Please indulge this quibble on a quibble

I don't buy the botanist's assumption that all the plants grown at the epilogue grew from that single plant.

A perfectly reasonable alternative interpretation was that the plant provided evidence that it was possible to grow plants (and animals and fungi) back on earth and that it was not the only source of genetic materials. One could reasonably assume that the spaceship had some sort of "ark" going on with the properly flora/fauna/fungi to produce/restore an ecosystem.

-Ari

Your interpretation is more than reasonable. It is supported by the captain saying, "Life is sustainable now." The film never claimed that the single plant gave way to farming on earth, just that Earth had become habitable again.

I find it hard to believe that the Slate author missed this.

Thank you for pointing this out so that I don't have to. I thought it was incredibly obvious as I was watching that the plant was merely an indicator of habitability. The writer even advocates seed banks, yet it doesn't cross her mind that they might have one in the movie.