With ever more mouths to feed, we really need to pull something special
out of the hat, says Debora MacKenzie
Take a look around you. All the organic things you see, from your hands
the leather of your shoes to the wood in your table, are built of
strings of carbon
atoms. So too is the petrol in your car and the coal in your local power
station. All this carbon came from thin air, from carbon dioxide in the
From termites to blue whales, virtually all life on Earth depends on
plants' ability to turn sunlight and carbon dioxide into food - and
without the waste product, oxygen, you would be dead in minutes. You
would think, then, that evolution would have honed the process to
perfection over the past 2 billion years or so.
Yet photosynthesis remains astonishingly inefficient in some ways.
In one respect, this is very good news. With the world's population set
to soar to 9 billion or so by 2050, we need to grow a lot more food.
That's a huge challenge, and as the climate changes, higher temperatures
and more severe droughts and floods will make it even harder. We got a
glimpse of what the future might be like this year, with drought in
Russia and floods in Pakistan devastating crops.
Improving photosynthesis itself could dramatically boost yields. The
sound like hubris, but there is no doubt it can be done-because some
plants have already achieved it. Many have evolved the same workaround
for the most serious glitch in photosynthesis, and researchers are now
trying to copy that advance in wheat and rice. And some smaller
improvements have already been made in the lab.
Surprisingly, one approach might be to reduce levels of the molecule
light energy from the sun. It turns out that plants make up to four
times as much
I chlorophyll as they need for photosynthesis. Why? Because if plants
produced only as much chlorophyll as they needed for themselves, more
light would be able to pass through their leaves and reach upstart
seedlings nearby. "Plants didn't evolve to optimise their yield," says
Don Ort of the University of Illinois at Urbana-Champaign. "They evolved
Plants with excess chlorophyll don't just shade out rivals, though. They
their own lower leaves. What's more, the excess chlorophyll may cause
some leaves to absorb light energy faster than it can be used, which can
damage them. So plants have evolved a "quenching system" to mop up the
surplus energy. All this is costly for the plant, which is one big
reason crops yield less food per light unit than they otherwise might. A
soybean mutant with half the usual level of chlorophyll can produce 30
per cent more biomass than normal.
More generally, the amount of food a plant can produce depends not only
on the amount of light it intercepts but also on how efficiently it
traps the energy of that light in chemical bonds within sugars and
starches, and on the proportion of those sugars and starches that end up
in edible parts of the plant. The green revolution of the 19605 came
from shifting the last factor, by producing short-stemmed wheat and rice
varieties that put half their photosynthetic output into seeds. There is
little scope for pushing this further: modem varieties can divert no
more energy from stems to seeds and still stand until harvest.
As for the first factor, modern crop varieties are bred for large,
optimally arranged leaves. They already intercept most light at the
right wavelengths during the growing season, Ort says. In theory, plants
could capture a wider range of wavelengths (see "Why aren't plants
black?", below), but such a fundamental redesign is beyond today's
That leaves more efficient photosynthesis. Here the scope for
=============WHY AREN'T PLANTS BLACK?
Plants are green because they don't absorb green light. The question is:
why? Why let these wavelengths go to waste? No one can say for sure, but
the most intriguing explanation was proposed by Andrew Goldsworthy
of Imperial College London (New Scientist, 10 December
1987. p 48).
When photosynthesis evolved, Goldsworthy suggests, the oceans were
full of a purple pigment called bacteriorhodopsin. Some simple cells
make this so they can exploit light energy in a primitive way, and it
looks purple because it absorbs green light. In fact, chlorophyll
absorbs precisely the wavelengths that bacteriorhodopsin does not. So
plants might be green
because photosynthesis evolved in bacteria that had to make do with
Because photosynthesis is so complex, by the time these cyanobacteria
started to dominate the oceans, it was impossible to make major changes
to chlorophyll without breaking the system. Some plants, particularly
marine algae, have evolved extra pigments that can capture other
but most remain stuck with the wavelengths chlorophyll
The big flaw
When photosynthesis first evolved there was little oxygen in the
Now there is lots of oxygen and very little CO2, and the process often
goes wrong, wasting vast amounts of energy
CO2-RICH ATMOSPHERE OXYGEN-RICH ATMOSPHERE
THE AIM THE FAULT
The enzyme rubisco add Rubisco sometimes adds oxygen
CO2 to a 5 carbon O2, instead of CO2. Creating
molecule. one three carbon molecule and
O-C-O one two carbon molecule
C-C-C-C-C O-O + C-C-C-C-C ---> C-C-C
creating 2 three carbon
molecules C-C-C C-C-C
Most of the 3-carbon SUGARS, The resulting unwanted
molecules are recycled PROTEINS, 2-carbon molecules can be
to make more 5-carbon ETC . partly recycled, but
molecules, but some are this is an energy hungry
siphoned off to make sugars process that results in the
and other compounds loss of one CO, molecule instead of
A SOLUTION €
Concentrating CO2 around rubisco reduces the energy loss in "C4" plants
such as maize. Mesophyll cells capture C02 and pump it into the bundle
sheaf cells, where photosynthesis takes place, recreating a C02-rich
enormous. Plants capture CO2 using an enzyme called rubisco, and it is
lackadaisical enzyme we know of. Whereas most enzymes catalyse thousands
of reactions per second, rubisco manages only a few - although to be
fair, it catalyses reactions involving gas molecules; which are harder
to grab hold ofttian larger molecules in solution. To compensate for its
sloth, plants have to produce huge quantities of rubisco, making it the
most abundant protein on the planet.
Slowness is not the only problem: rubisco is error-prone, too. The
intricate shuffling of organic molecules that photosynthesis uses to
make sugars begins with rubisco grabbing a molecule of CO2. Then, using
the energy captured by chlorophyll, rubisco adds the CO2 to a molecule
containing five carbon atoms,
yielding two 3-carbon molecules. Sometimes, though, rubisco grabs hold
of oxygen instead of CO2, which results in an unwanted 2-carbon molecule
(see diagram, above). The error is costly to fix, as it not only wastes
also leads to the net loss of one molecule of CO2.
This was not a problem a couple of billion years ago when rubisco
evolved, as there was no free oxygen around. Now there is lots of
"Just a few million years after they evolved, C4 plants dominated
environments like grasslands"
oxygen and less CO2, which means rubisco is far more likely to grab
oxygen by mistake, triggering the wasteful process known as
photorespiration. In theory, the energy from just eight photons should
be enough to capture a CO2 molecule, but in practice plants typically
need 13 photons because of all the energy lost during photorespiration.
The most obvious way to improve photosynthesis, then, is to tweak rubisco
to make it work faster or less likely to grab oxygen. But this is easier
said than done. "Evolution had several billion years to change that and
didn't," points out lulian Hibberd of the University of Cambridge. When
biologists tried making rubisco more selective, they found the enzyme
got even slower.
In fact, it is far from clear whether this "dreadful enzyme", as Ort
describes it, can be
improved. Some argue that rubisco is already as good as it can be. If
so, perhaps a better way of capturing CO2 can be designed from scratch
but again, this is beyond today's bioengineers.
Fortunately, plants have already found ways to compensate for rubisco's
weaknesses. Over the past 30 million years, as CO2 levels fell to their
lowest for at least 300 million years, some plants evolved a neat
solution: capturing and concentrating the gas to recreate the ancient
atmosphere. In these plants, photosynthesis usually takes place in cells
clustered around the leaf veins. They are surrounded by a layer of cells
that, instead of containing rubisco, contain another enzyme that grabs
CO2 molecules and attaches them to a 3-carbon
sugar. The resulting 4-carbon molecule is then transported into the
rubisco-containing cells. There the CO2 is released.
Although concentrating CO2 in this way takes a fair bit of energy, it
overall by reducing the amount wasted on photorespiration. So the
process, called C4 photosynthesis after the 4-carbon molecule that
transports CO2, is far more efficient than normal or "C3"
photosynthesis. C3 crops like wheat and rice typically produce 11 tonnes
of grain per hectare, while the main C4 crop, maize, can yield more
than 18 tonnes.
The extra CO2 we are pumping into the atmosphere would, if the climate
unchanged, reduce photorespiration and boost growth in C3 plants.
Unfortunately, global warming will counteract this effect, as night
temperatures increase photorespiration. What's more, in hot conditions
their pores to reduce water loss. This also results in less CO, entering
the leaves, boosting photorespiration further.
Wheat and rice yields are already falling in some areas as the
temperature rises, and this is just the start. Models point to wheat
yield falling 25 to 35 per cent in coming decades, says Hans Braun,
chief wheat breeder at the International Maize and Wheat Improvement
Center (CIMMYT) in Mexico.
By contrast, higher temperatures make little difference to C4 plants.
Because C4 plants have an internal store of CO2, they can keep
photosynthesising even when their leaf pores are closed, so they are
better at conserving water. And because they need less nitrogen
containing rubisco, they need less nitrogen fertiliser than C3 plants.
This is why, although, C4 plants only evolved recently, they already
dominate environments like grasslands.
This long list of advantages spurred CIMMYT to launch a consortium of
researchers last November, with the aim of switching wheat from C3 to C4
photosynthesis. But no one expects it to be easy. lust adding a few
to C3 plants won't do the trick. This has already been tried in rice
and, while it looked promising in the lab, the plants reverted to C3
photosynthesis in the field, says Rowan Sage of the University of
Toronto, Canada. For C4 photosynthesis to work, he says, rubisco has to
be kept in a separate "compartment". In other words, converting plants
from C3 to C4 requires not just the enzymes to capture and concentrate
CO2, but also the special arrangement of cells found in C4 plants,
known as Kranz anatomy. That's a tall order, given that how structures
develop is among the least well-understood things in biology.
As for which crops to work on, Matthew Reynolds, chief wheat
physiologist at CIMMYT, thinks wheat might be easier to switch to C4
than rice. That's because most wheat strains have six copies of their
genome rather than the usual two. "We can move genes around without
being scared we will delete something vital," he says.
Rice researchers are not giving up, though. In 2008 the International
Institute (IRRI) in Manila, the Philippines, launched a new drive to try
to engineer C4 rice, funded by the Bill and Melinda Gates
To shade out rivals, plants make much more chlorophyll than they need
Foundation. The IRRI is bombarding 'sorghum, a C4 grain, with gamma rays
see whether any of the resulting mutants partially revert to C3
if so, what genetic changes are responsible, says Susanne von Caemmerer
Australian National University in Canberra, a member of a consortium set
up by the IRRI. The institute is also screening rice strains and their
wild relatives to see if any have C4 characteristics. "Conversion of C3
to C4 has become realistic," von Caemmerer says, "because of the new
technologies for massive sequencing that make possible research we
previously could not attempt."
Such methods are revealing the scale of the challenge. Hibberd and
colleagues have been studying two closely related plants, one C3 and one
C4. They reported in June that the plants express 603 genes differently,
including 17 that regulate other genes. "C3 to C4 conversion is
incredibly ambitious, but we're pursuing it
because the rewards are so great," Hibberd says. His lab and others are
already adding C4 sequences to rice plant embryos and sending them to
IRRI to grow and assess.
They take heart from the fact that plants evolved C4 independently on at
45 occasions. "So this may not be a very difficult switch for plants to
Hibberd. In fact, Eleocharis vivipara, a sedge grown in freshwater
C3 photosynthesis when under water, and C4 - complete with Kranz anatomy
- on land.
THere is a huge scope for improving photosynthesis.
If we don't start trying now, we won't do it at all"
There are other, possibly easier, ways to reduce the bottlenecks in
"It would be silly not to try the simple things as well as C4," says
Christine Raines of the University of Essex, UK. "There are some points
in the cycle where increasing efficiency has boosted yield, although so
far not under
For instance, some photosynthetic bacteria have evolved a less wasteful
way of recycling the 2-carbon compounds produced by photorespiration.
Adding this pathway to plants results in faster growth and more biomass
(Nature Biotechnology, vol 25, p 593).
One relatively simple tweak would be to stick a rubisco from a C4 plant
into a C3 crop, rather than trying to convert it to C4. This could boost
photosynthetic efficiency by 25 per cent, Ort thinks. The reason is that
while evolution has not found a way to stop rubisco binding to oxygen,
it has produced the best
balance between selectivity and speed. The rubiscos in C3 plants are
optimised for the very low CO2 levels that have prevailed for the past
million years or more. Though levels will soon more than double, it will
take evolution a long time to catch up. The rubiscos inside C4 plants,
however, are optimised for higher
CO2 levels; they bind CO2 a little less strongly and hence work a bit
Problem: no one has yet managed to get a plant to make a foreign
rubisco. The enzyme consists of eight large proteins coded for by the
chloroplast's genome, and eight small proteins coded for by genes in the
cell nucleus. All these proteins have to be put together by special
chaperone proteins. It's all quite a challenge, but Ort is optimistic
that decent funding will overcome it. "That's a technical problem: $50
million over five years will buy that," says Ort. "C4 conversion needs
basic research - we don't know how to buy it."
"We should do the other improvements to photosynthesis too," counters
Hibberd. "But in 2050 we have to produce a lot more food. The only
biological precedent for a major step change in productivity is the
evolution of C4. We'll be lucky to do it in 15 or 20 years. But if we
don't start trying now, we won't do it at all." €
Debora MacKenzie is New/Scientist's
11 September 20101 NewScientist 143
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