Plants interact in their rhizosphere with a plethora of soil borne microorganisms such as fungi, bacteria or protists. Thereby, they have to differentiate between those that are pathogenic and those that support plant performance, by either microbial biocontrol (support of defence) or growth promotion (by hormonal stimulation, improved nutrient acquisition or increased tolerance). Thus, defence mechanisms have to be activated in case of pathogen attack and are related to the induction of defence compounds. At the same time, beneficial microbes should not be deterred but have to be recruited to the plant roots. Therefore, communication in both directions has to be fine-tuned by specific metabolite cocktails, which are released by plant roots and summarized as exudates.
Plants exude a large amount of fixed carbon through their roots
To better understand how plants shape the microbial community in their rhizosphere attempts have been made to analyse the composition of root exudation. Different analytical techniques have been used that revealed a complex diversity of metabolites, including a range of primary metabolites and variable secondary metabolites. Altogether, plants exude about 5-20% of their fixed carbon into the soil. Especially sugars and amino acids represent a valuable nutrient source for all soil borne microorganisms. Most probably, the specialized (secondary) metabolites are key players in the specific interaction with microbes. Until now, the knowledge of exudation is far from being well understood. Only few compounds have been identified and information on specificity towards microbial recruitment strategies is still elusive.
Microorganisms mineralise organically bound nutrients in soil
It is well known that microorganisms are capable to mineralise inorganic nutrients from organic soil matter for their growth, e.g. phosphate, ammonium or sulfate. These macronutrients are essential sources for plants that do not possess the ability to use the organic forms in an efficient manner. This makes the interaction of plants with microorganisms profitable so that plants evolved strategies to recruit beneficial microbes to their roots.
Recruitment of beneficial interaction partners requires specific signals
In CEPLAS, we aim to identify compounds that are crucial for the communication with beneficial microorganisms. Therefore, we use contemporary analytical techniques (mass spectrometry) that allow sensitive detection of metabolites. We use Arabidopsis thaliana as model organism that allows investigation of candidate genes on exudate composition, and to correlate the observed metabolite patterns to differences in microbial interaction. Furthermore, we apply nutrient limitations for nitrate, phosphate and sulfate to detect specific signals that are exuded under these selected limitations.
Especially under nutrient limitation it is supposed that plants stimulate microbial activity and influence the microbial community structure in their rhizosphere to eventually enhance nutrient uptake. Profound understanding of these mechanisms is essential to improve interaction efficiencies and therefore specific fertilization strategies leading to future applications in a sustainable agriculture.
Contribution by Manuela Peukert, AG Kopriva, Cologne Biocenter, UoC
Under the heading Planter’s Punch we present each month one special aspect of the CEPLAS research programme. All contributions are prepared by our young researchers.
Bai, Y., Müller, D.B., Srinivas, G., Garrido-Oter, R., Potthoff, E., Rott, M., Dombrowski, N., Münch, P.C., Spaepen, S., Remus-Emsermann, M., Hüttel, B., McHardy, A.C., Vorholt, J.A. and Schulze-Lefert, P. (2015) Functional overlap of the Arabidopsis leaf and root microbiota. Nature, 528, 364-369. [Abstract]
Carvalhais, L.C., Dennis, P.G., Fan, B., Fedoseyenko, D., Kierul, K., Becker, A., von Wiren, N. and Borriss, R. (2013) Linking plant nutritional status to plant-microbe interactions. PloS one, 8, e68555. [Abstract]
Haney, C.H., Samuel, B.S., Bush, J. and Ausubel, F.M. (2015) Associations with rhizosphere bacteria can confer an adaptive advantage to plants. Nature Plants, 1, 15051. [Abstract]