Fungal species of the Metarhizium genus colonize most land plants and help provide nitrogen to the plant root. The nitrogen source is unique – insects that the fungus has pathogenized and killed using enzymatic degradation of the insect’s shell.
Mike Bidochkaof Brock University investigated the phenomena by injecting labelled nitrogen into Galleria mellonella larvae (moth). They buried the larvae in soil and separated the larvae from either beans (Phaseolus vulgaris) or switchgrass (Panicum virgatum) plants using a screen with pores large enough for fungal mycelium to grow through but small enough to prevent plant root growth.
Fourteen days later, they found labelled nitrogen made up more than a quarter of nitrogen found in plant root tissue. Insects Larvae with labelled nitrogen not infected by the fungus did not act as nitrogen sources for the plant.
Good evidence for an ecosystem rich in biota, rather than one where selective human inputs alters it into a simpler set of relationships. In most cases, the soil environment becomes less sustainable.
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
As I’ve studied medicinal plants, one intriguing question keeps cropping up – what is the biological rationale for plants investing in several classes of structurally varied plant secondary metabolites?. Certainly this question drives a number of researchers in the field of plant chemical ecology. Edwards et al (2008) provided some striking evidence from their study of bacteria resistant cotton (Gossypium hirsutum).
They detected flavonoid pigments, chrysanthemin and isoquercitin, at unusually high levels in epidermal tissue of young leaves in response to Xanthomonas infection. These cells clustered around infected cells that had died because of the plant’s hypersensitive resistance response.
A very different chemical, a sesquiterpene, 2,7-dihyroxycadalene, acted as a light activated phytoalexin, destroying both bacteria and the infected plant cell. So what were the flavanoids doing to help out? The presence of the flavanoids in surrounding cells appeared to filter sunlight, limiting the generation of free radicals resulting from light activation of 2,7-dihyroxycadaline. How’s that for compartmentalizing your response to friends and enemies?
In an issue of New Phytologist, Claire Belcher provides a wonderful summary about work by Bond and Scott (from same issue) on the ecological role of fire in the spread of angiosperms (flowering plants). During the Cretaceous (145.5 to 65.5 mya, ending with extinction of dinosaurs), angiosperms were fast growing, weedy and largely understory herbs, shrubs and small trees more likely to colonize edge or disturbance sites. An improved plant vascular system, including a large increase in leaf vein density, doubled the photosynthetic rate and increased leaf mass in comparison with ferns and gymnosperms, which were the dominant land plants at the time. This adaption allowed angiosperms to work more efficiently fixing carbon from falling CO2 levels of the period. Fire also appeared to play a vital ecological role in the spread and ultimate dominance of flowering plants on earth.
Fossil charcoal demonstrated the presence of fire in the early ecosystem. The combination of highly flammable detritus from weedy angiosperms and increasing atmospheric oxygen levels created an angiosperm-fire cycle equivalent to modern prairie fire cycles. Once an ecosystem was fire damaged, the faster growing angiosperms out-competed both gymnosperms and ferns. The early fossil records indicate that fire activity was greater during the Cretaceous than in previous epochs. However, when oxygen levels dropped about 56 mya, fire-cycles decreased and angiosperm-dominated forests, such as tropical forests, expanded.
In fact, the resulting layers of charcoal helped preserve the fossil record of dinosaurs’ last days. Researchers were then able to predict what the Cretaceous forests looked like.
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.