By Kevin McFadden
Imagine a happy place where you’re laying out in the Caribbean sunshine on a beach with a mojito in hand. You jump into the crystal clear blue bathwater of the sea, then you lay down in your chair under an umbrella and doze off thinking about delicate and beautiful New England winters… you wake up and look over at a little saltbush that hasn’t been consuming delicious beverages, cooling off in the sea or taken cover under an umbrella to take a nap. In fact, this poor plant hasn’t moved an inch. First off, let’s hold the phone here. The truth is, this plant is loving life and it took over 3 billion years of evolution for it to become so happy! Saltbush is one of many plants who has adapted to warm, sunny, and dry conditions.
Long ago, when atmospheric carbon dioxide levels were high and oxygen levels were low, an enzyme called Rubisco started being made in plants which is an important driver in the Calvin cycle, without which food can not be made. Atmospheric carbon dioxide and oxygen levels started to fluctuate and before plants could say “ribulose-1,5-biphosphate carboxylase”, carbon dioxide levels were low and oxygen levels were high. A combination of high oxygen levels and warmer, sunnier, and dryer conditions drove Rubisco to start making toxic products instead of what it was supposed to. This drove plants to have to go through photorespiration fixing this terrible mistake Rubisco had made which required lots of carbon dioxide and energy. These plants are known as C3 plants. Although some C3 plants like palm trees can cope with this terrible phenomenon, most C3 plants can lose efficiency in productivity of up to 40% in warm, sunny, and dry conditions!
Some plants, such as saltbush have evolved a series of anatomical and biochemical modifications to this typical C3 pathway, increasing their productivity in these outrageous conditions—these plants are called C4 plants. This C4 pathway didn’t originate from one source however, but it evolved independently over 45 times in 19 families of flowering plants! Although there is much variation among the strategies of these independently evolved C4 plants, they all agreed upon needing to concentrate carbon dioxide around Rubisco so oxygen couldn’t approach it and make it do terrible things. C4 photosynthesis is responsible for about two slices of pie (one quarter) of the primary productivity on the planet, a large fraction of which humans consume either directly by plant material or indirectly by animal products. Because C4 plants are so awesome, and because us scientists feel the need to know absolutely everything, it’s important for us to know the history of this C3-C4 split.
The first plants to catch on to having to make these changes were grasses most likely in the Oligocene epoch between 24 and 35 million years ago, but how did we figure out that they were C4 plants? One of the most popular approaches to putting puzzles like this one together is to look at C4 plants now and to look for the same anatomical features of other plants in the paleo-record. We know that C4 photosynthesis requires the modification of C3 leaf structure to form the inner compartment where Rubisco is localized and where carbon dioxide can be concentrated. This results in a wreath like arrangement of palisade mesophyll around bundle-sheath cells where in C3 plants palisade mesophyll is found in a single layer in the upper part of the leaf (Figure 1). With this method scientists were able to track down C4 plants from about 12.5 million years ago! Molecular clock analyses can also be done where they can look at gene sequences in fossils and compare them to extant C4 species. By doing this, scientists have been able to conclude that C4 plants arose between 20-30 million years ago!
C4 plants and the future: Believe it or not, sooner or later, we will experience another Ice Age. Imagining carbon dioxide levels will decrease with the Ice Age like they have always done in the paleo record, some interesting things are going to happen with the recent rise in C4 dicots. It’s believed that we will observe the evolution of C4 forests! Forests are a major carbon sink and with the future low densities of atmospheric carbon dioxide, C3 forests will be at risk of major decline and future C4 forests could eliminate them completely. This will mark major changes in the nature of the biosphere… get excited!
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Pascal-Antoine, C, Osborne, CP, Chatelet, DS, Columbus, JT, Besnard, G, Hodkinson, TR, Garrison, LM, Vorontsova, MS & EJ Edwards (2012) Anatomical enablers and the evolution of C4 photosynthesis in grasses. PNAS 110: 1381-1386
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