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.
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?