The diversity of root forms among plants is mind-boggling. Plant roots come in all shapes, sizes and colors, which likely permits them to exploit unique niches in the soil and coexist. It was not until I started looking more closely at the roots of tropical trees that I was able to appreciated the diversity of root forms (and hypothetically function, although linking root morphologies to their functional trade-offs is an ongoing scientific endeavor). It is hypothesized that Angiosperm roots have divergently evolved from a common ancestor with an intermediate root (an inner stele) diameter (1,2). Some Angiosperms in the more-basal clades (e.g., Magnolids) developed thicker more-fleshy roots, while higher-order clades developed thinner, more-lignified roots.
Secondly, plant roots have evolved in the soil environment, which is loaded with a diversity of bacteria and fungi and other organisms (soil fauna, etc.). They, therefore, associate with both bacteria and fungi in a wide-variety of relationships, some of which are beneficial. One of the most-potentially beneficial relationships among roots and bacteria is nitrogen fixation. Nitrogen fixation occurs by bacteria (either those in the genus Rhizobium (rhizobial symbioses) or Frankia (actinorhizal symbioses), 3) housed in root nodules, or modified root structures specifically designed for the functional association with the bacteria. There is a whole body of literature on the biological mechanism by which the formation of root nodules is induced by the the bacterial infection/presence (see 4,5), which makes the interaction all that more intriguing. What's more is that the association is rare among plants.
Legumes are the masters of rhizobial nitrogen fixation (e.g., Peanuts, Soybeans, etc.), which has made them favorites in agricultural crop rotations for centuries because they replenish soil nitrogen. Rhizobium bacteria are found in about 80% of Legumes and the plants in the genus Parapsonia (3). Actinorhizal symbioses occur in about 200 species from 8 families (3). Both rhizobial and actinorhizal associations are found only in the Rosids 1 clade, which means they share a common ancestor (3). The Figs (in the Moraceae family) are also in this group, although it has been thought that bacterial associations with them are non-existent. I must say, that I seen a fair number of Ficus roots, and none have been nodulated. Futhermore, I found at least one reference that reports that plants in the Moraceae cannot form root nodules (see image below from 3):
That was why I was perplexed when I presumably dug up the roots of a Ficus in a forest near Xishuangbanna Tropical Botanical Garden and found nodules (shown below).
I say presumably dug up, because it is always possible that I got the root of a neighboring tree or liana or something other than the target tree, despite my best efforts to trace the root from the trunk of the target tree. Jury still out on whether this came from a Ficus or not. I will need to double check; a cool nodule none the less ∴ #nodular, bro! ♠️
Root tumor/nodule suspected to have been collected from a Ficus langkokensis Drake (Moraceae) individual collected near Xishuangbanna Tropical Botanical Garden in Yunnan, China. Grid squares are 1 mm.
1. Valverde‐Barrantes, Oscar J., et al. "A worldview of root traits: the influence of ancestry, growth form, climate and mycorrhizal association on the functional trait variation of fine‐root tissues in seed plants." New Phytologist 215.4 (2017): 1562-1573.
2. Maherali, Hafiz. "The evolutionary ecology of roots." New Phytologist 215.4 (2017): 1295-1297.
3. Pawlowski, Katharina, and Kirill N. Demchenko. "The diversity of actinorhizal symbiosis." Protoplasma 249.4 (2012): 967-979.
4. Schauser, Leif, et al. "A plant regulator controlling development of symbiotic root nodules." Nature 402.6758 (1999): 191.
5. Schultze, M., and Adam Kondorosi. "Regulation of symbiotic root nodule development." Annual Review of Genetics 32.1 (1998): 33-57.
James "Aaron" Hogan is a tropical plant ecologist interested in tropical plant biodiversity and global climate change.