BacterioFiles 322 - Parents' Partners Protect Plants

Cacao tree with fruit pods
By Luisovalles
Own work, CC BY 3.0

Happy New Year! This episode: Fungal endophytes transferred from healthy adult plant leaf litter help baby cacao plants resist disease!

Thanks to Dr. Natalie Christian for her contribution!

Download Episode (10.3 MB, 11.25 minutes)

Show notes:
Microbe of the episode: Lactobacillus virus Lb338-1

News item
Hear a CBC interview with Dr. Christian about this research

Journal Paper:
Christian N, Herre EA, Mejia LC, Clay K. 2017. Exposure to the leaf litter microbiome of healthy adults protects seedlings from pathogen damage. Proc R Soc B 284:20170641.

Other interesting stories:

Using bacteria to clean up radioactive strontium from water (paper)

Ocean bacteria make themselves slippery instead of sticky to avoid predators (paper)

Which plants grow in tropics depends on how fungi interact with their seeds

Using an antibiotic-sensing protein to control antibiotic production in bacteria

Symbiotic fungi can help plants tolerate stresses better

Post questions or comments here or email to bacteriofiles@gmail.com. Thanks for listening!

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Episode outline:

• Background: Our microbiota: complex communities of bacteria + more
• But in plants, fungi can do similar
• Statement 1: This is Dr. Natalie Christian, lead author on today's study.
• Specific to hosts, vary over time, can help plants grow/survive better in various ways
• Previously: 189 herbivore defense, 243 phytoremediation, 309 leaf-cutters
• Not really passed through seeds, more from leaf to leaf via spores
• Questions: From live leaves or fallen leaf litter? What affects which kinds go to different sites?
• What effect on disease resistance?
• What’s new: Now, Dr. Christian and her colleagues Allen Herre, Luis Mejia, and Keith Clay, publishing in Proceedings of the Royal Society B, found that exposure to leaf litter from healthy adult cacao trees help seedlings resist pathogens!
• Theobroma cacao, from which we get chocolate, so very important
• Methods: Got seeds from Panama, sterilized surface with bleach
• Then grew in sterile soil to get germ-free seedlings, lacking endophytes
• Then exposed to leaf litter from adult cacao, from mix of other species, or none
• Placed at various heights in canopy of forest
• Then collected and exposed to pathogen Phytophthora palmivora
• Causes black pod, most important/bad disease of cacao
• Also exposed some plants maintained germ-free
• Statement 2
• Based on significance cutoff of p = 0.05, was ~0.03
• Other groups not significantly different
• But trend of less disease from germ-free to no litter to mixed litter to cacao litter
• No difference from height in canopy
• Used sequencing and culture to ID endophytes
• Somehow No Litter group had more isolates than others
• Possible that species from litter colonized quickly and excluded others
• Height had effect too: fewer isolates from higher up
• Most common found was Colletotrichum tropicale
• Often found in healthy cacao, helps disease resistance even on its own
• Correlated here with less disease
• Sequencing found 5x more kinds than culturing, esp rare ones
• But mirrored culturing results
• Canopy has more heat, sun, less moisture; harsher conditions
• Summary: Cacao seedlings exposed to fungi from leaves of a healthy adult tree are more resistant to infection from serious pathogen
• Applications and implications: Understand effects of fungal endophytes
• Save our precious chocolate
• Could find good species to inoculate plants with, or just cultivation approaches
• Application to other crops too, probably, as Natalie explains: statement 3
• What do I think: Authors mention Janzen-Connell: offspring thrive when farther from parents
• Less competition, less pathogen pressure
• Better for parents too for same reasons
• But here seems like proximity to parents has advantage: good microbe transmission
• Endophytes, rhizosphere, etc
• Complex interactions, probably depends on specific plant/environment what is best
• Plant microbes vs. human microbes
• Transmitted parent to offspring after birth (or during)
• Statement 4
• No human leaf litter though
• Good microbes make good inheritance

Transcript:
1
Plants also have a diverse microbiome, and some of the most functionally important players in that microbiome are microscopic fungi, called endophytes. Endophyte communities have been isolated from every species of plant on our planet, but it remains unknown how these communities assemble, and how the resulting communities affect plant health.

2
What was especially interesting was that exposure to litter from cacao adults significantly reduced pathogen damage on cacao seedlings. And we think that is due to specific fungal microbes that are transferred from leaf litter to those seedlings, because in those seedlings we found a higher abundance of fungal species that we known to be very protective of cacao.

3
This process could have important implications for agriculture. In most monoculture settings, surrounding leaf litter is removed from crops, but if that leaf litter is the source of beneficial microbes, then we may need to start rethinking that practice.

4
the results that we documented in cacao have parallels with other, very different systems. For instance, evidence of seeding a healthy microbiome into hosts has been documented in human microbiome settings, including fecal transplants and maternal transmission of a healthy microbiome to infants.