In C4 plants, such an anatomical arrangement facilitates a more efficient transfer and processing of CO2 in the bundle sheath cells when CO2 is in relatively short supply. When temperatures get hot or plants become stressed, they stop taking in as much CO2, creating just such a shortage within the leaf.
So PACMADs as a group had developed an anatomical predisposition to C4 photosynthesis that BEP grasses didn't, said senior author Erika Edwards, an assistant professor of ecology and evolutionary biology at Brown.
"We found that consistently these PACMAD C3s are very different anatomically than the C3 BEPs," she said. "We think that was the evolutionary stepping stone to C4-like physiology."
When the new leaves turned over
It didn't used to be this way. Back around 60 or so million years ago, BEP and PACMAD grasses were more similar and both headed in the same direction. The distance between the leaf veins in both clades had been growing closer together. But then they started to diverge in a key way. The bundle sheath cells surrounding the veins in BEP grasses started to shrink down while those in PACMAD grasses stayed larger.
For a long time the climate didn't particularly punish or reward either of those directions. But then climate changed, and opportunity knocked, Edwards said. Only PACMAD was near the proverbial door.
"When atmospheric CO2 decreased tens of millions of years after the split of the BEP and PACMAD clades, a combination of shorter [distances between veins] and large [sheath] cells existed only in members of the PACMAD clade, limiting C4 evolution to this lineage," Christin and co-authors wrote in the paper.
The researchers also found that evolution among C4 grasses was anatomically nuanced. Some C4 grasses evolved because of advantageous changes in outer sheath cells, whil
|Contact: David Orenstein|