Plant Root Chemistry is Just an Invitation to Play

Flavonoid Basic Structure

Plants and organisms in the rhizosphere (area of soil surrounding plant roots) are living organisms and part of a complex ecosystem requiring communication skills. Two different classes of compounds are important communicators – flavonoids and strigolactones – both ubiquitous in plants across multiple taxa. Of note, strigolactones were originally detected (Bouwmeester et al.) in plant root exudate stimulating seed germination of parasitic plants (genera Striga and Orobanche). These weedy species parasites host plant roots for nutrients.

Flavonoid basal structures are highly varied, including flavones, flavonols, flavan-3-ols, flavanones, isoflavonoids, isoflavans and pterocarpans. They accumulate at root tips or root cap and makeup a large portion of root exudate. The fact these structures are easily modified and that their biosynthesis can be triggered by a numerous transcription factors points strongly to a role as elicited, signaling compounds. The conversation starters, the deal makers, they patrol the root neighborhood deciding who’s gonna join the party.

Hassan and Mathesius (2012) noted in more technical terms that this localization allows them to influence the rhizosphere environment – increasing the bioavailability of both phosphorous and iron, inducing Rhizobium nod genes (for nitrogen fixation), determining host specificity, and influencing bacterial quorum sensing. They also influence soil  fungi, both parasite and non-parasite, to investigate their environment by stimulating macronidial germination. These are spore structures that allow the fungus to remain in a dormant state until the surrounding soil supports their growth (Ruan et al.). Plants represent nutrient sources – the internal cell structures for parasites, and the substances released from plant root such as carbohydrates, organic acids, and proteins (root exudate) for non-parasitic fungi.

One of the most fascinating chemical conversations involves how both flavonoids and strigolactones trigger AM fungi to investigate the rhizosphere more actively, by stimulating sporulation (breaking out of their dormant state), hyphal branching (similar to send out runners), and root colonization. Interestingly, changes to flavonoid ratios in root exudate can alter the symbiotic relationship and defines how mature the relationship is developmentally.

Metabolic pathways in living organisms require dedicated gene expression. They tend to have been around a long time. Plants originated as aquatic organisms. They had no root systems. The prevalent theory on their terrestrial adaptation suggests that root exudate facilitated symbiosis with AM fungi, which allowed primitive land plants to survive by providing them an early “root system”.

A paper by Delaux et. al. (2012) tested whether the presence of strigolactones in the aquatic green algae lineage may have helped them adapt to and colonize terrestrial environments. The researchers used a bioassay to detect branching of a AM fungus, Gigaspora rosea, to show strigolactones were present in the green algae Charales corallina. They also applied a synthetic strigolactone to C. coralina, which stimulated rhizoid elongation in the algae. Rhizoids were early “root-like” structures. The results beg the question of whether strigolactone biosynthesis predates AM fungal colonization and reinforces the idea that what survives adapts to changes in habitat, since anchoring to land increased the plant’s ability to acquire water and nutrients.

Strigolactones General Chemical Structure

Experiments on potato found that  strigolactone may be involved in resource partitioning by maintaining phosphate and nitrogen homeostasis in plants (Pasare et al., 2013).  Researchers reported that strigolactones enhanced plant association with AM fungi by increased branching of the AM hyphae (Giovannetti et al.). We see the effect of this type of chemical conversation, both internally within the plant and externally, with the fungus, to stimulate exploration.

Strigolactones do also play a role as a plant hormone and appear to regulate axillary growth, lateral branching, and decreased apical dominance (Delavault et al.). This mimics the biological impact on fungi and suggestions that plants explore their aerial environment of air and sunlight.

Rasmussen et al. (2012) noted the ability of strigolactones to impact root exploration in plants, limiting adventitious rooting by inhibiting the initial formative divisions of founder cells. These phenomena may point to the plant directing both the timing and directionality of new root growth in response to the presence/absence of appropriate soil symbionts.

So then, what triggers strigolactone or flavonid secretions? What’s the chemical cross talk originating from the fungal side of the conversation?












Leaf lamina. The leaf architecture probably ar...
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In an issue of New Phytologist, Claire Belcher provides a wonderful summary about work by Bond and Scott (from same issue) on the ecological role of fire in the spread of angiosperms (flowering plants). During the Cretaceous (145.5 to 65.5 mya, ending with extinction of dinosaurs), angiosperms were fast growing, weedy and largely understory herbs, shrubs and small trees more likely to colonize edge or disturbance sites. An improved plant vascular system, including a large increase in leaf vein density, doubled the photosynthetic rate and increased leaf mass in comparison with ferns and gymnosperms, which were the dominant land plants at the time. This adaption allowed angiosperms to work more efficiently fixing carbon from falling CO2 levels of the period. Fire also appeared to play a vital ecological role in the spread and ultimate dominance of flowering plants on earth.

Fossil charcoal demonstrated the presence of fire in the early ecosystem. The combination of  highly flammable detritus from weedy angiosperms and increasing atmospheric oxygen levels created an angiosperm-fire cycle equivalent to modern prairie fire cycles. Once an ecosystem was fire damaged, the faster growing angiosperms out-competed both gymnosperms and ferns. The early fossil records indicate that fire activity was greater during the Cretaceous than in previous epochs. However, when oxygen levels dropped about 56 mya, fire-cycles decreased and angiosperm-dominated forests, such as tropical forests, expanded.

In fact, the resulting layers of charcoal helped preserve the fossil record of dinosaurs’ last days. Researchers were then able to predict what the Cretaceous forests looked like.

Sister Morphine

Although I’ve met and friended folks who claim they’ve seen it, I’ve never witnessed a plant that could run. Instead, they engage in chemical warfare or communication. The chemicals are the result of multi-step metabolic networks that provide the chemical apparatus to change the staring material into a bioactive substance. Such a chain of chemical reactions is controlled by a series of proteins, called enzymes. Each protein is coded for by a gene.Poster_papaver_3a

This research identified a large cluster of 15 genes that encode enzymes in the metabolic pathway with morphine as an endpoint. Approximately 50 alkaloids are found in Opium poppy (Papaver somniferum), with morphine the largest in concentration.

According to the study, the pathway for the painkilling drugs evolved around 7.8 million years ago (mya). Primates are presumed to have appeared 63 mya, Hominidae (precursors to modern humans) 15 mya, and humans 1.3-1.8 mya. First recorded humans use appears in 5000 BCE in the Neolithic age. The PBS show Frontline provides a timeline of human use. It’s history shows that Morphine has been a “wonder” drug for pain, and a bane for those addicted to it and it’s derivatives

The mechanism responsible for euphoria also kills. Morphine binds to receptors in the brain, inhibiting neurotransmitter release and resulting in among other physiological changes, pain relief, but also slowed breathing.  Overdose victims often stop breathing.

Since the plant has been around for quite some time, I wondered if there were any histories to show animals consuming this or other plants to reduce pain or to  give pleasure? A brief review showed the following:

  • Researchers discovered it was chili peppers. Next, they studied tree shrews in the wild and discovered they ate one particular pepper, the Piper boehmeriaefolium, and actually preferred to eat it over other plants and vegetation.
  • Scientific Reports provides evidence that Borneo based apes chew leaves of the Dracaena cantleyi plant to create a white lather, which they then rub onto to their bodies
  • A study of chimps found that they roll Aspilia leaves for a period of time (they are very bitter to chew). This plant material contain thiarubine A , which kills harmful bacteria, and fungi because they contain thiarubine A, a powerful antibiotic. Research also suggests these leaves act as a stimulant, since chimps ingest them first thing in the morning.

What’s more compelling is the rich association of the opium poppy with war.  with a few examples below. One aftermath of each – the trail of addiction that followed either the imposition of trade or the use of morphine on the battlefield to reduce pain from horrible damage.

  • During the 18th century, forcibly exported opium to China, even while it was banned in Britain because the government and industry knew it was not good for the populace in general. It took two opium wars, eventually disrupting the country and leading to the  collapse of the Qing Dynasty.
  • After World War I in remembrance of the fallen soldiers, the living commemorate the sight of thousands of blood-red poppies appearing on the battle-scarred fields of Flanders, in Northern France.
  • US wars in Vietnam and Afghanistan have been greatly affected by opium production supporting the opposing militaries ability to pay for the fighting.

Now America is facing a public health crisis of opioid addiction.  In an interesting turn, last November, President Donald Trump asked Chinese President Xi Jinping to help stop the “flood of cheap and deadly” fentanyl from China into the United States.  Fentanyl is a synthetic opioid, 50 to 100 times stronger than morphine.

For better or worse, this is an example of co-evolution – humans identifying and applying a plant to alleviate the pain they, themselves create.