Rhizosphere Influence on Plant Medicine

Einjähriger Beifuß (Artemisia annua)
Artemisia annua
Wikipedia

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).

Structures-of-ginsenosides-from-Panax-ginseng-Glc-glucose-Rha-rhamnose-Araf
Li, H, Lee, JH, and Ha, JM. (2008) Effective Purification of Ginsenosides from Cultured Wild Ginseng Roots, Red Ginseng, and White Ginseng with Macroporous Resins. Journal of Microbiology and Biotechnology. 18(11):1789-91. DOI: 10.4014/jmb.0800.192

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 RbRb2).

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?

 

Unique Germination Characteristics of Goldenseal Seed

goldensealUnderstanding dormancy requirements for woodland, medicinal plant species is a requirement for discovering how they initiate their relationship with soil fungi. Sanders & McGraw (2002) noted that despite wide geographic distribution, seedling establishment is a constraint in wild goldenseal (Hydrastis canadensis, L.) populations. Richo Cech, of Horizon Herbs, has been quite successful teasing recalcitrant, deep-forest medicinal species to break dormancy using forest propagation studies (2002). Baskin and Baskin (2014) noting that goldenseal was a two-phase germinator, with some seed germinating only as root tissue and lacking aerial development until the second year. They indicated that this appeared to be a developmental pattern found in several native plant species growing in the same habitat as goldenseal.

From the stand point of understanding the chemical ecology of medicinal plants, this may allow the roots to interact with the rhizosphere community of fungi longer and, in the case of goldenseal, to develop the anti-microbial alkaloid pool necessary for defense of aerial growth the following season. In a closely related plant family, Berberidaceae, alkaloid production in Berberis vulgaris occurs immediately after seed germination and increases with seedling age (Pitea et al. 1972). This suggests that the medicinally active alkaloids in root tissue are important to the defense of the plant seedling from the onset.

Data is lacking on goldenseal rootlet interactions with rhizosphere fungi post-germination. Initial screenings of mature, wild goldenseal root populations (Tims, 2008) indicated that arbuscular mycorrhizal fungal (AMF) were not associated with either root or seed tissue, and that AMF spores were not found in the rhizosphere soil of the plant. Goldenseal roots did appear to form an endophytic relationship with a zearalenone (ZON) producing Fusarium oxysporum, normally associated with pathogenic characteristics (Tims and Bautista, 2007).

Zearalenone (ZON) has reputed auxin-like (plant hormone) effects on plant tissue, promoting development of lateral roots (Celenza et al. 1995), and stimulating root tip growth (Bean et al. unpublished). These are regions of the root where soil fungi would attempt to enter the plant tissue. In contrast, this particular endophyte appeared to co-exist within the plant root without causing obvious signs of ill health. It is possible that the production of ZON by Fusarium may affect meristematic activity in H. canadensis emerging from dormancy by initially stimulating rootlet formation and root exudation. Rootlet interaction with ZON during the early phase may benefit the plantlet by increasing plant secondary metabolite formation.

Tims and Bautista found that an alkaloid in mature goldenseal root tissue, hydrastine, inhibited ZON production in the endophyte. Cech (private communication) collected cultivated goldenseal from Kentucky and Oregon, and found that berberine levels were higher in the leaf and hydrastine levels higher in the root.  This too would corroborate the results by Shitan et al. (2005) that an ABC pump moved berberine from root tissue to aerial portions of the plant. The interplay between ZON produced by the fungus and hydrastine found in goldenseal root would appear to  have a two-fold effect. Hydrastine may limit more aggressive pathogenesis of the root tissue by F. oxysporum, while ZON may stimulate the production of hydrastine in developing goldenseal root. An additional developmental question that needs to be explored is to what degree might the presence of ZON, or similar microbial compounds in the rhizosphere, favor or induce two-phase germination?

References:

  1. Bean, GA. (1999) Unpublished.
  2. Baskin, CC and Baskin, JM. (2014) Seeds: Ecology, Biogeography, and, Evolution of Dormancy and Germination, 2nd Edition. Academic Press: Cambridge, MA.
  3. Cech, R. (2002) Growing At-Risk Medicinal Herbs: Cultivation, Conservation, and Ecology. Horizon Herbs Publication, Williams, Oregon, pp.41-51.
  4. Celenza JL, Grisafi PL, Fink GR (1995) A pathway for lateral root formation in Arabidopsis thaliana. Gene Dev 9: 2131–2142.
  5. Pitea, M, and Margineanu, C. (1972) Correlations between Chemical Structure and Antibacterial Activity of Berberine. Clujul Med 45:465.
  6. Sanders, SM, and McGraw, JB. (2002) Distribution, Abundance, and Population Dynamics of Goldenseal (Hydrastis canadensis L.) in an Indiana Nature Preserve, USA. Nat Areas J 22:129.
  7. Shitan, N, Kiuchi, F, Sato, F, Yazaki, K, and Yoshimatsu, K. (2005) Establishment of Rhizobium- Mediated Transformation of Coptis japonica and Molecular Analyses of Transgenic Plants. Plant Biotech 22:113.
  8. Tims, M.C. (2008) The Chemical Ecology of Goldenseal (Hydrastis canadensis L., Ranunculaceae): How medicinal plants roots control the microbial communities in their rhizosphere. VDM Verlag, Saarbracken Germany.
  9. Tims M.C. and Bautista C., (2007) Effects of Root Isoquinoline Alkaloids from Hydrastis canadensis on Fusarium oxysporum isolated from Hydrastis Root Tissue, Journal of Chemical Ecology, 33:1449–1455.