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Mining the biomass deconstructing capabilities of rice yellow stem borer symbionts

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JournalBiotechnology for biofuels
DateAccepted/In press - 24 Oct 2019
Original languageEnglish

Abstract

Efficient deconstruction of lignocellulosic biomass into simple sugars in
an economically viable manner is a prerequisite for its global acceptance as a
feedstock in bioethanol production. This is achieved in nature by suites of enzymes
with the capability of efficiently depolymerizing all the components of lignocellulose.
Here we provide detailed insight into the repertoire of enzymes produced by
microorganisms enriched from the gut of the crop pathogen rice yellow stem borer (
Scirpophaga insertulas ).
Results: A microbial community was enriched from the gut of the rice yellow stem
borer for enhanced rice straw degradation by sub-culturing every 10 days, for one
year, in minimal medium with rice straw as the main carbon source. The enriched
culture demonstrated high cellulolytic and xylanolytic activity in the culture supernatant.
Metatranscriptomic and metaexoproteomic analysis revealed a large array of enzymes
potentially involved in rice straw deconstruction. The consortium was found to encode
genes ascribed to all 5 class of carbohydrate active enzymes (GHs, GTs, CEs, PLs
and AAs), including Carbohydrate-Binding Modules (CBMs), categorized in the
Carbohydrate-Active enZYmes (CAZy) database. The GHs were the most abundant
class of CAZymes. Predicted enzymes from these CAZy classes have the potential to
digest each cell wall components of rice straw i.e. cellulose, hemicellulose, pectin,
callose and lignin. Several identified CAZy proteins appeared novel, having an
unknown or hypothetical catalytic counterpart with a known class of CBM. To validate
the findings, one of the identified enzymes that belongs to the GH10 family was
functionally characterized. The enzyme expressed in E. coli efficiently hydrolyzed
beechwood xylan, and pretreated and untreated rice straw.
Conclusions: This is the first report describing the enrichment of lignocellulose
degrading bacteria from the gut of the rice yellow stem borer to deconstruct rice straw,
identifying a plethora of enzymes secreted by the microbial community when growing
on rice straw as a carbon source. These enzymes could be important candidates for
biorefineries to overcome the current bottlenecks in biomass processing.

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