Semi-overexpressed OsMYB86L2 specifically enhances cellulose biosynthesis to maximize bioethanol productivity by cascading lignocellulose depolymerization via integrated rapid-physical and recyclable-chemical processes

Abstract

Genetic engineering of plant cell walls has been implemented in bioenergy crops, but the tradeoff between biomass production and lignocellulose recalcitrance remains to be resolved. Although OsMYB86L2 overexpression caused a defective phenotype in a homozygous Ho86 mutant, this study found that its semi-overproduction could up-regulate cellulose biosynthesis and down-regulate non-cellulosic polymer assembly into cell walls in a heterozygous He86 mutant, which not only generated a desirable substrate that consists of a high level of cellulose and low-recalcitrance lignocellulose but also resulted in the accumulation of a much higher level of fermentable sugars (a 1.6-fold increase) with a similar grain yield to the wild type. After incubation with a recyclable alkali (CO) or organic acid (oxalic acid) and a brief (1–2 min) microwave irradiation pretreatment, the He86 mutant showed near-complete biomass saccharification from ultrasound-assistant enzymatic hydrolysis, leading to either a high yield of cellulosic ethanol (15–17% dry matter) or maximum total ethanol (25–26% dry matter) via engineered yeast fermentation. As these two optimal integrated pretreatments could largely co-extract the wall polymers to reduce cellulose polymerization and increase lignocellulose accessibility and porosity, accompanied by a distinct reduction in chemical inhibitor release, this study finally proposed a novel mechanism to elucidate how the modified lignocellulose can be completely digested and efficiently converted via integrated biomass processes, providing insights into precise lignocellulose modification and effective biomass engineering.