All five of my Darwinian Agriculture lectures at the International Rice Research Institute are now available on YouTube. My talks were prepared in advance, so I was only able to incorporate a small fraction of the interesting things I learned during my visit.
My last talk discussed the tendency (not necessarily by scientists themselves) to exaggerate research progress. For example:
"The researchers have already... successfully introduced 10 out of the 13 genes needed for C4 rice."
-- Rice Today, January-March 2013, p. 5
Wheat, rice, soybean and tomato use C3 photosynthesis, named for the number of carbon atoms in the first product of photosynthesis. Maize ("corn" in the US) and sugar-cane use C4 photosynthesis. In hot climates, C4 photosynthesis can support higher rates of crop growth, using less water.
My book (p. 62) uses C4 photosynthesis as an example of "something that may have been easy for natural selection (given millions of years) [but] extremely difficult for humans." So I was surprised to learn, before arriving at IRRI, that C4 photosynthesis only needs 13 genes and that they have already transferred 10 of them. Maybe "skeptical" would be a better word.
I should have asked about this when I met with Paul Quick, who is leading the C4 rice project at IRRI. I'm guessing that he told the magazine that they've identified 13 key genes so far, and transferred 10 of them. My impression, from our discussions, is that they don't yet know the total number of genes they will need to transfer.
They have a lot of smart people, at IRRI and around the world, collaborating on C4 research. C4 rice will need :
* Some way to pump CO2 into bundle sheath cells around the leaf veins, from adjacent mesophyll cells.
* A diffusion barrier around the bundle sheath cells to keep the CO2 from leaking out again.
* More photosynthetic chloroplasts in the bundle sheath cells than rice has now.
* Ideally, closer vein spacing. The assumption is that CO2 can't be pumped very far, so if veins are widely spaced, only a fraction of the leaf will have C4 photosynthesis. But Paul Quick told me that corn husks have C4 photosynthesis throughout the leaf, despite widely-spaced veins. Interesting.
They seem to have made considerable progress on most of the above. I don't think he mentioned any progress on the diffusion barrier, though, which seems more critical than vein spacing, at least to me.
One of the clever approaches they are using is to knock out genes in a C4 plant, at random, to see which of them are essential to C4 photosynthesis. How do they tell if they've knocked out C4? Because of their CO2-concentrating mechanism, C4 plants can survive at much lower CO2 concentrations than C3 plants can. So they grow the random-knockout plant population at 15 ppm CO2 -- the atmosphere is 390 ppm and rising -- and look for plants that don't grow. Sounds like cruelty to plants, but they rescue them before they die, by transferring them to a high-CO2 tent. They're also drawing on IRRI's huge (100,000 genotype) collection of rice varieties and rice's wild relatives.
I don't know if they'll succeed, but this seems like a reasonable test of our current ability to improve complex traits in crops. At a minimum, they should get a lot of useful information about photosynthesis, leaf structure, the evolution of complex traits, etc. This information could have applications beyond improving photosynthesis. For example, the ability to develop crops with wider or narrower vein spacing would have applications in developing more-digestible crop leaves (for cows or for biofuel production). Vein spacing may also affect drought tolerance. Whether spending the same amount of money on other kinds of agricultural research would make more sense is a more-complex question. But the Gates Foundation is funding this "high-risk, high-potential-reward" research, so it doesn't come at the expense of their other work.
For more information, see IRRI's C4 rice page. For an interesting history of the project, see this video interview with John Sheehy, former head of the C4 rice project, who back visiting IRRI the same week I was there talking about Darwinian Agriculture.