A paper out of the Hawes Lab noted that root border cells respond to or probe the surrounding soil by releasing a multitude of proteins, amino acids, sugars and secondary metabolites, and they do so even after the cells detach from the root cap. This material, called exudates, appears to alter the surrounding rhizosphere community of microorganisms. Curlango-Rivera et al., (2010) investigated the consequence of these plant exudates on the growth and production of root border cells. Exposure to pisatin, at concentrations capable of inhibiting fungal growth, stimulated production of root border cells, while exposure to a plant cell wall component, ferulic acid, inhibited growth of root border cells. Pisatin is a phytoalexin, a secondary metabolite made by the plant to limit microbial attack.
Does this indicate that plant root investment in root border cells as sensory organs occurs under the protective influence of a plant phytoalexins, but that cell wall fragments resulting from the rupture of root cap cells under microbial attack may limit plant root border cell proliferation? I am curious about experimental results if simultaneous exposure to the compounds occurred? Ferulic acid is often bound to a matrix of polymer structures found in plant cell walls. Would it take time for diffusion of these compounds into the rhizosphere to gain concentrations able to impart a biological effect?
Bilberry (Vaccinium myrtillus) contains varying levels of phenolic compounds – anthocyanins, chlorogenic acid derivatives, hydroxycinnamic acids, flavonol glycosides, catechins, and proanthocyanidins. Research by Martz et al (2010) elucidated how levels of bilberry leaf phenolics differed along an ecological gradient in boreal forests running north to south in Finland. These regions differ in latitude, altitude, over story cover, levels of continuous light, temperature and associated frost spells.
An analysis of bilberry leaves showed that major phenolic changes in bilberry leaves appeared in the first stages of leaf development. As important, synthesis and accumulation of flavonoids was delayed in the forest compared to the high light sites. Two-fold higher flavonoid levels appeared in leaf tissue growing in high-light intensity sites, higher latitudes, and/or higher altitudes compared to in lower altitudes and low-light intensity sites.
Close and Mcarther (2002) previously theorized that the presence of greater phenolic levels in leaf tissue found in northern regions was a response to colder temperatures, which would limit essential enzyme function, during periods of maximal photo-oxidative stress (Close and Mcarther, 2002). However, Martz et al (2010) also showed that leaf flavanoid genes were highly expressed in shade, but that the timing of expression appeared to alter the relative metabolite levels in shade compared to sun exposed bilberry leaf.
Mudge et al., (2016), researched phenolic profiles of wild elderberry fruits (Sambucusnigra subsp. canadensis) over two years in eastern US, noting that flavanols (quercitin, isoquercitin, rutin) and chlorogenic acid metabolite concentrations were higher in the southeast, particularly interior. They suggested the variation of phytochemical profiles of the berries were impacted by genetic or environmental factors without understanding on which was more important.
What’s missing from the data picture includes a more complex measurement of ecological influences, such as response to herbivory and rhizosphere fungal associations? This type of whole community data would help to build a more complete picture of plant response.
This requires sampling, sampling, sampling.
Martz, F., Jaakola, L., Julkunen-Tiitto, R. and Stark, S. (2010). Phenolic Composition and Antioxidant Capacity of Bilberry (Vaccinium myrtillus) Leaves in Northern Europe Following Foliar Development and Along Environmental Gradients. J Chem Ecol, published online, 19 August 2010
Close, D.C., and Mcarther, C. (2002). Rethinking the role of many plant phenolics—protection from photodamage not herbivores? Oikos. 99:166–172
Mudge, E., Applequist, W. L., Finley, J., Lister, P., Townesmith, A. K., Walker, K. M., & Brown, P. N. (2016). Variation of Select Flavonols and Chlorogenic Acid Content of Elderberry Collected Throughout the Eastern United States. Journal of food composition and analysis : an official publication of the United Nations University, International Network of Food Data Systems, 47, 52–59.
Mycorrhization leads to nutrient and information flow, often in both directions. The plant root supplies sugars to the fungus, while the fungus induces Jasmonic Acid biosynthetic enzymes in the plant, leading to an increase in jasmonate levels that enhance the accumulation of soluble sugars in plant root and the production of plant root defense compounds.
From a research article, the presence of mycorrhizal fungus, Glomus mosseae and nitrogen fixing Bacillus subtilis on the roots influenced the levels of plant biomass growth, and the yield of an important medicinally active phytochemical, artemisinin, from Artemisia annua L and used as an anti-malarial treatment.
Gabriele et al. (2016) investigated the effect of mycorrhizal soil inoculation of various Sangiovese wine grapes and found the presence of the fungus increased levels of 14 polyphenols compared to un-inoculated plants. Here the presence of symbiotic relations in the soil altered the phytochemical makeup of fruit.
So how are the plant roots attracting mycorrhizal symbionts? Plant produced flavanoid compounds accumulate at root tips/cap and make up a large portion of root exudate (the portion of the root sap excreted to the external environment). These phytochemicals are easily modified and their biosynthesis is triggered by transcription factors, which suggests a role as elicited signal compounds – compounds that are made specifically in response to conversation from rhizosphere fungi and bacteria. Interestingly, their presence in the rhizosphere soil triggers mycorrhizal fungi to explore their surroundings (Hassan and Mathesius, 2012), perhaps increasing the likely hood of contact with plant roots.
Given the high price of American wild grown ginseng, the ecological influence on ginsenoside formation, and ultimately, the therapeutic value, points to optimizing the rhizosphere cross talk by way of forest farming.
The highest ginsenoside content occurs (from highest to lowest) in the root hairs > lateral roots > cortex > interior taproot (Li and Wardle, 2002), exactly where we should expect a chemical conversation to occur.
Within this class of compounds we designate as ginsenosides, two molecular forms are dominant, protopanaxadiols and protopanaxatriols. Data from two different papers (Zhu et al., 2004: Wang et al., 2010) compared levels of diols and triols in different species and sources of ginseng. American ginseng (Panax quinquefolia) had higher levels of the triols (especially Rg1) compare to Chinese ginseng (P. ginseng), which had higher levels of diols (especially Rb1 Rd).
Comparing wild grown versus cultivated plants within each species, a similar pattern emerged, with wild plants showing a higher concentration of triols (especially Rg1 Re), while cultivated plants had higher concentration of diols (especially Rb1 Rb2).
James, et al. (2013) investigated levels of diols and triols in wild sourced P. quinquefolia leaf and root in a North Carolina collection, finding that there was no relationship between age and ginsenoside content. However total ginsensosides were higher in the leaf, as was Rb2 and Rd (diols), In the root tissue, Rb1(diol) and Rg1 (triol) was found to be higher.
This has implications for how we “farm” medicine and speaks to a long held tenet; complex interactions in native ecologies, including the soil, produce medicinal plant crops that are more biologically active. Farm versus wild grown ginseng is only one example. What’s been your experience as a imbiber, herbalist, researcher, plant grower or manufacturer?
Surrounded by material excreted (exudate) by their own root border cells, the growing root tips (apical region) of plants move through soil regions where important biological interactions occur with a community of soil microbes. This exudate not only helps define the soil microbiome (microbial community), but also changes the physical and chemical characteristics of rhizosphere soil.
Hiltpold et al (2011) provided evidence of systemic, volatile signals in maize roots in response to herbivore attack. From 2013 research on Arabidopsis suggests that soil microbes can alter plant leaf chemistry to inhibit insect feeding. They posited a role for microbial-derived volatile organic compounds acting as a deterrence signal, and noted the presence of Actinobacteria, Firmicutes and Proteobacteria in soil and within Arabidopsis root tissue.In a 2013 Tansley Review, Turnbull and Lopez-Cobello noted that despite localized cellular communication found in the root apical meristem, communication via vascular transport to the rest of the plant did not seem to occur. That left me wondering how plant roots communicated changes throughout the entire plant (systemic).
Those microbes are often associated with “soil odors”. On a sensorial level, “smelling” the earth may help us appreciate the complex, unseen communication happening under foot.
Fungal species of the Metarhizium genus colonize most land plants and help provide nitrogen to the plant root. The nitrogen source is unique – insects that the fungus has pathogenized and killed using enzymatic degradation of the insect’s shell.
Mike Bidochkaof Brock University investigated the phenomena by injecting labelled nitrogen into Galleria mellonella larvae (moth). They buried the larvae in soil and separated the larvae from either beans (Phaseolus vulgaris) or switchgrass (Panicum virgatum) plants using a screen with pores large enough for fungal mycelium to grow through but small enough to prevent plant root growth.
Fourteen days later, they found labelled nitrogen made up more than a quarter of nitrogen found in plant root tissue. Insects Larvae with labelled nitrogen not infected by the fungus did not act as nitrogen sources for the plant.
Good evidence for an ecosystem rich in biota, rather than one where selective human inputs alters it into a simpler set of relationships. In most cases, the soil environment becomes less sustainable.
The search for chemical mediators in plant root rhizosphere interactions with symbiotic and pathogenic organisms found in the soil continues to generate interesting research. Martha Hawes group at the University of Arizona reported on the role of sugars, proteins and small molecules found in root cap secretions – a mucilaginous mixture that covers the growing root tip and “converses” with the surrounding matrix of living organisms. The cap is rich in root border cells, which detach from the growing root tip. Curlango-Rivera et al (2010) provides us a bit more detail about which metabolites are biologically active. Neither sugars nor amino acids triggered root growth or border cell production. Transient exposure to biologically active concentration levels of the isoflavonoid pisatin, a phytoalexin, stimulated root border cell production but not root tip growth. I wonder if inhibition of root elongation may “reset” plant growth patterns as root border cells, acting as chemical sense organs, define the nature of the environment?
A second paper used histochemcial methods to profile root metabolites in plants from the Rose family (Hoffman et al., 2010). They found flavan-3-ol molecules in the root tip and border cells. Their findings suggest that the distribution of flavan-3-ols in Fragaria and Malus is under tight developmental control. These molecules are found in plants as catechin and epicatechin derivatives and in long chain (polymeric) form. They influence the taste and medicinal potential of green tea and wine, to name a few well-known plants. Previous researchers summarized their role in chelating toxic cations (metals) in the soil, establishing mycorrhizal interactions and priming plant root defense. This paper suggests a role in the transport of the long distance plant hormone auxin, which would link the chemical cross talk at root border cells with responses that occur in tissue distal to root tips. Hoffman’s research lacked a clear distinction of whether the monomeric or polymeric flavan-3-ol forms where the active species. This has plagued plant research for some time, since the analytical methods for detecting the polymeric forms have been crude and ineffective. All of their samples were from a botanical garden. I wonder if the flavan-3-ol profile would differ compared to native wild grown species?
Curlango-Rivera, G. et al. (2010) Plant Soil 332:267-275
Hoffmann, T. et al. (2011) Plant Biology, 13: no. doi: 10.1111/j.1438-8677.2011.00462.x
As I’ve studied medicinal plants, one intriguing question keeps cropping up – what is the biological rationale for plants investing in several classes of structurally varied plant secondary metabolites?. Certainly this question drives a number of researchers in the field of plant chemical ecology. Edwards et al (2008) provided some striking evidence from their study of bacteria resistant cotton (Gossypium hirsutum).
They detected flavonoid pigments, chrysanthemin and isoquercitin, at unusually high levels in epidermal tissue of young leaves in response to Xanthomonas infection. These cells clustered around infected cells that had died because of the plant’s hypersensitive resistance response.
A very different chemical, a sesquiterpene, 2,7-dihyroxycadalene, acted as a light activated phytoalexin, destroying both bacteria and the infected plant cell. So what were the flavanoids doing to help out? The presence of the flavanoids in surrounding cells appeared to filter sunlight, limiting the generation of free radicals resulting from light activation of 2,7-dihyroxycadaline. How’s that for compartmentalizing your response to friends and enemies?