Signaling molecules from either plant or fungi are perceived by the other using receptors. Many plants monitor their ecosystem for bacteria or fungi using receptor-kinases, which as cell surface proteins activate a signaling cascade in the cell to change it’s function in some way. Research groups continue to unearth various themes on this mechanisms for plant/mycorrhizal communication.
One model, identified Lipochitooligosaccharides (LCO) as signal molecules used by nitrogen fixing bacteria (rhizobia) to alter how plant roots form a symbiotic relationship. Communication using LCOs allows plants to gain nitrogen from soil bacteria and bacteria to gain carbon in the form of plant sugars. Similar molecules are excreted by arbuscular mycorrhizal (AM) fungi. This research noted that a mixture of sulphated and non-sulphated lipochitooligosaccharides (LCOs) secreted from the AM fungi, Glomus intraradices, stimulated root branching and growth in the legume Medicago truncatula. Apparently, the diffusible chemicals activated plant root genes that code for a series of receptor kinase. In M. truncatula, rhizobium LCO secretions also stimulate the same symbiotic pathway. The researchers found this signaling effect active in diverse plant species.
In other experiments, scientists found a hydrolase protein (D14L), which functions deep within the cell, modulating plant communication with AM fungi. This receptor had originally been characterized as a receptor for Karrikin, a plant hormone produced when plant material is burned. In species such as eucalyptus and the tobacco family, this hormone detects smoke and stimulates seed germination after fire has decimated an ecosystem. It allows those plants, known as fire chasers, to outcompete in the newly altered environment. What is particularly interesting – the same protein is part of early plant developmental interaction with light, and may have played an evolutionary role in plant emergence on to land.
So burn a little incense, light a candle, offer up something sweet and see if your mycorrhizal fungus responds. You don’t need to burn down the entire house!
Dr. Martha Hawes has been a pioneering researcher on plant root border cells. I became fascinated with their role while researching the fungal/plant communication in the rhizosphere of goldenseal (Hydrastis canadendis) during my doctorate. I called her lab hoping someone might speak with me. She answered and spent an hour pointing out important research papers and suggesting approaches I might take to incorporate root border cell research. She was always open to helping anyone with a curious mind and passion for the subject into which she’d immersed her career efforts. I’m grateful to her for showing me generosity and kindness.
Plant root border cells are formed at the root tip where physical and biological interactions occur with the soil and microbe communities. The cells are genetically programmed to separate from the rest of the root structure and from each other. Cell-wall degrading enzymes dissolve cell wall matrix material that holds plant cells together. These “outpost” remain biologically active, excreting proteins and smaller molecules into the surrounding environment. Both types of molecules act as signals turning on/off gene expression to stimulate or prevent the growth of soil-borne bacteria and fungi. One important role appears to be in establishing a symbiotic relationship with mycorrhizal fungi (see previous post).
Few plants such as the Arabidopsis thaliana, which do not produce root border cells, also do not form mycorrhizal associations. In most plants, the content of border cells are accessible only to microorganisms able to recognize and respond to specific root signals. Among the compounds located in root border cells of various plants, medicinally valuable isoflavonoids modulate stable ecological relationships between mycorrhizal fungi and plant root tissue. These fungi stimulate the production of isoflavonoid in plant root tissue, while simultaneously the isoflavonoids increase mycorrhizal spore germination. The spores are an important survival mechanism used by the fungi. Measuring the activity in root border cells in “real time” as they interact with fungi is one of the great challenges to plant biologists.
Here’s a short video showing the release of border cells from a plant root cap:
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?
This paper in Science, investigated molecular conversation between a parasitic plant, dodder (C. pentagona), and two host plants, by sequencing all three transcriptomes.
Genes, defined segments of DNA (deoxyribonucleic acid), must be “read” and copied (transcribed) into RNA (ribonucleic acid). These gene readouts are called transcripts, and a transcriptome is a collection of all the gene readouts present in a cell, The major type, of gene readout is called messenger RNA (mRNA), which plays a vital role in making proteins that can have a profound impact on an organism. The production of these proteins can vary, depending on both environmental and genetic influences.
The researchers found thousands of mRNAs moving in a bidirectional manner between species. These transcripts represented thousands of different genes. Researchers think this molecular conversation might allow the parasitic plant to direct the host plant to dampen its defense responses.
Since we also contain ancient, and potentially active viral or bacterial transcriptomes in our genes, at least some of the voices we hear are real.
Is it really so odd to consider kin recognition in plants? Plant roots grow more in proximity to genetically related plants (Bhatt) and the recognition of kin is based on chemicals secreted by the roots (Biedrzycki). In a recent paper in New Phytologist, Crepy and Casal noted that plants also react to kin in the aerial portions; first by reorienting leaf growth when growing near kin, but not near unrelated plants of the same species; and secondly, by producing more seeds when interacting with kin vs. nonkin.
Can we then consider plants connected via mycorrhizal associations step families?
Bhatt, MV, Khandelwal, A, Dudley, DA. (2011) Kin recognition, not competitive interactions, predicts root allocation in young Cakile edentula seedling pairs. New Phytologist. 189: 1135-=1142.
Biedrzycki, ML, Jalany, TA, Dudley, SA, Bais, HP. (2010) Root exudates mediate kin recognition in plants. Communicative and Integrative Biology. 3. 28-35.
Crepy, MA and Casal, JJ. (2014) Photreceptor-mediated kin recongition in plants. New Phytologist. 205: 329-338.
Well, dirt plus nutrient content. Organic farmers know that it’s really about the soil. In particular, the “living” component of the soil. Researchers are now catching up with findings that help explain why soils on organic farms and in native woodland ecologies have greater concentrations of fungal spores in the soil and greater levels fungal colonization of plant roots – particularly the symbiotic or helpful fungi.
Mycorrhizal fungi form a symbiotic relationship with plant roots, each exchanging benefits with the other. The plant gains phosphorous from the extended “root-like” threads of fungal hyphae, while the fungi absorb glucose stored in plant root cells, which was originally metabolized (made) by the plant during photosynthesis. Additional benefits these fungi provide the plants include enhanced disease resistance, soil stability and structure, as well as nitrogen fixation.
However, the fungus cannot be cultivated in the absence of a host plant root. Commercial farming often suffers from dead soil. The USDA’s Eastern Regional Research Center (ERRC) focuses research on the use of mycorrhizal fungi to improve crop quality and yield. Researchers at this facility try to understand the necessary chemical signal exchanged between plant and fungus required during the various stages of fungal development. Their aim is to grow the fungus on artificial media without the presence of plant roots. Because of the numerous benefits that mycorrhizal fungi provide, commercial farmers hope that a fungal inoculum could then be used to limit the amount of fertilizers applied to large scale crops while still improving plant growth and health.
I’ll come back to the way plant and fungus woo each other, whispering sweet chemical cross talk…
Understanding 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?
Bean, GA. (1999) Unpublished.
Baskin, CC and Baskin, JM. (2014) Seeds: Ecology, Biogeography, and, Evolution of Dormancy and Germination,2nd Edition. Academic Press: Cambridge, MA.
Cech, R. (2002) Growing At-Risk Medicinal Herbs: Cultivation, Conservation, and Ecology. Horizon Herbs Publication, Williams, Oregon, pp.41-51.
Celenza JL, Grisafi PL, Fink GR (1995) A pathway for lateral root formation in Arabidopsis thaliana. Gene Dev 9: 2131–2142.
Pitea, M, and Margineanu, C. (1972) Correlations between Chemical Structure and Antibacterial Activity of Berberine. Clujul Med 45:465.
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.
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.
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.
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.