Plant Root Exudates
Variation between Species and Reaction to Water Deficit
von Pervin Akter
Datum der mündl. Prüfung:2016-11-17
Erschienen:2017-02-09
Betreuer:Prof. Dr. Petr Karlovsky
Gutachter:Prof. Dr. Petr Karlovsky
Gutachter:PD Dr. Franz Hadacek
Zum Verlinken/Zitieren:
http://dx.doi.org/10.53846/goediss-6115
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Name:Ph.D. thesis_Pervin Akter.pdf
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Zusammenfassung
Englisch
Root exudates represent complex mixtures of low-molecular-weight and high-molecular-weight compounds. The former comprise predominantly primary and secondary plant metabolites, the latter mucilage precursors and some proteins. Different collection approaches exist. The traditionally most widely used one (also applied in this study) is soaking roots for several hours in distilled water that have been thoroughly cleaned from soil. Other collection methods comprise hydroponic cultures and rhizoboxes with microsuction devices. The former are more used by molecular biologists in attempts to characterize phenotypes reproducibly, the latter by ecologists in efforts to explore specific regions of the rhizosphere. The classical approach is one that still allows plant to be cultured in soil. This is important because mucilage formation is affected by microbial soil communities and soil physicochemical properties. Primary and secondary plant meta¬bolites provide the majority of low-molecular precursors for mucilage development. Quality and quantity of extractable root-exuded plant metabolites is most probably affected by these parameters which are completely absent in hydroponic cultures. Six model plants were chosen on basis of their status as crop plant and on their tolerance of the uniform culture conditions to which all model plants were subjected to in the only available climate chamber. These included the Brassicaceae Arabidopsis and Rapeseed, the Fabaceae Phaseolus and Pisum, the Solanaceae Tobacco and the grass Maize. In all experi¬ments, the model plants received identical amounts of light, the same water supply and nutrient provision. In attempts to simulate drought stress, one-half of the plants was not deprived of water for two consecutive weeks. One aim was to explore which and to what amounts primary as well as secondary plant metabolites do occur in the root exudates and if they differ from those present in the roots. All six plant species showed similar primary metabolite profiles that, however, varied quanti¬tatively between the plant species. A prominent root exudate metabolite was myo-inositol, a sugar alcohol. Root tissues and root exudates showed different profiles with amino acids showing the most profound differences. The found primary metabolites agree with those reported in the literature. By contrast, secondary metabolites showed characteristic profiles, in which only few com-pounds were common to more than one species. One metabolite that was detected in all species was cinnamic acid. Structure elucidation was focussed especially on those secondary plant metabolites that were pointed out by non-parametric multivariate statistics as substantial contributors to similarity and dissimilarity of root exudates and root tissues. Root exudates were found to contain chalcones, flavanols, isoflavones, cinnamides, a cinnamoyl spermidine, indoles, stilbenes, a hydroxamic acid benzoxazine and a gibberellic acid deri¬vative, amongst others. Notably, no glycosides were detected among the elucidated metabolites and a considerably high proportion of aldehydes was noted. In case of Arabidopsis, an extensive analysis of hydroponically obtained root exudates exists in the literature. Many dimeric structures were reported, most of which could not be detected in the present study. Another explored aspect was the effect of water deficit on root exudation. Primary metabolite patterns changed in a more similar way, sugars such as glucose and sucrose increased and myo-inositol proportions decreased. Amino acid pattern changes, by contrast, were more species-specific. Generally, the amounts of detectable secondary metabolites decreased as shoot: root ratios in the affected plants increased. Only Phaseolus and Maize showed higher shoot: root ratios after water deficit. This suggests a different, more opportunistic strategy to survive stress. Only Pisum exuded a new class of secondary metabolites that was absent in the regularly watered plants. Altogether 24 different root exudate samples were available. Bases on the variability of primary and secondary root exudate metabolites correlations with nutrient supply in leaves was explored by Spearman rank correlation. Interestingly and as once suggested in a previous review, weak correlations between secondary metabolite profiles and leaf nutrients were found. Especially more unsaturated metabolites with vicinal oxygen functions correlated with the uptake of several nutrients, most of them being metal cations. The structural properties of the identified secondary metabolites allows them to act as ligands in coordination complexes in which the nutrient represents the central atom. This chemistry can add to the mobilization and uptake of nutrients by plant roots.
Keywords: Plant root exudates; Primary metabolites; Secondary metabolites; Water deficit; Arabidopsis; Rapeseed; Green bean; Pea; Tobacco; Maize