Promiscuous Plants Make Problems

An acacia. Photo from Flickr

An acacia. Photo from Flickr

Beautiful isn’t it?  This is an acacia tree, a member of the pea family from Australia. Its good looks have led to its introduction around the world.  California is one of those places, and 16 acacia species have been introduced for the horticultural trade.

These new plants on the block are non-native species, meaning they aren’t from around these parts.  Regardless, at least three species of non-native California acacias have begun to exploit the landscape. These pushy plants are called invasive species.  Invasive species crowd out native plants and wildlife, negatively affect the environment, and cause over $120 billion in damages each year.  But what makes these acacia plants so successful as invaders?

Microbes.  That’s right, tiny organisms that live just about anywhere, including the soil and roots of plants.

These microbes are a special type of soil bacteria called rhizobia. Rhizobia colonize pockets in the roots of certain plants, like those in the pea family. The pockets of rhizobia, or nodules as scientist call them, convert unusable Nitrogen into a form of Nitrogen that plants can use.  Nitrogen is one of the important chemicals that determine plant growth and success.

Dr. Metha Klock, doctoral student at Louisiana State University, has found that the relationship between rhizobia soil microbes and certain acacias help them spread to new areas.  If an acacia species can associate with more types or strains of rhizobia, it is more likely to spread and become invasive. Dr. Klock calls these invasive acacia plants “promiscuous hosts.” Getting friendly with more rhizobial strains increases the chances of getting lucky and spreading to new habitats.  

Of course everything is fine until someone (or something) gets hurt, and California’s ecosystems take the blow. Prior to her days of research, Klock was a restoration professional in California.  She was on the ground every day ripping out invasive plants like acacias and replacing them with natives.  But she could see the battle couldn’t be won by force alone, we needed to know why these plants were spreading and how to prevent future introductions.

In a recently published study, Klock grew over 1600 acacia plants of multiple species and harvested the root nodules for analysis.

An example of a bacterial culture. Photo from Flicker.

An example of a bacterial culture. Photo from Flicker.

 She also sampled acacias invading California.  She cultured the bacteria and through molecular analysis was able to determine the different rhizobial strains associated with different acacia species.  Her results show that invasive acacias associate with more rhizobial strains then non-invasive acacias. The implications of which span a variety of topics:

  1. We’ve identified that rhizobia-plant interactions can contribute to invasion.  We can use this knowledge to identify future invasive species before they become a problem.
  2. Acacias are native to Australia, and in areas where restoration is being conducted, these promiscuous plants can be a benefit.  They may spread and restore areas more efficiently than other native plants.  
  3. We can select agricultural crops that are more promiscuous host, and therefore grow better. Or;
  4. There is a potential to develop soil inoculants for improve growth of pea family agricultural plants (a.k.a. probiotics for plants).   

There is more research to be done, but we’ve taken a step in the right direction. Observing how below ground activities affect plant growth and establishment is not an easy task, but we have now identified one mechanism of how acacias spread in new ranges. Klock hopes future studies will include a comparison of more genetically diverse rhizobia strains, and she has more publications to come.  Check out this recent story from one of her presentations, and keep an eye out for her work!

Note to the reader: Invasive species are not just a problem in California.  All over the world, invasive species are altering ecosystems, impacting landscapes, and threatening ecosystem services that both wildlife and people depend on.  The first step to preventing invasion is people.  Before planting a plant, or releasing an animal, do some homework.  If it’s not found there naturally, don’t introduce it.  Check with you local governments for policies on invasive species management, and share what you learn with others.  Invasive species reportings can be sent to local agencies or uploaded here.


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