Biology

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:
[youtube https://www.youtube.com/watch?v=2ocGwAoMU-M?rel=0]
More in-depth readings:
Harrison, M. and Dixon, R. (1993) Isoflavonoid accumulation and expression of defense gene transcripts during establishment of vesicular-arbuscular mycorrhizal associations in roots of Medicago truncatula. Mol. Plant Microbe Interact. 6:643-654
Hawes, M,C. et al (1998) Function of root border cells in plant health: Pioneers in the Rhizosphere, Annual Review of Phytopathology, 36:311-327.
Hawes, M.C. et al (2003) Root Caps and Rhizosphere. J. Plant Growth Reg. 21:353.
Kape, R. et al (1992) Legume root metabolites and VA-mycorrhiza development. J. Plant Physiol. 141:54-60.
Phillips D.A. et al (2004) Microbial products trigger amino acid exudation from plant roots. Plant Phys. 136: 2887-2894

How do you interact with plants? ? Why do you interact with plants?

brussel sprouts

We respond to a spectrum of sensory effects

visual – pigments
taste – spice
smell – aromatic oils
effect – pharmacologically active

Evolutionarily speaking, it remains unclear whether pharmacological use of plants by humans was more prevalent before or after the development of agriculture led to cultivars with reduced biological activity compared to the wild types. Dr. Fatimah Jackson, at the University of Maryland, College Park, argues succinctly that cultural evolution – driven by language – became the driver influencing the extent of human interaction with plants. Dietary preferences are central to how cultures self identify and define. According to Daniel Moerman at the University of Michigan, Native Americans used plants in a 5:1 ratio as medicine and food. Over time humans have learned how to limit their exposure to toxic plants. I imagine a group of early humans going out as a group and asking ‘Mikey’ to try the plant first. If he lived, ‘Mikey’ discovered how to modifying plants’ palatability, nutrition, toxins and to amplify beneficial effects through various means – extraction, heating, drying, and fermentation to name a few. What examples exist from your own cultural heritage of unique use of plants and their chemistry?

Is this a form of co-evolutionary symbiosis between humans and plants? I would argue that humans have had profound effect on the genotype and phenotype of cultivated plants, while plants have provided nutrition, medicine, and the early stimulus for our enzymatic detoxification system and possibly for language development in the brain (synaesthesia – discussed in a future post). Dietary exposure to continuous low levels of plant mutagens would certainly effect mutation rates or genetic drift. I would highly recommend an article by Dr. Jackson on human-plant-parasite triads as evidence for coevolution.

Consider the next you avoid eating your bitter tasting brussel sprouts – if you don’t eat them, are you de-evolving?

Bhodi Tims

Outpost Communication

Co-Evolution of Humans and Plants