eXtra Botany |
Root secretions: from genes and molecules to microbial associations
1Department of Plant and Soil Sciences, University of Delaware, Newark, DE 19716, USA
2Delaware Biotechnology Institute, 15 Innovation Way, Newark, DE 19711, USA
* E-mail: hbais{at}udel.edu
As the field of root biology continues to gain momentum, researchers have begun to recognize the importance of root secretions in innumerable plant–plant and plant–microbe interactions hidden beneath the ground. It is well documented that diverse plant species form both beneficial and harmful associations with soil bacterial communities. Understanding all of the factors involved in these rhizosphere communications is the daunting task that has been laid before root biologists. However, Micallef et al. (2009) have made progress in this field by describing the influence of genetic background on root secretions in Arabidopsis thaliana and the implications of the unique root secretion patterns on associated bacterial communities.
In the past, root secretion patterns of rice, wheat, and barley have been analysed and compared, revealing variations between species (Mazzola et al., 2004). In addition, root secretion profiles of the model plant A. thaliana (Columbia-0) have been analysed after treatment with biological elicitors and signalling molecules to reveal 289 possible different secondary metabolites present in the secretions (Walker et al., 2003; Narasimhan et al. 2003). These studies point to the countless combinations of secondary metabolites that may be present in the rhizosphere at any given time. Micallef et al. (2009) took the current research one step further and were interested in determining whether genetic influences within a species can produce unique root secretion cocktails. In their recent paper, they compared the differences between root secretion profiles, through HPLC, of eight different A. thaliana accessions (ecotypes) and determined that root secretion profiles did indeed differ in the compounds present and in the relative abundance of many of these compounds (Micallef et al., 2009). As all of the plants from different accessions were grown under the same conditions, natural genetic variation is attributed to cause differences in the secreted compounds. These data support the research of Clark et al. (2007) where it was demonstrated that when 20 A. thaliana accessions were compared, approximately 4% of the genome differed or were deleted with reference to the control (Columbia-0). As the eight different accessions tested by Micallef et al. (2009) were originally collected from a wide geographical range, it supports the suggestion that allelic variations between these plants may confer a selective advantage in certain environments, as has been demonstrated for traits such as flowering time and plant growth (Koornneef et al., 2004). Along these lines, Micallef et al. (2009) further investigated as to whether these unique secretion cocktails resulted in variations in bacterial community associations.
Previous research by Broeckling et al. (2008) demonstrated that, in both model species, Medicago truncatula and A. thaliana, plants were able to maintain resident soil fungal populations but were not able to maintain non-resident populations, demonstrating that root exudates are able to regulate some soil microbe populations. Micallef et al. (2009) continued this line of research and demonstrated that bacterial populations are also influenced by root secretion compounds as they found that each of the eight accessions tested has distinct and reproducible bacterial community associations. These bacterial community associations differed in the species present and in the abundance of the species associated with the A. thaliana roots. Although a direct link was not examined, strong evidence has been presented to support that differences in bacterial community assemblages is a result, in part, of root secretions compounds. Other root traits that differ between accessions, such as root architecture are likely to play a role as well.
It has long been known that microbial communities in the soil contribute valuable nutrients to plants (e.g. rhizobia and Azolla-associated cyanobacteria provide nitrogen to legumes and to rice, respectively). In addition, beneficial microbes can also protect plants from disease, as shown by the recognized ‘suppressive soil" effect (Schroth and Hancock, 1982). Yet relatively little is known about the diversity of microbes that associate with plants, i.e. the microbiome, and their combinatorial interactions and effects on performance and plant yields. Root-derived microbial colonization initiation and development is complex and not well understood due to the dynamic nature of plant root surfaces and microbial diversity. However, it remains unclear whether specific plant derived compounds influence colonization structures and if microbial colonization patterns affect the plant genomic and metabolic responses. Although, the studies by Micallef et al. (2009) do not deal, in detail, about the direct involvement of a specific compound(s) in recruitment of the microbiome, it very well sets the path forward to understand the effect of plant genomic background on positive feedback.
A recent paper by Rudrappa et al. (2008), utilized A. thaliana roots to show that plant roots selectively secret organic compounds and signal beneficial rhizospheric microbes. This unprecedented non-symbiotic interaction establishes and supports the work of Micallef et al. (2009) to reveal the regulatory role of root metabolites in the recruitment of beneficial microbes as well as underscores the breadth and sophistication of plant–microbial interactions. The question that still awaits an answer is specifically how plants could structure the surrounding rhizosphere and what host genomic functions are affected upon microbial association?
In summary, the study by Micallef et al. (2009) is significant for presenting information in two areas. First, by supporting that genetic differences between ecotypes within a species are diverse and play an important role in determining overall plant functions. Also, this study by Micallef et al. (2009) further investigates the role of genetically controlled root secretions and their effect on the surrounding rhizosphere and bacterial communities. Further research in this area may lead to a greater understanding of how root secretions are regulated and allow for the manipulation of these secretions to produce beneficial associations in agricultural species. A comprehensive understanding of the effects of the microbiome on plants will enable the development of agricultural technologies that exploit the natural alliances among microbes and plants, and provide new avenues to increase yields beyond conventional plant genetics and breeding.
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Broeckling CD, Broz AK, Bergelson J, Manter DK, Vivanco JM. Root exudates regulate soil fungal community composition and diversity. Applied Environmental Microbiology (2008) 74:738–744.
Clark RM, Schweikert G, Toomajian C, et al. Common sequence polymorphisms shaping genetic diversity in Arabidopsis thaliana. Science (2007) 317:338–342.
Koornneef M, Blanco CA, Vreugdenhil D. Naturally occurring genetic variation in Arabidopsis thaliana. Annual Review of Plant Biology (2004) 55:141–172.[CrossRef][Medline]
Mazzola M, Funnell DL, Raaijmakers JM. Wheat cultivar-specific selections of 2,4 diacetylphloroglucinol-producing fluorescent Pseudomonas species from resident soil populations. Microbial Ecology (2004) 48:338–348.[CrossRef][Web of Science][Medline]
Micallef SA, Shiaris MP, Colon-Carmona A. Influence of Arabidopsis thaliana accessions on rhizobacterial communities and natural variation in root exudates. Journal of Experimental Botany (2009) 60:1729–1742.
Narasimhan K, Basheer C, Bajic VB, Swarup S. Enhancement of plant–microbe interactions using a rhizosphere metabolomics-driven approach and its application in the removal of polychlorinated biphenyls. Plant Physiology (2003) 132:146–153.
Rudrappa T, Czymmek KJ, Pare PW, Bais HP. Root-secreted malic acid recruits beneficial soil bacteria. Plant Physiology (2008) 148:1547–1556.
Schroth MN, Hancock JG. Disease-suppressive soil and root-colonizing bacteria. Science (1982) 216:1376–1381.
Walker TS, Bais HP, Halligan KM, Stermitz FR, Vivanco JM. Metabolic profiling of root exudates of Arabidopsis thaliana. Journal of Agricultural and Food Chemistry (2003) 41:2548–2554.
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