Scarecrows and Wreaths: Genetic Secrets of Efficient Food Crops

Scarecrows and Wreaths: Genetic Secrets of Efficient Food Crops

 

• Ancient plants, like rice, wheat and barley, originating in the Mesozoic and Paleozoic eras, still form 95% of the Earth’s plant biomass. They use an enzyme known as RuBisCo (the most abundant protein on the planet!) to fix atmospheric carbon dioxide on to a 5-carbon sugar (ribulose bis-phosphate) to make 2 molecules of a 3-carbon sugar that eventually becomes sucrose. This is the C3 pathway, but it’s not too efficient: the enzyme RuBisCo also catalyzes a competing reaction called “photorespiration” that adds oxygen to the 5-carbon sugar making a byproduct that takes many tedious and expensive steps to convert back to the useful sugar. These plants can also lose 97% of the water absorbed by the roots through stomata or pores on the underside of the leaves. If they close their stomata, they limit the diffusion of CO2 into leaves, so they have limited growth in hot, dry areas.

• Fortunately, in the last 6-7 million years, another group of plants (sugarcane, maize, grasses) began to flourish that bypassed this problem. They evolved from the C3 plants independently, more than 60 times- a spectacular example of convergent evolution.  In these plants, a different enzyme is used to fix CO2 to make a 4-carbon sugar in the leaf cells, that is then shuttled into special wreath-like layer around the veins, known as Kranz sheath (German for wreath).  Kranz cells release CO2 from this intermediate, insulating and concentrating it around the Rubisco enzyme so that the wasteful side reaction does not occur. This highly effective C4 pathway boosts productivity by 50%. Even though C4 plants make up only 3% of plant species, they account for 30% of all carbon fixation on land.

• How does one coax C3 plants to follow C4 pathways and boost food production in hot, dry areas, while removing more CO2 from the atmosphere? C3 plants have all the enzymes needed, but lack the specialized anatomy of the wreaths and the tight spacing between veins. It was assumed that engineering Kranz anatomy would be exceptionally difficult. In a breakthrough study, scientists noted common features of the Kranz sheath with root and stem bundles, suggesting a common developmental pathway. Working on a hunch, they showed that a gene called Scarecrow, regulates the special anatomy in both roots and leaves.  “Recapitulating the evolution of C4 structure in C3 plants is likely to be a much more manageable goal if the underlying regulatory components are already in place in roots and stems”.

Image: Kranz anatomy in French Millet, a C4 plant. Note the bundle sheath, packed with green chloroplasts, around the central vein, and the tight spacing of less than 4 cells between the bundles. http://goo.gl/J004P

 

Read More: http://www.news.cornell.edu/stories/Jan13/Scarecrow.html

http://en.wikipedia.org/wiki/C4_carbon_fixation

Paper: Scarecrow plays a role in establishing Kranz anatomy in maize leaves. Slewinsky, T.L., et al. Plant Cell Physiol. 2012 Dec;53:2030-7. doi: 10.1093/pcp/pcs147.

#ScienceEveryday when it’s not #ScienceSunday .

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108 Responses to Scarecrows and Wreaths: Genetic Secrets of Efficient Food Crops

  1. Kevan Hayes says:


    Wow, this is really cool! Thanks for the post


  2. Very interesting. I am typically looking for good cellulosic sources of C3 sugars for our biochemistry projects, but it’s interesting to think about overall plant efficiency in these ways.

  3. Rajini Rao says:


    I didn’t know of your interest in C3 sugars, Jeffrey J Davis . Both types of plants use the C3 sugars, but the C4 pathway temporarily converts pyruvate to malate (a 4 carbon sugar) as a shuttle. 


  4. If I remember well, sugars from C3 and C4 plants present different isotopic composition so you can determine their origin.

  5. Rajini Rao says:


    Zephyr López Cervilla yes, they are. But it’s complicated..feel free to explain 🙂


    http://en.wikipedia.org/wiki/Isotopic_signature

  6. Rajini Rao says:


    Re. isotopes, this is the interesting part: “The different isotope ratios for the two kinds of plants propagate through the food chain, thus it is possible to determine if the principal diet of a human or an animal consists primarily of C3 plants (rice, wheat, soybeans, potatoes) or C4 plants (corn, or corn-fed beef) by isotope analysis of their flesh and bone collagen.”


  7. We search for viable routes to replace propylene as a feedstock for superabsorbent polymers

  8. Rajini Rao says:


    I see, such as acrylic acid in baby diapers and other hygiene products.


  9. Exactly, Bio -AA is the holy grail for us

  10. Sandi D says:


    Rajini Rao really enjoy your post

  11. Kevan Hayes says:


    Roelf Renkema Maybe she meant “plant food biomass”

  12. Rajini Rao says:


    Yes, I did mean plant food biomass, thanks for that Kevan Hayes . Although, now I’m curious Roelf Renkema ! Do bacteria really constitute a huge biomass? I’m off to read your link.


  13. Roelf Renkema, perhaps bacteria is in the form of living biomass whereas most biomass produced by plants isn’t forming part of living cells, and a significant fraction of it is decaying leaves and wood.

  14. Rajini Rao says:


    Edited to clarify plant biomass, thanks! C3 plants “still represent approximately 95% of Earth’s plant biomass” from Wiki on C3 plants.

  15. Robert Moser says:


    I would not have even begun to think that coaxing the preferred pathway would be possible, considering the complexities involved.  Very encouraging.  

  16. Rajini Rao says:


    Well, I learned something about bacterial biomass from you, Roelf Renkema 🙂

  17. Rajini Rao says:


    Robert Moser there is a large global initiative to make rice more efficient via the C4 pathway. They chose rice because 50% of the world’s population subsists on rice. http://irri.org/index.php?option=com_k2&view=item&id=10435:supercharging-the-rice-engine&lang=en

  18. Robert Moser says:


    Makes sense.  If C4 Golden Rice were available, it would be a massive public health boon.

  19. Rajini Rao says:


    Bacteria are great as factories to produce biomolecules, clean up waste or produce fuel, but we need photosynthesis (green plants) to fix carbon from the air into carbohydrates (sugars). We animals are heterotrophs and can’t do that!  Algae would be great, since we can farm them in the open ocean.

  20. Rajini Rao says:


    Now that is a story that impinges on science fiction. Asimov would have loved it! Imagine a photosynthetic layer in our skin..I wouldn’t have to cook dinner tonight 🙂


  21. 90%? a bit overrated, specially considering that corn is the crop with largest global yields (granted, quite often used to feed farm animals rather than people) but it’s also the staple in most of the American countries and Africa, whereas in Europe and Mid East that place is occupied by wheat (and the yellow river basin in China). Nevertheless, rice is still the most consumed grain by humans over corn and wheat, but not by the 90% of the world population.

  22. Rajini Rao says:


    I fixed that already, Zephyr López Cervilla . 3 billion people depend on rice (90% of the population of Asia), so it is more like 50%.

  23. Kevin Clift says:


    You may have already seen this Jeffrey J Davis


    Once the polymer is dissolved, warming it causes the tails to squeeze water molecules away and form links with neighbouring polymer strands. Above a certain temperature, the solution transforms into a gel in seconds as the strands self-assemble into bundles roughly 10 nanometres wide. As with the biopolymers in a living cell, or the fibres in a rope, the bundling stiffens the whole structure. “The nanoscale mechanism is the same as at the macroscale,” says Rowan. http://www.nature.com/news/polymer-can-turn-swimming-pool-to-jelly-1.12275


  24. Rajini Rao I guess the next step is to transform and overexpress this gene in rice. Surely won’t be that easy with many phenotypic abnormalities but it could give some clues as to a direction forward. Then as cell type specific promoters become available and usable you could express it only in the right cells around the vascular tissue.


  25. Kevin Clift I had not. Very interesting and potentially useful for me. Was in a global r&d meeting in Germany all this week with my guys, and I can think of at least one problem we discussed which could benefit from this.

  26. Rajini Rao says:


    David Nicholl yes, that’s exactly right. The hopeful part is that the Scarecrow gene is already driving development in the root and shoot of rice and other C3 plants. It would need to be expressed in the leaf vasculature, presumably under control of the same signals found in C4 plants. Before this, there were many attempts to get to the differences between the two types of plants without much success.


  27. Awesome post! I have vague memories of learning this during undergrad biochem 😛

  28. Rajini Rao says:


    I had to quickly re-learn this again, Buddhini Samarasinghe . It is horrendously complex biochemistry. I’m grateful to all the nice people who have bothered to read through this 😀

  29. Bill Collins says:


    It’s fascinating that the older plants are more digestible and valuable to us. I wonder if that’s a byproduct of evolution? (By valuable, I mean that grains have less overall sugar, are easier to digest/cook/obtain, etc.)

  30. Rajini Rao says:


    Bill Collins , when I put together the post on gluten allergies, I learned that wheat and other ancient crops have been bred to produce more protein/nutrients by selecting polyploids…multiple sets of chromosomes. This also increased gluten, which is essentially indigestible to everyone and allergic to some. So plant breeding can be a double edged sword.

  31. John Bump says:


    Bill Collins — either I’m reading you backwards or you’re reading this backwards.  Grains are grasses, which are C4 plants, the newer ones, right?

  32. Bill Collins says:


    Sorry, yes. Grasses. Duh.

  33. Rajini Rao says:


    “Of the monocot clades containing C4 plants, the grass (Poaceae) species use the C4 photosynthetic pathway most. Forty-six percent of grasses are C4 and together account for 61% of C4 species. These include the food crops maize, sugar cane, millet, and sorghum.” All the other cereal grains (wheat, rice, etc.) use the C3 pathway.

  34. John Bump says:


    Huh — I didn’t know there were any cereal grains that used C3.  It’s been waaaay too many years since I took plant metabolism.

  35. Rajini Rao says:


    John Bump


    me too 🙂 The generalization is that all cool season cereals like wheat, oats, rye, barley, triticale are C3. C4 evolved in warm season grasses, with the notable exception of rice.


  36. Wow!


    “C4 pathway boosts productivity 50%”!

  37. Arun Shroff says:


    Fascinating stuff! Although curious after reading about Scarecrow –   have scientists switched from using exotic Latin/Greek words to common words and pop culture references to name their new discoveries ? If so I am all for it 🙂 

  38. Rajini Rao says:


    David Andrews what is cool is that the C4 pathway costs more in the short term and is more complicated: “the C3 pathway requires18 molecules of ATP for the synthesis of one molecule of glucose, whereas the C4 pathway requires 30 molecules of ATP. This energy debt is more than paid for by avoiding losing more than half of photosynthetic carbon in photorespiration as occurs in some tropical plants, making it an adaptive mechanism for minimizing the loss.” Also, Wiki mentions the huge number of water molecules lost in the C3 pathway, although I admit that I can’t trace the reactions they originate from 😀

  39. Rajini Rao says:


    Arun Shroff , gene names can be a lot of fun and a way for scientists to channel their humor. They vary depending on species and on the discoverer. Fly gene names are particularly funny. Buddhini Samarasinghe had a post on this that you will enjoy: http://goo.gl/UNfFk

  40. Rajini Rao says:


    Roelf Renkema , yes, Chlamy is an awesome organism! But it is an algal cell, not a bacterium. It’s also a eukaryote, like yeast, so a lot more sophisticated than a bacterium which is a prokaryote. I get what you mean: I think you are saying that single celled microorganisms have great potential. I agree.


  41. Rajini Rao Thank you for the explaination. Great post, very educational as usual.

  42. Arun Shroff says:


    Rajini Rao Thanks – I did enjoy that link. Very creative indeed! I had no idea that names for genes allow scientists to go wild and bring out  their inner poet/punster! Is it just genes or other areas of science are benefiting from this novel idea as well?


  43. Rajini Rao: “Wiki mentions the huge number of water molecules lost in the C3 pathway, although I admit that I can’t trace the reactions they originate from”


    – The reduced water loss is because of the much higher affinity to CO2 rate of enzymatic activity of the pyruvate phosphate dikinase (PPDK) than the RuBisCO, what allows the plant to keep its stomata almost closed all day (since it has already enough supply of CO2) thus losing less water.

  44. Rajini Rao says:


    I’m going to revisit Asimov and photosynthesis, Roelf Renkema 🙂


    Arun Shroff , wondering what else we scientists should admit too without completely destroying our gravitas 😉 There are funny names of chemicals and molecules, for example: http://www.chm.bris.ac.uk/sillymolecules/sillymols.htm

  45. Rajini Rao says:


    Zephyr López Cervilla , that does make sense in general but the numbers can’t be that precise. “When grown in the same environment, at 30°C, C3 grasses lose approximately 833 molecules of water per CO2 molecule that is fixed, whereas C4 grasses lose only 277 water molecules per CO2 molecule fixed.”


  46. I see what you mean. Still I think that the difference in water loss isn’t directly related to water consumed in some enzymatic reaction considering that the substrates (CO2, H2O) and end products (sugars, O2) of the C3 and C4 pathways are the same. Some water is hydrolyzed in the light phase of the photosynthesis, but the downstream efficiency of that photolysis depends on many factors and if I remember well the number of H2O molecules hydrolyzed isn’t two orders of magnitude greater than the number of CO2 molecules fixed.

  47. Arun Shroff says:


    Rajini Rao Wow, I had no idea chemists had such a naughty sense of humor – some of those molecule names are hilarious and some are  quite – shall we just say NSFW  🙂  

  48. Rajini Rao says:


    Haha, Arun Shroff , I think I’ll save the naughty molecular structures for another day 😀

  49. Rajini Rao says:


    Zephyr López Cervilla I was wondering if that water consumption was from photorespiration and the salvage pathway for phosphoglycolate. It does generate H2O2, presumably from H2O.

  50. Jon Hiller says:


    The C4 bond in the materials world is quite a strong bond, similar to that of diamond if I’m reading this right.  So, what is the breakdown or biodegradability of C4 compared to the C3?  If they account for 30% of all carbon fixation they have a very high carbon footprint.


  51. Wishing you All Happy Republic Day….2013

  52. Rajini Rao says:


    Jon Hiller , the C3/C4 nomenclature is a biochemical shorthand for the number of carbons in the compound, not the bonds (they are all single or double bonded in chemistry). Glucose, a 6-carbon sugar is made by a cyclic process in which carbon atoms are added by attaching one carbon, in the form of CO2, to a recipient 5 carbon sugar  (they form 2 x 3-carbon sugars, from which the term C3 comes). The C4 plants add CO2 to a 3-carbon compound to make a 4-carbon intermediate. Sorry about the confusion.

  53. Rajini Rao says:


    harshad yagnik , thank you. Happy 64th Republic Day celebrating the Constitution of the largest democracy!

  54. Jon Hiller says:


    Thanks for the clarification I get it now. I thought it referred to the bonds. Just shows how we need to get our nomenclatures together across scientific fields.


  55. If the C4 is more efficient for the plant in most conditions, and seems easy to evolve (as it has been reinvented many times), should it not have swamped out C3. Has it survived  just because we have frozen it in cultivated varieties, perhaps with a starting point from conditions where C3 is better?

  56. Rajini Rao says:


    That’s a good point, Siddhartha Gadgil , why are do they constitute only 3% of the species? This report indicates that they are even more ancient than once thought: http://www.sciencedaily.com/releases/2010/11/101115142007.htm


     

  57. Rajini Rao says:


    From what I’ve been able to gather, C4 evolution required whole gene duplications from C3 plants more than once. There appears to be “quite a long time-lag between the gene duplication events and the appearance of C4 grasses. These findings suggest a long transition process, including different modes of functional innovation, before the eventual establishment of C4 photosynthesis.”


    http://genomebiology.com/2009/10/6/R68

  58. Eric Hopper says:


    What I suspect is that this extra carbon capture doesn’t actually do the plant a lot of good. It doesn’t make it especially more fit. If the plant built a whole ton of cellulose and extra leaves with the carbon then it would have a growth advantage. But the various C4 species mentioned are not really extremely fast-growing plants.


    I wonder if blackberries or bamboo is a C4 species?

  59. Rajini Rao says:


    Bamboo is C3 species. It’s not the rate of growth, but the ability of C4 plants to survive under poor conditions: high temperature, less water, CO2 limitation. These are found in grasslands where the grasses have successfully colonized. C3 plants grow fine in moderate conditions, until they are challenged.


  60. Eric Hopper: “But the various C4 species mentioned are not really extremely fast-growing plants.”


    – Corn is a C4 and is the most productive crop in the world (in warm and sunny climates they can produce several yields a year). Sugar cane, sorghum and millet are also very productive.

  61. Eric Hopper says:


    Zephyr López Cervilla – Thank you! I was worried I was wrong. Hmm, well that shoots that theory.


  62. something to stop global warming


  63. And remember when we are talking about fitness there is a big difference between a wild plant surviving and reproducing and a cultivated crop where humans specifically mold certain characteristics and it’s fitness depends on how much we plant it.


  64. Even at its best, photosynthesis is not that efficient.  Yields could be higher (in theory) using photovoltaics to generate hydrogen and then use bacteria go convert H2 + CO2 into organic molecules.  Maybe not for food, but for fuel this could be effective. 


    Grasses also suffer predation from herbivores, so they require fast regrowth.  Only other plants that can regrow as fast could compete with them under that kind of pressure. 


  65. wow, great agriculture evolution


  66. Silly question: In theory, can you just grow huge swaths of C4 plants and make an efficient CO2 sink to compensate for the CO2 being released from trapped ice and emissions? Or are these happening on completely different scales?

  67. Rajini Rao says:


    I don’t think that’s a silly question at all. It’s called biosequestration (http://en.wikipedia.org/wiki/Biosequestration) and carbon fixing plants are probably the easiest way to remove atmospheric carbon dioxide. After all, the evidence from fossil fuels shows that this has happened on a massive scale in the past. C4 plants like sugarcane or grasses would work best. 


  68. Great post Rajini Rao! This is my subject – my PhD is on C4 and my lab works on understanding C4 evolution and how it’s controlled at the molecular level.


    Sum an C4 plants are indeed optimum for bioseqeustration in most of the world. Miscanthus is the C4 grass used most often. Papyrus too.


    David Nicholl we’re working on the C4 rice project – putting C4 into rice. Scarecrow has long been suspected to be involved in krantz anatomy, but it’s just one tiny piece in a puzzle that involves hundreds of genes. We are constantly generating and refining lists of candidate genes to test.


    For those discussing the greater water use efficiency of C4 plants; the most productive C4 plants are on average about 50% more water efficient than the most efficient C3 and this derives from the fact that, because CO2 is concentrated around RuBisCO, less CO2 uptake is required so the stomata can remain closed more of the time. Thus there is less transpiration.


    Because C4 plants need much less RuBisCO than C3 plants, and RuBisCO is the most abundant protein in most plants, C4 plants are also ~50% more efficient in their use of nitrogen, so they need less fertiliser.

  69. Rajini Rao says:


    Richard Smith I was wondering if I should have pinged you on this post, given my rusty memories of plant biochemistry! Delighted to hear that you work on this topic, I had no idea. Putting this post together, I had fun looking at pretty images of vascular bundles on the web and figuring out whether they were C3 or C4 by counting the number of cells between bundles 😀


    Also, thanks for explaining the nitrogen connection..I was wondering where that came from.


    The one question we got stuck on, was why C4 plants do not make up a larger biomass than 5% when they are noticeably more energy efficient. Shouldn’t this be a selective advantage (they have been around for some 30 million years so they’ve had time to expand).


  70. Rajini Rao actually C4 plants account for ~40% of terrestrial biomass (despite being found in only 4% of species), and close to 100% in biomes where photorespiration is a limiting factor, such as in high temp high light environments. I’m out atm but can provide references later.


  71. Richard Smith: “C4 plants account for ~40% of terrestrial biomass”


    – Have been human crops included in that 40%? (if I had to guess I’d say they have been so perhaps C4 plants aren’t so abundant in the wild.) Is there any tree species among the C4 plants?


    I come up with a few other questions that you may know and be in the mood to answer:


    1. Since C4 plants seem to have appeared independently several times over evolution, which C4 group appeared most recently and in which period? Is it the most widespread of them?


    2. Were there some particular environmental conditions during that period that could have favored their emergence, evolution and dissemination (warmer global or regional temperatures, lower CO2 atmospheric concentration, less abundant rainfall, more common drought, etc.)? 


    3. Is there any candidate in the fossil record of an ancient group of C4 plants that went extinct?


    4. As for the gas exchange through stomata, could carbonated water (with disolved CO2 / H2CO3) in the soil reduce the volume of water required for the Calvin Cycle/Dark Phase of the photosynthesis? If there’s usually some contribution to the CO fixation from the CO2 disolved in the soil, what is the approximate percentage of that contribution?

  72. Jim Carver says:


    As far as CO2 sequestration is concerned I think trees are much more efficient at converting atmospheric CO2 into biomass.  Grasses and the like tend to cycle rather quickly and are at best usually neutral. If you take the trees and convert them into bio-char you can use it as an amendment to improve soil structure and increase yields. The residency time for this carbon is on the order of 100s of years and seems to be the only practical way I know of to lock up carbon for any substantial length of time.


    Of course you could always grow C4 plants in hot areas and make bio-char that way also.

  73. Jim Carver says:


    Anyone interested in the bio-char initiative can visit the IBI website and of course google around, there is a lot of information out there and I have posted quite a few things on here myself.


    http://www.biochar-international.org/

  74. Rajini Rao says:


    Thanks for the link, Jim Carver . This is good stuff! Didn’t you also have a post on the global rice initiative at some point?


    Siromi Samarasinghe may be interested in the Tea plantation project of Biochar in Sri Lanka: http://www.biochar-international.org/profiles/Sri_Lanka

  75. Jim Carver says:


    Rajini Rao Thanks, yeah pretty sure I did. I’ll see if I can dredge that one up.


    Bio-char is a fun project if you’re a pyro like me. (hehe). Probably the easiest way to do it in the backyard is take two barrels (steel, of course), one smaller than the other so it fits inside. Fill the smaller one up with biomass, say chunks of wood, leaves, lawn clippings, whatever. Place the larger barrel over the top and invert the whole thing. Start a fire in the space between the inner and outer barrels.


    When the thing catches you will know it as alcohols etc. light up. This is a destructive distillation. When your homemade jet engine shuts down, you will know it and just spray some water on it to cool it down. There is a critical temperature where it should reach for best results. I would have to look that up. But marginal char is better than no char at all.


    In a production situation this could be a self-sustaining process requiring no input from fossil fuels.


    Edit: Btw, make sure the biomass is dry.

  76. Rajini Rao says:


    I once set fire to my compost heap! Little did I know that I could have been making Biochar. I had one of those rotary barrels behind the house on top of the hill. It was full of the usual grass clippings and leaves when I added some spent charcoal from a small Hibachi grill. I didn’t know that there were live embers in there still. So a day later, my daughter looks out to see the whole thing burning merrily. I called 911 and they sent our local (all-volunteer) fire department consisting of a nice old lady and a teenage boy. Somehow we managed between us to not set the house on fire 🙂


  77. you’re lucky you didn’t have a methane explosion!!

  78. Jim Carver says:


    Haha! That’s funny! (Well since it turned out okay.) That reminds me of this story from a guy in Australia. He had heard it’s a good idea to burn your Pampas grass in the dormant season to reinvigorate it. Well, he didn’t realize how much oil is in that stuff and it was rather close to his house. The flames shot up 12 feet or more and started his house on fire. Nobody was hurt and they came and got the fire put out. 🙂


    I’ve had some scary instances when we would burn the weeds in the irrigation ditches in Spring. Some of those weeds have a lot of energy in them and it’s a flash fire. I’m thinking we could be doing a lot more with biofuels in this area. Some of them go up like you poured gas on them.

  79. Jim Carver says:


    Say, I wonder…is Pampas grass C4? I bet it is…it doesn’t like cold weather at all.

  80. Rajini Rao says:


    According to Google, it is.

  81. Jim Carver says:


    Yep, looks like they are and you can grow them in Denver, (I never did.): “A great deal of research is being done on these because of their famous C4 metabolism which allows them to accrue enormous biomass in short periods of time. Miscanthus have nevertheless been practically banned in much of the eastern U.S. due to their propensity to produce colossal seed crops. I have yet to see a seedling appear in Denver outside, so I think we can grow these without guilt–except that these are the most water demanding of these larger grasses. Don’t try them in your Xeriscape!”


    http://www.botanicgardensblog.com/2013/01/24/will-the-real-pampas-grass-please-stand-up/


  82. This one is way over my head and so foreign Rajini Rao but I am reading, learning and following. 

  83. Rajini Rao says:


    Shame on me for inflicting this on my unsuspecting friends, Cheryl Ann MacDonald 🙂

  84. Jim Carver says:


    You know I did read all the comments but I can’t remember if this was mentioned. As I recall, there are certain plants that can switch back and forth between C3 and C4. If that’s been covered I apologize.

  85. Rajini Rao says:


    We have not covered it. As you know, no stone remains unturned here, so good call Jim Carver ! This paper is a great read: http://www.plantphysiol.org/content/127/4/1524.full.pdf


    The sedge grass Eleocharis uses C3 pathways when it is submerged under water but switches to C4 when it is terrestrial. The paper also mentions environmental conditions like high salt, triggering a switch between the two.

  86. Jim Carver says:


    We pretty much always surmised it was environmentally stress related, but there seems to be a variety of methods that can be employed to achieve this, for example:


    “There is no evidence that a single gene is capable of


    setting in motion the entire C4 machinery. Thus, C4


    photosynthesis appears to be a combination of independently


    inherited characteristics (Brown and Bouton,


    1993). Our understanding of the molecular basis


    of the control of C4 differentiation is still limited.”


    Nice paper Rajini Rao thanks. 🙂

  87. Rajini Rao says:


    Good point that no single gene is going to switch entire pathways of metabolism, as Richard Smith said as well. Identifying one gene, such as Scarecrow is like pointing in the right direction.

  88. Jim Carver says:


    Plant hormones can have effects that aren’t easily discernible at first glance. Many complex reactions with abscisic acid (ABA), for example are just now coming to light.  (Wups!)

  89. Jim Carver says:


    Yes, yes, a pun a day will keep the doctor at bay.

  90. Jim Carver says:


    Rajini Rao Boy, you’ve got a good memory. I think it was this one from almost exactly a year ago. https://plus.google.com/u/0/110582671305643479452/posts/Vv4ifp3uJ1W
    “Codename: C4 rice project. Mission: to modify photosynthesis in rice to boost crop yields. Duration: 15 to 25 years. This might sound like science fiction, but it is already under way at the International Rice Research Institute (IRRI) in the Philippines, where William Paul Quick and his team have been working since 2008 on changing the photosynthesis process in rice from the C3 carbon-fixation mechanism, common to 98% of all plants, to its much more efficient C4 counterpart.”


    http://www.guardian.co.uk/science/2012/jan/24/scientists-boost-rice-crop-yield


  91. I love your posts, Rajini Rao.  Clearly you are one with critical thought.  Carry on!


  92. Jim Carver Rajini Rao


    Are we at the inflection point in history of mankind where inefficient processes of large variations followed by natural selection is predominantly replaced by engineered variations as proposed above? My guess is that it is still some decades away but not for too long.


    We are now modifying the some building blocks of fundamental evolutionary forces.


    On the other note which is more relevant to the post-


    How does this CO2 fixing cycle affect Nitrogen fixing cycle?


    I know that the high yield cycle as described fixes more carbon, but what about nitrogen?


    Nitrogen cycles are very critical to the atmosphere too. The balance of nitrogen to air is critical to sustaining of life.

  93. Jim Carver says:


    mandar khadilkar It looks like CAM cycles are the most important in warming environments and higher CO2 levels. I think that much is pretty clear in general. But there are certainly exceptions that totally blow that out of the water.


    It could be that this is really something to look at. Now about the nitrogen cycle? No, I wouldn’t worry. That’s fairly stable and it would take something that would kill us long before them.

  94. Rajini Rao says:


    mandar khadilkar , the C4 plants have greater photosynthetic rates per unit nitrogen than C3 plants because they don’t need as much of the enzyme Rubisco. Enzymes are proteins and all proteins are made up of nitrogen. Rubisco is a highly abundant protein, so C4 plants can get away using less nitrogen. 


    Nitrogen fixing is as important as carbon fixing…not so much to regulate levels in the air, but to convert gaseous N2 to the organic form used in amino acids (proteins), nitrogen base of DNA, etc. Ultimately all the nitrogen in our macromolecules comes from nitrogen in the air. Bacteria can fix nitrogen and make them available to roots of plants (especially legumes/dals). 


    We already affect what animals and plants flourish by breeding and farming, as well as our influence on the environment. Genetic engineering is only a tiny part of that so far. 

  95. Jim Carver says:


    I went and looked at Cannabis sativa because I couldn’t remember…but yeah, I was right; it’s C3.


    Does well in hot and fairly drought resistant. So, I was wondering…how many dicots are C4? I can only think of one off the top…okra is. Any takers? 🙂

  96. Rajini Rao says:


    The Wiki link in the post has the number of dicots that are C4 under the section “Plants that use C4 fixation”. Amaranths, daisies, sedges..

  97. Jim Carver says:


    Oh, sorry…this is another old subject that I really liked and trying to get caught up with. I got in late and never really fully caught up. Some days are like that ya know. 😉

  98. Rajini Rao says:


    The paragraph in the Wiki link was a bit long for a cut/paste so I though it would be easier to direct people to it, that’s all 🙂


  99. I see Rajini Rao. So the point you are making is N fixation is unaffected by c4 or c3?


    If that is so then, there would be no ill effect of predominant c4 based agriculture.


    I vaguely remember that oxygen reactivity in the air and thus availability of free oxygen is tightly coupled with N2 in the air. N2 allows O2 to stay as is.


    Any change is N2 in the air will make O2 combine with other elements very reactively. Decrease in N2 is linked to higher corrosion.

  100. Jim Carver says:


    mandar khadilkar Oh yeah man, this has really nothing to do with N fixation. Mostly Nitrobater spp. do that. This is just details in photosynthetic pathway(s).

  101. Rajini Rao says:


    I’m saying that C4 plants are more efficient in using nitrogen. I don’t think they directly affect N fixation. 


    N2 is inert, so you must be right about influencing the reactivity of oxygen. Although, as a biologist, I more interested in the efficiency of N fixation to make proteins and DNA/RNA. Small changes in N2 concentration in the air are secondary…there would be no organic life if N was not fixed into proteins or nucleic acid! 


  102. I’ve believed for a long time that there’s too much nitrogen in the atmosphere compared to the amount of nitrogen present in crops to make a change in the crops any significant difference.


    Besides there’re four other details that make that hypothetic scenario even more unlikely:


    1. Nitrogen is majorly fixated by microorganisms in the soil, then plants will take it in some absorbable form (ammonia, nitrite, nitrate). Regardless of the action of plants, the rate of nitrogen fixation shouldn’t change much.


    2. C4 plants need less nitrogen than C3 for the same amount of biomass produced, so if something, C4 plants will contribute less to “deplete” the nitrogen of atmospheric origin.


    3. A significant part of the nitrogen required by crops is supplied in fertilizers, so in fertilized fields the rate of nitrogen fixation becomes partially uncoupled to the rate of nitrogenous compounds absorbed by the crops.


    4. The rate of nitrogen fixation carried out by free-living organisms present in the soil will be probably negatively affected by the lower requirements of the C4 crops (in both nitrogenous compounds and water) since these plants will leave a larger amount of fixated nitrogen in the soil, and that most microorganisms don’t synthesize any particular compound if they can find it in their surrounding medium in enough concentration.

  103. Jim Carver says:


    Rajini Rao Yes, but there are also other inputs in the nitrogen cycle besides the bacteria, which are major, but we also have lightening which fuses N and O and falls to the earth creating nitrates. Man-made pollution is also a prblem, just banging molecules together in an internal combustion engine does it. So, the nitrogen cycle is pretty complex and there are other things I didn’t mention. A lot of things really.


    I just can’t figure out why I’m thinking Rhizobium  would have just a little greater effect. 🙂

  104. Raman M P says:


    The secret of plants revealed very nicely.I’d like to use the photo for my lecture pl.- mpr

  105. Rajini Rao says:


    Raman M P , sure, please feel free to use any of the material in this post. The image source is listed in the post, so you can cite it in your lecture. 

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