Abstract: Interest in the problem of plant nitrogen nutrition is increasing. Certain plants can use not only inorganic nitrogen, but also intact amino acids and short peptides. According to our studies, the roots of several agricultural and wild-living plants are able to exude proteases and by using them to create a pool of accessible N. This mini-review offers an overview of the problem of protease exudation by plant roots and its potential role in plant nitrogen nutrition.
Abstract: Our aim was to compare enzyme activities (tannase, polyphenol oxidase and protease) with concentrations of tannins and their ability to precipitate proteins in the litter layer and the humus layer under silver birch (Betula pendula Roth.) and Norway spruce (Picea abies L.). We also estimated the influence of these enzymes on protein–tannin complexes and the influence of tannins on proteolytic activity. The study site was a tree species experiment in Eno, middle-eastern Finland, having three replicated plots dominated by 42-year-old silver birch and Norway spruce. Our hypotheses were (1) tree species and soil layer have an influence on tannin concentrations and enzyme activities, (2) that tannin and protein concentrations in soil organic horizon are positively correlated with enzyme activities and (3) that the enzymes studied have the ability to degrade tannin–protein complexes and that tannins can inhibit proteolytic activity. Concentrations of total tannins and hydrolysable tannins, and tannase and proteolytic activities were higher in the humus layer than in the litter layer. In general the highest values of concentrations of total tannins and hydrolysable tannins and enzyme activities were obtained for the birch humus layer, but the concentrations of condensed tannins and proteins were highest in the litter layer and under spruce. A strong correlation between substrate concentration and enzyme activity was found between hydrolysable tannins and tannase activity. Polyphenol oxidase showed similar activities in both layers. To study the influence of enzymes on protein–tannin complex we synthesized such complexes using bovine serum albumin and either condensed tannins from silver birch and Norway spruce needles or a hydrolysable tannin, tannic acid. Studies with commercial enzymes and enzymes extracted from the soil showed some decrease in tannin concentration of the tannin–protein complex over time, but surprisingly, only a negligible decrease in protein concentration. Complexes of protein with condensed tannins were more recalcitrant than tannic acid–protein complexes. Tannins, depending on the concentration and chemical structure, tended to inhibit proteolytic activity. Our results indicate that protein–tannin complexes are relatively recalcitrant since the enzymes studied here do not effectively release protein from the complexes. Also proteolytic activity and the concentration of extractable proteins seem to be low in soil. However, tannin-degrading enzymes showed high activities.
Abstract: Nitrogen is one of the crucial elements that regulate plant growth and development. It is well-established that plants can acquire nitrogen from soil in the form of low-molecular-mass compounds, namely nitrate and ammonium, but also as amino acids. Nevertheless, nitrogen in the soil occurs mainly as proteins or proteins complexed with other organic compounds. Proteins are believed not to be available to plants. However, there is increasing evidence to suggest that plants can actively participate in proteolysis by exudation of proteases by roots and can obtain nitrogen from digested proteins. To gain insight into the process of organic nitrogen acquisition from proteins by leek roots (Allium porrum L. cv. Bartek), casein, bovine serum albumin and oxidized B-chain of insulin were used; their degradation products, after exposure to plant culture medium, were studied using liquid chromatography–mass spectrometry (LC–MS). Casein was degraded to a great extent, but the level of degradation of bovine serum albumin and the B-chain of insulin was lower. Proteases exuded by roots cleaved proteins, releasing low-molecular-mass peptides that can be taken up by roots. Various peptide fragments produced by digestion of the oxidized B-chain of insulin suggested that endopeptidase, but also exopeptidase activity was present. After identification, proteases were similar to cysteine protease from Arabidopsis thaliana. In conclusion, proteases exuded by roots may have great potential in the plant nitrogen nutrition.
Abstract: This paper reports on the role of proteases secreted by roots in nitrogen capture by plants. The study was conducted on aseptically cultivated wheat seedlings (Triticum aestivum cv. Tacher) obtained from embryos isolated from grains. Seedlings were cultivated for 21 days on deionised water, Murashige Skoog medium (MS), MS without inorganic nitrogen (IN), and MS without IN, in which IN was replaced by casein (0.01%, 0.1% or 1%). Comparison of seedlings grown on these media showed that casein entirely compensated for the lack of inorganic nitrogen in the medium. Shoots and roots of seedlings cultivated on MS medium with this protein had higher fresh weight than those cultivated on MS medium without casein. The increase in fresh weight of seedlings was correlated with casein concentration and proteolytic activity in the medium. In conclusion, wheat that uses proteases secreted by the roots can directly utilise proteins in the medium as a source of nitrogen without prior digestion by microbial proteases and without protein mineralisation. These results suggest the important role of organic nitrogen fertilisers in increasing wheat yield.
Abstract: The aim of this study was to compare the concentration of tannins and their capacity to precipitate proteins in the dominant species of ground vegetation (Deschampsia flexuosa (L.) Trin., Pleurozium schreberi (Brid.) Mitt., Vaccinium myrtillus (L.), and Vaccinium vitis-idaea (L.)) and in different layers of the soil organic horizon (litter layer—L, fermentation layer—F, humified layer—H) under silver birch (Betula pendula Roth.), Norway spruce (Picea abies (L.) Karst.), and Scots pine (Pinus sylvestris L.). Total tannin concentrations were also measured in leaves or needles of birch, spruce, and pine. The study site is located in Kivalo, northern Finland, close to the Arctic Circle. Differences in total tannin concentrations in ground vegetation were due mainly to species, with Vaccinium species having the highest values. The influence of the dominant tree species was less important. Protein precipitating capacity was dependent on plant species; the highest values occurred in Vaccinium species and spruce. Because of their relatively high protein precipitating capacity but low total tannin concentration, D. flexuosa and P. schreberi seemed to have more astringent tannins. Concentrations of total tannin and hydrolyzable tannin in the soil organic horizon differed depending on the layer and tree species. In general, the highest concentrations of total tannins were found under birch and spruce in the L layer and the lowest concentrations under pine. Protein precipitating capacity was usually the lowest in the H layer and highest under birch and spruce in the F and H layers. We showed that lignin from rotted pine wood can also precipitate proteins but only small amounts; additionally, lignin can be an important source of error for soil total tannin measurements.
Abstract: The aim of our study was to find out if the culture medium of aseptically cultivated seedlings exhibits proteolytic activity and if this event is universal in angiospermous plants. Seedlings of 15 agricultural and wild-living plant species were cultivated for 14 days without any addition of nutrients. Our studies showed that roots of higher plants could secrete proteases and that levels of proteolytic activity in the culture medium of individual species (and cultivars of the same species) could be significantly different. The differences between quantities of the secreted proteases were connected neither with the fresh weight of the growing seedlings nor with the surface of the root system. No proteins were required to induce secretion of proteases. The culture medium of a few studied species (Allium porrum, Zea mays, Helianthus annuus) showed the highest proteolytic activity at pH 7. Studies of the influence of standard protease inhibitors showed that examined proteases belong to the cysteine protease family. The results suggest that the apical parts of roots exuded proteases more intensively than mature parts. Our studies suggest that some plant species could develop a strategy to actively increase the level of free amino acids in the soil solution as a source of N. Our results may contribute to studying plant N nutrition in natural ecosystems and to increasing yield after organic fertilization of agricultural species.
