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Scientists Explain How Cacti Grow On Bare Rocks

August 20, 2009

Scientists have worked out how some species of desert cactus manage to grow on nothing but bare rock, BBC News reported.

They say the plants have evolved a symbiotic relationship with rock-dissolving bacteria, which they allow to grow in their roots. The cacti even pass this on to future generations.

Dr. Yoav Bashan, a biologist at the Northwestern Center for Biological Research in La Paz, Mexico, said they were working in the desert when they observed that many individual cacti grew on sheer rocks.

The researchers noted the cacti looked good and green in habitats where usually plants do not grow, but the enigma was such plants need minerals and nitrogen to survive.

However, neither are available from rock, which binds in minerals and contains no accessible nitrogen.

Bashan said the only explanation they could think of was a possible involvement of microorganisms assisting the plant to grow, fixing nitrogen and dissolving mineral.

“We looked for them and found them,” he added.

Bashan and colleagues then discovered that cardon cactus (Pachycereus pringlei) growing in the volcanic region of the Baja California Sur mountain range harbor bacteria that are capable of breaking down rock.

The scientists reported in the journal Environmental and Experimental Botany that the bacteria can be found in the surface of the plant’s roots and within cells that make up the root itself.

According to lab tests, the endophytic bacteria can also grow in the cactus fruit where it is then transferred into seeds. The bacteria can also weather rock, dissolving particles into smaller sizes.

Bashan said they believe they have found a new symbiosis between bacteria and plants, since the cactus is the carbon provider for the bacteria and the bacteria indirectly provide the minerals and nitrogen for the plant.

The bacteria dissolve the rock, allowing the cactus seed to take purchase. The roots then drill into the weathered rock, fracturing it further.

Bashan explained that below the plant is a small cave where the rocks were consumed and washed as soil and the roots are literally in the air.

Without the bacteria, the cacti couldn’t survive because they are able to pass the bacteria onto the next generation, according to further testing.

Bashan said when a seed falls in bats and bird droppings onto barren rock, it contains all the bacteria it needs to pioneer colonization of that rock.

“The seed is the lucky one, as there is no other competition from other plants that do not have these bacteria,” he said.

Experts say the bacteria and cactus likely evolved together, with their ancient ancestors developing their symbiotic relationship. Meanwhile, the cactus also helps produce soil from the rock, which other plants can use to colonize what was once an extreme habitat.

Bashan called them pioneering plants. “They formed soil in accelerated speed that otherwise will take millions of years to form,” he said.

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