William Bartram describes his encounter with Franklinia alatmaha, growing in the wilds of Georgia along the Altamaha River in the late 1700’s.
Thus, when William Bartram came through Georgia 250 years ago, the Franklinia alatmaha grew on the banks of the Altamaha River.
At the same time, collecting exotic plants was all the rage in Europe, a fad that William’s father, John Bartram made a livelihood from. Shipment of these exotics around the world eventually brought an oomycyte, or water mold, phytophthora cinnamomi to the Southeastern United States. P. cinnamomi may have originated in Southeast Asia, but after colonizing agricultural sites degraded by cotton, it naturalized in Southeastern forests, killing susceptible species. Infrequently mentioned in articles online, one of those susceptible trees is Franklinia alatmaha, which while extinct in the wild, still survives. But where?
Well right here in Colbert, GA on my front porch! And up in Clayton, GA and Highlands, NC for a few others. Yet all these came from a single source, Bartram’s Garden in Philadelphia.
Thus does this source in Bartram’s Garden have enough genetic diversity to grow a cultivar that can resist the deadly seductions of phytophthora?
That’s what PhD. student, Heather Gladfelter is figuring out for her thesis. She has two powerful tools at her disposal. In laymen’s terms they are cloning and genetic engineering. In scientific terms, cloning is somatic embryogenesis, and genetic engineering is transformation. On Tuesday November 17, 2015 Heather generously walked me through the greenhouse and labs, explaining the processes in great detail.
Heather has decided not to use transformation to modify the genetics of the Franklin Tree to resist phythophthora, as she wishes to preserve the species. However she refers to somatic embryogenesis as her Holy Grail. The process is complex, but perhaps not as difficult as other processes, because the embryo grows into an entire plant, with leaves and roots. Heather can get leaves to grow from cells in the plant, but getting the leaves to convert to a plant with roots proves difficult, depending on the species, and it adds a step to the process.
As I wandered through the outdoor greenhouse with the Franklinias showing their fall colors, to the enclosed greenhouse with powerful LED lights, to the lab with petri dishes and microscopes, I wondered why Heather didn’t start at the beginning, but as I sit to write this, I realize its a chicken and egg issue, and why not start with the whole plant?
Without the plant, somatic embryogenesis would not be possible. There would be no starting point. The Franklin Tree, all grown up, blossoming, attracting bees, ants, flies, even butterflies to its sweet nectar, pollinates. Did it pollinate itself? Perhaps. Or perhaps those insects carried some other Franklin tree’s pollen to the blossom. In any case, the following year, instead of bearing blossoms, the Franklin Tree bears fruit.
The fruit holds the key to somatic embryogenesis. But when to harvest? Ripe in August, for the right stage of development, the fruit must be harvested early. But how early? That’s what Heather hopes to determine in the 2016 harvest.
She knows what to look for. An invisible plant embryo. If she cuts open the seed in the unripe fruit, puts it under a microscope, and sees an embryo, then all bets are off. However, if she cuts open the seed, puts it under the microscope, and does not see an embryo, then she knows one of 2 things has occurred. Either the seed didn’t get fertilized, and thus didn’t form a plant embryo, or the fruit was harvested at just the right stage of development for somatic embryogenesis, or perhaps I should say non-development. Because the embryo Heather looks for has yet to go down a developmental pathway. Thus she can use growth regulators on tissue culture to direct its development.
Hoping to harvest the not-yet-visible embryonic cells, Heather plates them on media that contains an herbicide, 24D, which at very low levels works as a growth regulator. Each cell grows a callous, like a scab, from the insult. These callouses are then placed in a liquid bath of low level 24D. Gently shaken in the dark for days, tiny specks become visible in the bath. These are proembroygenic masses, or PEM’s.
These masses are place on a nylon filter, and the 24D washed off. No additional growth regulators are needed from this point forward. The PEMs, left in darkness, becomes an embryo.
The embryos are harvested and plated in rows on nutrient media. Next, the embryos sit in a cold, dark place, called stratification, simulating Winter, before they are brought out of the cold to germinate. Germination, length of time in cold, or whether placed in the cold, all vary with species and thus the process has to be tweaked each time a new species enters the mix.
Not all embryos are created equal however. Some germinate, growing new leaves, but never convert to a plant by growing roots. Some shrivel up and die. And some thrive like the chestnut below, which proceeded to become a plant in a covered greenhouse, with LED lights. Will the Franklinia ever survive placement in the phytophthora cinnamomi infested soil of the Southeast?
That remains to be seen…
Many thanks to Heather Gladfelter who led me on a tour of the greenhouses and labs and explained the science to me. Any mistakes wholly mine.
*Opening stanza from “The Slacks” by Trip Shakespeare.