Chinese chestnut,
to introduce resistant genes to the hybrid,
and then backcrossed them
to try to restore much of the American chestnut’s typical look.
This method takes a long time
and results in 1/16 of the 38,000 genes being foreign.

Looking up our  Chinese chestnut tree

Chinese chestnut husks and leaves
are very similar to those of the American chestnut

Another method to control C. parasitica
is to introduce a beneficial fungal virus,  hypovirus,
into the chestnut’s system.
This method was tried in Michigan
on a pocket of two to three thousand American chestnut trees
that had avoided the blight for decades.
They were planted a century ago,
far from of the natural Appalachian range of chestnuts,
and had only recently become infected.
The hypovirus worked—too well.
Scientists had hoped that the virus-infected C. parasitica
would spread itself from tree to tree,
but instead the infected C. parasitica died 
before it could spread to other trees.
Using hypovirus,
chestnut trees can only be saved on an individual basis.

Recently, I read of a third hope
that made me dance wildly across the kitchen floor—
transgenic chestnut trees!
William Powell, Ph.D.,
of Syracuse’s SUNY College of Environmental Science and Forestry,
and his colleagues are using a bacterium,
Agrobacterium tumefasciens,
to transfer a resistant gene from wheat to the American chestnut.
This wheat gene produces an enzyme,
oxalate oxidase,
which strengthens a plant’s ability to fight off C. parasitica
by breaking down the oxalic acid secreted by the fungus
into carbon dioxide and hydrogen peroxide,
both non-toxic byproducts.
Bacteria have been moving genetic material between species
since the beginning of time;
Dr. Powell’s research team simply
(... or not so simply)
introduced the wheat gene that resists C. parasitica
to the chestnut’s genome via Agrobacterium.
He and his colleagues have developed transgenic American chestnuts
that now have full blight resistance to C. parasitica.
Because they are only adding two genes out of 38,000,
the American chestnut’s genome is essentially the same,
and backcrossing is not necessary.

Kitchen floor without the dancing feet

This is how transgenic American chestnuts are being produced:
wheat-gene-containing Agrobacterium,
along with a common selectable marker gene,
are mixed with chestnut embryos in a test tube,
then moved to a petri dish
and given time for the genetic transfer to take place.
Next, embryos are moved to a medium containing antibiotics
that kill the Agrobacterium
and selects the embryonic cells that have incorporated the new genes.
Introduced hormones then cause new embryos
to multiply from the transgenic chestnut embryos.
Hormonal and nutrient changes in medium allow shoots to develop.
When the shoots are two to three inches tall,
they are dipped into rooting hormone
and put into potting mix.
These rooting American chestnut shoots
are placed in a controlled-environment growth chamber
and covered with plastic bags
to provide the high humidity levels of previous tissue culture stages.
The bags are gradually removed
to accustom the young chestnut trees
to the coming natural field conditions.

Plastic bags, miracle workers in transgenic chestnut labs.
Who knew?

several of the earlier versions
of transgenic American chestnut trees
with intermediate levels of blight resistance
are growing
in the New York Botanical Garden,
just across the street
from where the blight was discovered over a century ago.
With approval from the USDA,
the Food and Drug Administration,
and the Environmental Protection Agency,
the newest transgenic American chestnut trees
with high levels of blight resistance
should be ready for planting in restorative locations
such as old strip mines,
abandoned fields,
and public lands, where they can spread naturally.
In one hundred years,
our grandchildren may again
see adventurous squirrels
traveling through forests of American chestnut trees.

Potential site of future American chestnut trees.
Adventurous squirrels are waiting...


For chestnuts American and Chinese,
Lord, we thank thee.
For miniature porcupines on the forest floor
and the joy that accompanies hope,
Lord, we thank thee.
For grandmothers who share the good news
and grandfathers who come to faith very late in life,
Lord, we thank thee.
For creation and recreation and restoration,
for what has been and what will be,
Lord, we thank thee.
For minds that imagine and minds that organize,
for left brains and right brains working together,
Lord, we thank thee.
For Dr. Powell and the shoulders on which he stands,
Lord, we thank thee.
For patient colleagues and plastic bags,
Lord, we thank thee.
For what we know about fungi and bacteria and gene transfer
and all that we do not yet know,
and for the glorious coming day when all will be known,
Lord, we thank thee.

Bernard H. Rensel and Maud Waugaman Rensel,
original tour guides to the ancient American chestnut

This essay is dedicated to the memory of Fern Allen Hart, who studied Castanea dentata with me.


Joanie Kissell said...

Excellent essay, Sue, with your usual charm and down home warmth and with a wealth of knowledge. Thanks for sharing! And yes, you should have won the prize!

Emmett Hoops said...

Beautifully written, sincerely felt. Thanks for posting this.