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:
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