Bacteria from the endosphere and rhizosphere of Quercus spp. use mainly cell wall-associated enzymes to decompose organic matter

Abstract

Due to the ability of soil bacteria to solubilize minerals, fix N2 and mobilize nutrients entrapped in the organic matter, their role in nutrient turnover and plant fitness is of high relevance in forest ecosystems. Although several authors have already studied the organic matter decomposing enzymes produced by soil and plant root-interacting bacteria, most of the works did not account for the activity of cell wall-attached enzymes. Therefore, the enzyme deployment strategy of three bacterial collections (genera Luteibacter, Pseudomonas and Arthrobacter) associated with Quercus spp. roots was investigated by exploring both cell-bound and freely-released hydrolytic enzymes. We also studied the potential of these bacterial collections to produce enzymes involved in the transformation of plant and fungal biomass. Remarkably, the cell-associated enzymes accounted for the vast majority of the total activity detected among Luteibacter strains, suggesting that they could have developed a strategy to maintain the decomposition products in their vicinity, and therefore to reduce the diffusional losses of the products. The spectrum of the enzymes synthesized and the titres of activity were diverse among the three bacterial genera. While cellulolytic and hemicellulolytic enzymes were rather common among Luteibacter and Pseudomonas strains and less detected in Arthrobacter collection, the activity of lipase was widespread among all the tested strains. Our results indicate that a large fraction of the extracellular enzymatic activity is due to cell wall-attached enzymes for some bacteria, and that Quercus spp. root bacteria could contribute at different levels to carbon (C), phosphorus (P) and nitrogen (N) cycles.

Fig 1. Activity of cell-associated and freely-released enzymes from genus Luteibacter.

Box and whiskers were represented in the style of Tukey. Hinges correspond to the 1^st and 3^th quartiles and medians of the activity of each enzyme recorded for all strains are represented by horizontal lines. Red points correspond to those data beyond 1.5 x Inter Quartile Range. An ‘*’ indicates significant differences between the activity of those enzymes detected in both fractions calculated with the non-parametric Mann-Whitney U test at a confidence level of 95%. Abbreviations of enzymes: bG: β-glucosidase; Pho: acid phosphatase; bX: β-xylosidase; bGal: β-galactosidase; CBH: cellobiohydrolase; aG: α-glucosidase; ChTN: chitinase; Lip: lipase; bGlu: β-glucuronidase; aA: α-arabinosidase; bM: β-mannosidase; aGal: α-galactosidase. All the data were log-transformed for graphical clarity by fitting the same scale.

Fig 2. Activity of cell-associated and freely-released enzymes from genus Pseudomonas.

Box and whiskers were represented in the style of Tukey. Hinges correspond to the 1^st and 3^th quartiles and medians of the activity of each enzyme recorded for all strains are represented by horizontal lines. Red points correspond to those data beyond 1.5 x Inter Quartile Range. An ‘*’ indicates significant differences between the activity of those enzymes detected in both fractions calculated with the non-parametric Mann-Whitney U test at a confidence level of 95%. Abbreviations of enzymes: bG: β-glucosidase; Pho: acid phosphatase; bX: β-xylosidase; bGal: β-galactosidase; CBH: cellobiohydrolase; aG: α-glucosidase; ChTN: chitinase; Lip: lipase; bGlu: β-glucuronidase; aA: α-arabinosidase; bM: β-mannosidase; aGal: α-galactosidase. All the data were log-transformed for graphical clarity by fitting the same scale.

Fig 3. Activity of cell-associated and freely-released enzymes from genus Arthrobacter.

Box and whiskers were represented in the style of Tukey. Hinges correspond to the 1^st and 3^th quartiles and medians of the activity of each enzyme recorded for all strains are represented by horizontal lines. Red points correspond to those data beyond 1.5 x Inter Quartile Range. No significant differences were detected between both fractions according to the non-parametric Mann-Whitney U test at a confidence level of 95%. Abbreviations of enzymes: bG: β-glucosidase; Pho: acid phosphatase; bX: β-xylosidase; bGal: β-galactosidase; CBH: cellobiohydrolase; aG: α-glucosidase; ChTN: chitinase; Lip: lipase; bGlu: β-glucuronidase; aA: α-arabinosidase; bM: β-mannosidase; aGal: α-galactosidase. All the data were log-transformed for graphical clarity by fitting the same scale.

Fig 4. Relationship of the number of enzymatic activities and bacterial strains.

(a) Percentage of Luteibacter, Pseudomonas and Arthrobacter strains exhibiting different enzymatic activities. (b) Enzymatic specialization of Luteibacter, Pseudomonas and Arthrobacter strains. The degree of specialization of each genus was measured as the total amount of strains producing different number of enzymes in liquid cultures. Abbreviations of enzymes: aA: α-arabinosidase; aG: α-glucosidase; aGal: α-galactosidase; CBH: cellobiohydrolase; bG: β-glucosidase; bGal: β-galactosidase; bM: β-mannosidase; ChTN: chitinase; Pho: acid phosphatase; bGlu: β-glucuronidase; bX: β-xylosidase; Lip: lipase.

Fig 5. Enzymatic profiles of Luteibacter Pseudomonas and Arthrobacter strains.

Data represent means of the total activity of three replicates and were log-transformed to fit the same scale. The clustering was constructed by using UPGMA algorithm based on Euclidean distances. Bacterial genera are colour-coded. Abbreviations of enzymes: Pho: acid phosphatase; Lip: lipase; ChTN: chitinase; bGlu: β-glucuronidase; bG: β-glucosidase; aA: α-arabinosidase; CBH: cellobiohydrolase; bX: β-xylosidase; bM: β-mannosidase; aGal: α-galactosidase; aG: α-glucosidase; bGal: β-galactosidase.