Flavonoids are ubiquitous in plants and their 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, acting as phytoalexins and influencing bacterial quorum sensing.
What tugs my boat is they also trigger mycorrhizal fungi to explore the rhizosphere more actively, by stimulating sporulation, hyphal branching, and root colonization. Interestingly, changes to flavonoid ratios in root exudate can alter the symbiotic relationship.
The prevalent theory on terrestrial adaptation of plants suggests that root exudate facilitated symbiosis with arbuscular mycorrhizal (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. 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.
The anchoring to land, the ability to acquire water and nutrients, like a surfer learning to ride the big waves, must have been a rush.