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
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…