Peat substrate amended with chitin modulates the N-cycle, siderophore and chitinase responses in the lettuce rhizobiome

C De Tender,J Debode,A Haegeman,E E Kuramae,P Dawyndt,T Ruttink,B Vandecasteele,B Mesuere,F Van Der Jeugt

doi:10.1038/s41598-019-46106-x

C De Tender, J Debode + Show 7 more

Open Access

https://doi.org/10.1038/s41598-019-46106-x

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Abstract

Chitin is a valuable peat substrate amendment by increasing lettuce growth and reducing the survival of the zoonotic pathogen Salmonella enterica on lettuce leaves. The production of chitin-catabolic enzymes (chitinases) play a crucial role and are mediated through the microbial community. A higher abundance of plant-growth promoting microorganisms and genera involved in N and chitin metabolism are present in a chitin-enriched substrate. In this study, we hypothesize that chitin addition to peat substrate stimulates the microbial chitinase production. The degradation of chitin leads to nutrient release and the production of small chitin oligomers that are related to plant growth promotion and activation of the plant’s defense response. First a shotgun metagenomics approach was used to decipher the potential rhizosphere microbial functions then the nutritional content of the peat substrate was measured. Our results show that chitin addition increases chitin-catabolic enzymes, bacterial ammonium oxidizing and siderophore genes. Lettuce growth promotion can be explained by a cascade degradation of chitin to N-acetylglucosamine and eventually ammonium. The occurrence of increased ammonium oxidizing bacteria, Nitrosospira, and amoA genes results in an elevated concentration of plant-available nitrate. In addition, the increase in chitinase and siderophore genes may have stimulated the plant’s systemic resistance.

Highlights

In soil, degradation of chitin is mediated by enzymes produced by certain bacteria and several fungi, such as Streptomyces and Trichoderma[2]
After eight weeks of plant growth in chitin-amended peat substrate, there was a significant increase in NO3−, total mineral N and Ca2+ levels and a significant decrease in pH and water-extractable P in the chitin-amended peat substrate
Na concentration was similar in the peat, the peat-chitin mixture and the pure chitin, while Cl content was higher in pure chitin than peat-amended with chitin and peat substrate (Table 1; Supplementary Table S1)

Summary

Introduction

Degradation of chitin is mediated by enzymes produced by certain bacteria and several fungi, such as Streptomyces and Trichoderma[2]. Besides its importance as structural polymer, chitin and its deacetylated form chitosan display biological activity Beneficial effects of both compounds have been reported regarding plant growth and disease resistance. The reduction in Salmonella survival can be related to chitin mediated elicitation of the plant defense response[6] For both responses, the production of microbial chitin-catabolic enzymes is essential for the release of plant-available nutrients and small chitin-oligomers to the peat substrate. Sequencing of bacterial (16S rRNA gene) and fungal (ITS2 gene region) phylogenetic markers showed that chitin increased the relative abundance of PGPR and other rhizosphere microorganisms reported to be involved in the N-cycle and chitin degradation (e.g., Cellvibrio, Pedobacter, Dyadobacter, Streptomyces, Lecanicillium and Mortierella spp.) This information is based on taxonomy, ; no functional information of the microbiota on rhizosphere in peat substrate with chitin is available so far. To evaluate the release of N and other plant-available nutrients to the peat substrate, we evaluated the nutrient content in unamended and chitin-amended peat substrates

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Conclusion