New Scientist - USA (2020-07-18)

(Antfer) #1
44 | New Scientist | 18 July 2020

Center for Agricultural Research in the Dry
Areas in Tunisia observed that hedges
comprising the cactus prevent erosion and
boost the soil’s nitrogen content. In South
Africa, which has seen extreme droughts over
the past few years, some farmers are growing
another CAM crop called spekboom to revive
their parched land.
CAM plants are often thought to be slow
to grow, but they don’t necessarily deserve
this reputation. Annual crops like maize and
soya beans grow fast, but only for one season,
typically four to six months. Most CAM
plants are perennials that grow continuously
for years. “If you take seasonality out of the
equation, some cultivated CAM species are
just as productive,” says Cushman.

Pollination problems
That isn’t the full story, however. Some CAM
crops, including agave, flower and produce
seeds only once towards the end of their lives.
And their lives are long; one species of agave is
known as the “century plant”, though in truth
it lives about 30 years. To be commercially
successful, agave must be propagated not by

germinating seeds, but by cloning.
This creates several problems, including
a world shortage of tequila in 2018 (see
“Tequila sunset?”, page 42). A more serious
issue is that the pollinators that feed on agave
flower nectar – in Mexico this is largely bats –
are threatened with extinction. The cloned
crops, being so genetically similar, are also
vulnerable to pests and disease. Prickly pears
are at risk of infection by a stunting disease
called “macho”. We don’t know its cause.
Cushman’s team is sequencing the DNA
of prickly pear plants afflicted by macho to
investigate the disease and prevent it from
spreading. And several sustainable tequila
projects have been established that allow
a portion of the agave plantation to go
untouched, so that the plants flower and
can be pollinated naturally. It is estimated
that if 5 per cent of the agave planted
on a hectare of land is allowed to flower,
that will provide enough food for about
90 bats each night.
Some are wondering if we can go further,
with an effort akin to the C4 rice project that
aims to combine the traits of C3 and CAM
crops into the ultimate supercrop. Over the
past five years, scientists have sequenced
the genomes of several CAM plants. But
there is a long road ahead. While we broadly
understand how the CAM photosynthesis
pathway operates, important details such
as how regulatory enzymes fluctuate over
time remain unclear.
For the moment, Cushman and his team
are piecing together an understanding of
CAM genetics with a view to developing a
prototype CAM soya bean. He thinks we
could have one in about five years, so it may
be a while before we see them in the fields.
In the meantime, more and more of Earth’s
semi-arid land looks set to be planted with
crops like agave. Its tall, teal leaves are going
to become a lot more familiar. ❚

Larissa Fedunik is a
science writer based
in Canberra, Australia

Sugar 'n' nice


You may have heard of agave syrup, the
trendy sweetener that is an alternative to
sugar. But plants that, like agave, employ
crassulacean acid metabolism (CAM)
photosynthesis (see main story) can
end up on your plate in other ways.
Take the prickly pear cactus, which
is rich in carbohydrates, minerals and
vitamins. Segments of its stem can be
cooked like string beans and cactus
pears are the sixth most popular fruit in
Mexico. It also contains pectins, complex
carbohydrates used as thickening agents
in processed foods.
Agave itself provides more than just
syrup, which, by the way, isn’t necessarily
a healthier option than sugar because it
contains high levels of fructose, which
can increase blood sugar levels. The
stalks and hearts can be roasted and the
seeds can be ground into gluten-free
flour. Fermented agave sap can be used
to make the spirits tequila and mezcal
and pulque, a sour, beer-like drink.

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The prickly pear cactus
could be a more sustainable
source of biofuels
Free download pdf